content: {-# LANGUAGE BangPatterns        #-}
  {-# LANGUAGE CPP                 #-}
  {-# LANGUAGE DefaultSignatures   #-}
  {-# LANGUAGE DeriveDataTypeable  #-}
  {-# LANGUAGE DeriveGeneric       #-}
  {-# LANGUAGE GADTs               #-}
  {-# LANGUAGE OverloadedStrings   #-}
  {-# LANGUAGE ScopedTypeVariables #-}
  
  {-# OPTIONS_HADDOCK not-home #-}
  
  #include "version-compatibility-macros.h"
  
  -- | __Warning: internal module!__ This means that the API may change
  -- arbitrarily between versions without notice. Depending on this module may
  -- lead to unexpected breakages, so proceed with caution!
  --
  -- For a stable API, use the non-internal modules. For the special case of
  -- writing adaptors to this library’s @'Doc'@ type, see
  -- "Prettyprinter.Internal.Type".
  module Prettyprinter.Internal (
      -- * Documents
      Doc(..),
  
      -- * Basic functionality
      Pretty(..),
      viaShow, unsafeViaShow, unsafeTextWithoutNewlines,
      emptyDoc, nest, line, line', softline, softline', hardline,
  
      -- ** Primitives for alternative layouts
      group, flatAlt,
  
      -- * Alignment functions
      align, hang, indent, encloseSep, list, tupled,
  
      -- * Binary functions
      (<+>),
  
      -- * List functions
      concatWith,
  
      -- ** 'sep' family
      hsep, vsep, fillSep, sep,
      -- ** 'cat' family
      hcat, vcat, fillCat, cat,
      -- ** Others
      punctuate,
  
      -- * Reactive/conditional layouts
      column, nesting, width, pageWidth,
  
      -- * Filler functions
      fill, fillBreak,
  
      -- * General convenience
      plural, enclose, surround,
  
      -- ** Annotations
      annotate,
      unAnnotate,
      reAnnotate,
      alterAnnotations,
      unAnnotateS,
      reAnnotateS,
      alterAnnotationsS,
  
      -- * Optimization
      fuse, FusionDepth(..),
  
      -- * Layout
      SimpleDocStream(..),
      PageWidth(..), defaultPageWidth,
      LayoutOptions(..), defaultLayoutOptions,
      layoutPretty, layoutCompact, layoutSmart,
      removeTrailingWhitespace,
  
      -- * Rendering
      renderShowS,
  
      -- * Internal helpers
      textSpaces
  ) where
  
  
  
  import           Control.Applicative
  import           Data.Int
  import           Data.List.NonEmpty  (NonEmpty (..))
  import           Data.Maybe
  import           Data.String         (IsString (..))
  import           Data.Text           (Text)
  import qualified Data.Text           as T
  import qualified Data.Text.Lazy      as Lazy
  import           Data.Typeable       (Typeable)
  import           Data.Void
  import           Data.Word
  import           GHC.Generics        (Generic)
  
  -- Depending on the Cabal file, this might be from base, or for older builds,
  -- from the semigroups package.
  import Data.Semigroup
  
  #if NATURAL_IN_BASE
  import Numeric.Natural
  #endif
  
  #if !(FOLDABLE_TRAVERSABLE_IN_PRELUDE)
  import Data.Foldable    (Foldable (..))
  import Data.Traversable (Traversable (..))
  import Prelude          hiding (foldr, foldr1)
  #endif
  
  #if FUNCTOR_IDENTITY_IN_BASE
  import Data.Functor.Identity
  #endif
  
  import Prettyprinter.Render.Util.Panic
  
  
  
  -- | The abstract data type @'Doc' ann@ represents pretty documents that have
  -- been annotated with data of type @ann@.
  --
  -- More specifically, a value of type @'Doc'@ represents a non-empty set of
  -- possible layouts of a document. The layout functions select one of these
  -- possibilities, taking into account things like the width of the output
  -- document.
  --
  -- The annotation is an arbitrary piece of data associated with (part of) a
  -- document. Annotations may be used by the rendering backends in order to
  -- display output differently, such as
  --
  --   - color information (e.g. when rendering to the terminal)
  --   - mouseover text (e.g. when rendering to rich HTML)
  --   - whether to show something or not (to allow simple or detailed versions)
  --
  -- The simplest way to display a 'Doc' is via the 'Show' class.
  --
  -- >>> putStrLn (show (vsep ["hello", "world"]))
  -- hello
  -- world
  data Doc ann =
  
      -- | Occurs when flattening a line. The layouter will reject this document,
      -- choosing a more suitable rendering.
      Fail
  
      -- | The empty document; conceptually the unit of 'Cat'
      | Empty
  
      -- | invariant: not '\n'
      | Char !Char
  
      -- | Invariants: at least two characters long, does not contain '\n'. For
      -- empty documents, there is @Empty@; for singleton documents, there is
      -- @Char@; newlines should be replaced by e.g. @Line@.
      --
      -- Since the frequently used 'T.length' of 'Text' is /O(length)/, we cache
      -- it in this constructor.
      | Text !Int !Text
  
      -- | Hard line break
      | Line
  
      -- | Lay out the first 'Doc', but when flattened (via 'group'), prefer
      -- the second.
      --
      -- The layout algorithms work under the assumption that the first
      -- alternative is less wide than the flattened second alternative.
      | FlatAlt (Doc ann) (Doc ann)
  
      -- | Concatenation of two documents
      | Cat (Doc ann) (Doc ann)
  
      -- | Document indented by a number of columns
      | Nest !Int (Doc ann)
  
      -- | Invariant: The first lines of first document should be longer than the
      -- first lines of the second one, so the layout algorithm can pick the one
      -- that fits best. Used to implement layout alternatives for 'group'.
      | Union (Doc ann) (Doc ann)
  
      -- | React on the current cursor position, see 'column'
      | Column (Int -> Doc ann)
  
      -- | React on the document's width, see 'pageWidth'
      | WithPageWidth (PageWidth -> Doc ann)
  
      -- | React on the current nesting level, see 'nesting'
      | Nesting (Int -> Doc ann)
  
      -- | Add an annotation to the enclosed 'Doc'. Can be used for example to add
      -- styling directives or alt texts that can then be used by the renderer.
      | Annotated ann (Doc ann)
      deriving (Generic, Typeable)
  
  -- |
  -- @
  -- x '<>' y = 'hcat' [x, y]
  -- @
  --
  -- >>> "hello" <> "world" :: Doc ann
  -- helloworld
  instance Semigroup (Doc ann) where
      (<>) = Cat
      sconcat (x :| xs) = hcat (x:xs)
      stimes n x
        | n <= 0    = Empty
        | n == 1    = x
        | otherwise =
            let n' = fromIntegral n
                nx = hcat (replicate n' x)
            in case x of
                Fail            -> Fail
                Empty           -> Empty
                Char c          -> Text n' (T.replicate n' (T.singleton c))
                Text l t        -> Text (n' * l) (T.replicate n' t)
                Line            -> nx
                FlatAlt{}       -> nx
                Cat{}           -> nx
                Nest{}          -> nx
                Union{}         -> nx
                Column{}        -> nx
                WithPageWidth{} -> nx
                Nesting{}       -> nx
                Annotated{}     -> nx
  
  -- |
  -- @
  -- 'mempty' = 'emptyDoc'
  -- 'mconcat' = 'hcat'
  -- @
  --
  -- >>> mappend "hello" "world" :: Doc ann
  -- helloworld
  instance Monoid (Doc ann) where
      mempty = emptyDoc
      mappend = (<>)
      mconcat = hcat
  
  -- | >>> pretty ("hello\nworld")
  -- hello
  -- world
  --
  -- This instance uses the 'Pretty' 'Text' instance, and uses the same newline to
  -- 'line' conversion.
  instance IsString (Doc ann) where
      fromString = pretty . T.pack
  
  -- | Alter the document’s annotations.
  --
  -- This instance makes 'Doc' more flexible (because it can be used in
  -- 'Functor'-polymorphic values), but @'fmap'@ is much less readable compared to
  -- using @'reAnnotate'@ in code that only works for @'Doc'@ anyway. Consider
  -- using the latter when the type does not matter.
  instance Functor Doc where
      fmap = reAnnotate
  
  -- | Overloaded conversion to 'Doc'.
  --
  -- Laws:
  --
  --   1. output should be pretty. :-)
  class Pretty a where
  
      -- | >>> pretty 1 <+> pretty "hello" <+> pretty 1.234
      -- 1 hello 1.234
      pretty :: a -> Doc ann
  
      default pretty :: Show a => a -> Doc ann
      pretty = viaShow
  
      -- | @'prettyList'@ is only used to define the @instance
      -- 'Pretty' a => 'Pretty' [a]@. In normal circumstances only the @'pretty'@
      -- function is used.
      --
      -- >>> prettyList [1, 23, 456]
      -- [1, 23, 456]
      prettyList :: [a] -> Doc ann
      prettyList = align . list . map pretty
  
  -- $
  -- Issue #67: Nested lists were not aligned with »pretty«, leading to non-pretty
  -- output, violating the Pretty class law.
  --
  -- >>> pretty (replicate 2 (replicate 4 (1, replicate 8 2)))
  -- [ [ (1, [2, 2, 2, 2, 2, 2, 2, 2])
  --   , (1, [2, 2, 2, 2, 2, 2, 2, 2])
  --   , (1, [2, 2, 2, 2, 2, 2, 2, 2])
  --   , (1, [2, 2, 2, 2, 2, 2, 2, 2]) ]
  -- , [ (1, [2, 2, 2, 2, 2, 2, 2, 2])
  --   , (1, [2, 2, 2, 2, 2, 2, 2, 2])
  --   , (1, [2, 2, 2, 2, 2, 2, 2, 2])
  --   , (1, [2, 2, 2, 2, 2, 2, 2, 2]) ] ]
  
  instance ann ~ Void => Pretty (Doc ann) where
    pretty = vacuous
  
  instance Pretty a => Pretty (Const a b) where
    pretty = pretty . getConst
  
  #if FUNCTOR_IDENTITY_IN_BASE
  -- | >>> pretty (Identity 1)
  -- 1
  instance Pretty a => Pretty (Identity a) where
    pretty = pretty . runIdentity
  #endif
  
  -- | >>> pretty [1,2,3]
  -- [1, 2, 3]
  instance Pretty a => Pretty [a] where
      pretty = prettyList
  
  instance Pretty a => Pretty (NonEmpty a) where
      pretty (x:|xs) = prettyList (x:xs)
  
  -- | >>> pretty ()
  -- ()
  --
  -- The argument is not used:
  --
  -- >>> pretty (error "Strict?" :: ())
  -- ()
  instance Pretty () where
      pretty _ = "()"
  
  -- | >>> pretty True
  -- True
  instance Pretty Bool where
      pretty True  = "True"
      pretty False = "False"
  
  -- | Instead of @('pretty' '\\n')@, consider using @'line'@ as a more readable
  -- alternative.
  --
  -- >>> pretty 'f' <> pretty 'o' <> pretty 'o'
  -- foo
  -- >>> pretty ("string" :: String)
  -- string
  instance Pretty Char where
      pretty '\n' = line
      pretty c = Char c
  
  #ifdef MIN_VERSION_text
      prettyList = pretty . (id :: Text -> Text) . fromString
  #else
      prettyList = vsep . map unsafeTextWithoutNewlines . T.splitOn "\n"
  #endif
  
  -- | Convenience function to convert a 'Show'able value to a 'Doc'. If the
  -- 'String' does not contain newlines, consider using the more performant
  -- 'unsafeViaShow'.
  viaShow :: Show a => a -> Doc ann
  viaShow = pretty . T.pack . show
  
  -- | Convenience function to convert a 'Show'able value /that must not contain
  -- newlines/ to a 'Doc'. If there may be newlines, use 'viaShow' instead.
  unsafeViaShow :: Show a => a -> Doc ann
  unsafeViaShow = unsafeTextWithoutNewlines . T.pack . show
  
  -- | >>> pretty (123 :: Int)
  -- 123
  instance Pretty Int    where pretty = unsafeViaShow
  instance Pretty Int8   where pretty = unsafeViaShow
  instance Pretty Int16  where pretty = unsafeViaShow
  instance Pretty Int32  where pretty = unsafeViaShow
  instance Pretty Int64  where pretty = unsafeViaShow
  instance Pretty Word   where pretty = unsafeViaShow
  instance Pretty Word8  where pretty = unsafeViaShow
  instance Pretty Word16 where pretty = unsafeViaShow
  instance Pretty Word32 where pretty = unsafeViaShow
  instance Pretty Word64 where pretty = unsafeViaShow
  
  -- | >>> pretty (2^123 :: Integer)
  -- 10633823966279326983230456482242756608
  instance Pretty Integer where pretty = unsafeViaShow
  
  #if NATURAL_IN_BASE
  instance Pretty Natural where pretty = unsafeViaShow
  #endif
  
  -- | >>> pretty (pi :: Float)
  -- 3.1415927
  instance Pretty Float where pretty = unsafeViaShow
  
  -- | >>> pretty (exp 1 :: Double)
  -- 2.71828182845904...
  instance Pretty Double where pretty = unsafeViaShow
  
  -- | >>> pretty (123, "hello")
  -- (123, hello)
  instance (Pretty a1, Pretty a2) => Pretty (a1,a2) where
      pretty (x1,x2) = tupled [pretty x1, pretty x2]
  
  -- | >>> pretty (123, "hello", False)
  -- (123, hello, False)
  instance (Pretty a1, Pretty a2, Pretty a3) => Pretty (a1,a2,a3) where
      pretty (x1,x2,x3) = tupled [pretty x1, pretty x2, pretty x3]
  
  --    -- | >>> pretty (123, "hello", False, ())
  --    -- (123, hello, False, ())
  --    instance (Pretty a1, Pretty a2, Pretty a3, Pretty a4) => Pretty (a1,a2,a3,a4) where
  --        pretty (x1,x2,x3,x4) = tupled [pretty x1, pretty x2, pretty x3, pretty x4]
  --
  --    -- | >>> pretty (123, "hello", False, (), 3.14)
  --    -- (123, hello, False, (), 3.14)
  --    instance (Pretty a1, Pretty a2, Pretty a3, Pretty a4, Pretty a5) => Pretty (a1,a2,a3,a4,a5) where
  --        pretty (x1,x2,x3,x4,x5) = tupled [pretty x1, pretty x2, pretty x3, pretty x4, pretty x5]
  --
  --    -- | >>> pretty (123, "hello", False, (), 3.14, Just 2.71)
  --    -- ( 123
  --    -- , hello
  --    -- , False
  --    -- , ()
  --    -- , 3.14
  --    -- , 2.71 )
  --    instance (Pretty a1, Pretty a2, Pretty a3, Pretty a4, Pretty a5, Pretty a6) => Pretty (a1,a2,a3,a4,a5,a6) where
  --        pretty (x1,x2,x3,x4,x5,x6) = tupled [pretty x1, pretty x2, pretty x3, pretty x4, pretty x5, pretty x6]
  --
  --    -- | >>> pretty (123, "hello", False, (), 3.14, Just 2.71, [1,2,3])
  --    -- ( 123
  --    -- , hello
  --    -- , False
  --    -- , ()
  --    -- , 3.14
  --    -- , 2.71
  --    -- , [1, 2, 3] )
  --    instance (Pretty a1, Pretty a2, Pretty a3, Pretty a4, Pretty a5, Pretty a6, Pretty a7) => Pretty (a1,a2,a3,a4,a5,a6,a7) where
  --        pretty (x1,x2,x3,x4,x5,x6,x7) = tupled [pretty x1, pretty x2, pretty x3, pretty x4, pretty x5, pretty x6, pretty x7]
  
  -- | Ignore 'Nothing's, print 'Just' contents.
  --
  -- >>> pretty (Just True)
  -- True
  -- >>> braces (pretty (Nothing :: Maybe Bool))
  -- {}
  --
  -- >>> pretty [Just 1, Nothing, Just 3, Nothing]
  -- [1, 3]
  instance Pretty a => Pretty (Maybe a) where
      pretty = maybe mempty pretty
      prettyList = prettyList . catMaybes
  
  #ifdef MIN_VERSION_text
  -- | Automatically converts all newlines to @'line'@.
  --
  -- >>> pretty ("hello\nworld" :: Text)
  -- hello
  -- world
  --
  -- Note that  @'line'@ can be undone by @'group'@:
  --
  -- >>> group (pretty ("hello\nworld" :: Text))
  -- hello world
  --
  -- Manually use @'hardline'@ if you /definitely/ want newlines.
  instance Pretty Text where pretty = vsep . map unsafeTextWithoutNewlines . T.splitOn "\n"
  
  -- | (lazy 'Text' instance, identical to the strict version)
  instance Pretty Lazy.Text where pretty = pretty . Lazy.toStrict
  #endif
  
  -- | Finding a good example for printing something that does not exist is hard,
  -- so here is an example of printing a list full of nothing.
  --
  -- >>> pretty ([] :: [Void])
  -- []
  instance Pretty Void where pretty = absurd
  
  
  
  -- | @(unsafeTextWithoutNewlines s)@ contains the literal string @s@.
  --
  -- The string must not contain any newline characters, since this is an
  -- invariant of the 'Text' constructor.
  unsafeTextWithoutNewlines :: Text -> Doc ann
  unsafeTextWithoutNewlines text = case T.uncons text of
      Nothing -> Empty
      Just (t,ext)
          | T.null ext -> Char t
          | otherwise -> Text (T.length text) text
  
  -- | The empty document behaves like @('pretty' "")@, so it has a height of 1.
  -- This may lead to surprising behaviour if we expect it to bear no weight
  -- inside e.g. 'vcat', where we get an empty line of output from it ('parens'
  -- for visibility only):
  --
  -- >>> vsep ["hello", parens emptyDoc, "world"]
  -- hello
  -- ()
  -- world
  --
  -- Together with '<>', 'emptyDoc' forms the 'Monoid' 'Doc'.
  emptyDoc :: Doc ann
  emptyDoc = Empty
  
  -- | @('nest' i x)@ lays out the document @x@ with the current nesting level
  -- (indentation of the following lines) increased by @i@. Negative values are
  -- allowed, and decrease the nesting level accordingly.
  --
  -- >>> vsep [nest 4 (vsep ["lorem", "ipsum", "dolor"]), "sit", "amet"]
  -- lorem
  --     ipsum
  --     dolor
  -- sit
  -- amet
  --
  -- See also
  --
  --   * 'hang' ('nest' relative to current cursor position instead of
  --      current nesting level)
  --   * 'align' (set nesting level to current cursor position)
  --   * 'indent' (increase indentation on the spot, padding with spaces).
  nest
      :: Int -- ^ Change of nesting level
      -> Doc ann
      -> Doc ann
  nest 0 x = x -- Optimization
  nest i x = Nest i x
  
  -- | The @'line'@ document advances to the next line and indents to the current
  -- nesting level.
  --
  -- >>> let doc = "lorem ipsum" <> line <> "dolor sit amet"
  -- >>> doc
  -- lorem ipsum
  -- dolor sit amet
  --
  -- @'line'@ behaves like @'space'@ if the line break is undone by 'group':
  --
  -- >>> group doc
  -- lorem ipsum dolor sit amet
  line :: Doc ann
  line = FlatAlt Line (Char ' ')
  
  -- | @'line''@ is like @'line'@, but behaves like @'mempty'@ if the line break
  -- is undone by 'group' (instead of @'space'@).
  --
  -- >>> let doc = "lorem ipsum" <> line' <> "dolor sit amet"
  -- >>> doc
  -- lorem ipsum
  -- dolor sit amet
  -- >>> group doc
  -- lorem ipsumdolor sit amet
  line' :: Doc ann
  line' = FlatAlt Line mempty
  
  -- | @softline@ behaves like @'space'@ if the resulting output fits the page,
  -- otherwise like @'line'@.
  --
  -- Here, we have enough space to put everything in one line:
  --
  -- >>> let doc = "lorem ipsum" <> softline <> "dolor sit amet"
  -- >>> putDocW 80 doc
  -- lorem ipsum dolor sit amet
  --
  -- If we narrow the page to width 10, the layouter produces a line break:
  --
  -- >>> putDocW 10 doc
  -- lorem ipsum
  -- dolor sit amet
  --
  -- @
  -- 'softline' = 'group' 'line'
  -- @
  softline :: Doc ann
  softline = Union (Char ' ') Line
  
  -- | @'softline''@ is like @'softline'@, but behaves like @'mempty'@ if the
  -- resulting output does not fit on the page (instead of @'space'@). In other
  -- words, @'line'@ is to @'line''@ how @'softline'@ is to @'softline''@.
  --
  -- With enough space, we get direct concatenation:
  --
  -- >>> let doc = "ThisWord" <> softline' <> "IsWayTooLong"
  -- >>> putDocW 80 doc
  -- ThisWordIsWayTooLong
  --
  -- If we narrow the page to width 10, the layouter produces a line break:
  --
  -- >>> putDocW 10 doc
  -- ThisWord
  -- IsWayTooLong
  --
  -- @
  -- 'softline'' = 'group' 'line''
  -- @
  softline' :: Doc ann
  softline' = Union mempty Line
  
  -- | A @'hardline'@ is /always/ laid out as a line break, even when 'group'ed or
  -- when there is plenty of space. Note that it might still be simply discarded
  -- if it is part of a 'flatAlt' inside a 'group'.
  --
  -- >>> let doc = "lorem ipsum" <> hardline <> "dolor sit amet"
  -- >>> putDocW 1000 doc
  -- lorem ipsum
  -- dolor sit amet
  --
  -- >>> group doc
  -- lorem ipsum
  -- dolor sit amet
  hardline :: Doc ann
  hardline = Line
  
  -- | @('group' x)@ tries laying out @x@ into a single line by removing the
  -- contained line breaks; if this does not fit the page, or when a 'hardline'
  -- within @x@ prevents it from being flattened, @x@ is laid out without any
  -- changes.
  --
  -- The 'group' function is key to layouts that adapt to available space nicely.
  --
  -- See 'vcat', 'line', or 'flatAlt' for examples that are related, or make good
  -- use of it.
  group :: Doc ann -> Doc ann
  -- See note [Group: special flattening]
  group x = case x of
      Union{} -> x
      FlatAlt a b -> case changesUponFlattening b of
          Flattened b' -> Union b' a
          AlreadyFlat  -> Union b a
          NeverFlat    -> a
      _ -> case changesUponFlattening x of
          Flattened x' -> Union x' x
          AlreadyFlat  -> x
          NeverFlat    -> x
  
  -- Note [Group: special flattening]
  --
  -- Since certain documents do not change under removal of newlines etc, there is
  -- no point in creating a 'Union' of the flattened and unflattened version – all
  -- this does is introducing two branches for the layout algorithm to take,
  -- resulting in potentially exponential behavior on deeply nested examples, such
  -- as
  --
  --     pathological n = iterate (\x ->  hsep [x, sep []] ) "foobar" !! n
  --
  -- See https://github.com/quchen/prettyprinter/issues/22 for the  corresponding
  -- ticket.
  
  data FlattenResult a
      = Flattened a
      -- ^ @a@ is likely flatter than the input.
      | AlreadyFlat
      -- ^ The input was already flat, e.g. a 'Text'.
      | NeverFlat
      -- ^ The input couldn't be flattened: It contained a 'Line' or 'Fail'.
  
  instance Functor FlattenResult where
      fmap f (Flattened a) = Flattened (f a)
      fmap _ AlreadyFlat   = AlreadyFlat
      fmap _ NeverFlat     = NeverFlat
  
  -- | Choose the first element of each @Union@, and discard the first field of
  -- all @FlatAlt@s.
  --
  -- The result is 'Flattened' if the element might change depending on the layout
  -- algorithm (i.e. contains differently renderable sub-documents), and 'AlreadyFlat'
  -- if the document is static (e.g. contains only a plain 'Empty' node).
  -- 'NeverFlat' is returned when the document cannot be flattened because it
  -- contains a hard 'Line' or 'Fail'.
  -- See [Group: special flattening] for further explanations.
  changesUponFlattening :: Doc ann -> FlattenResult (Doc ann)
  changesUponFlattening = \doc -> case doc of
      FlatAlt _ y     -> Flattened (flatten y)
      Line            -> NeverFlat
      Union x _       -> Flattened x
      Nest i x        -> fmap (Nest i) (changesUponFlattening x)
      Annotated ann x -> fmap (Annotated ann) (changesUponFlattening x)
  
      Column f        -> Flattened (Column (flatten . f))
      Nesting f       -> Flattened (Nesting (flatten . f))
      WithPageWidth f -> Flattened (WithPageWidth (flatten . f))
  
      Cat x y -> case (changesUponFlattening x, changesUponFlattening y) of
          (NeverFlat    ,  _          ) -> NeverFlat
          (_            , NeverFlat   ) -> NeverFlat
          (Flattened x' , Flattened y') -> Flattened (Cat x' y')
          (Flattened x' , AlreadyFlat ) -> Flattened (Cat x' y)
          (AlreadyFlat  , Flattened y') -> Flattened (Cat x y')
          (AlreadyFlat  , AlreadyFlat ) -> AlreadyFlat
  
      Empty  -> AlreadyFlat
      Char{} -> AlreadyFlat
      Text{} -> AlreadyFlat
      Fail   -> NeverFlat
    where
      -- Flatten, but don’t report whether anything changes.
      flatten :: Doc ann -> Doc ann
      flatten = \doc -> case doc of
          FlatAlt _ y     -> flatten y
          Cat x y         -> Cat (flatten x) (flatten y)
          Nest i x        -> Nest i (flatten x)
          Line            -> Fail
          Union x _       -> flatten x
          Column f        -> Column (flatten . f)
          WithPageWidth f -> WithPageWidth (flatten . f)
          Nesting f       -> Nesting (flatten . f)
          Annotated ann x -> Annotated ann (flatten x)
  
          x@Fail   -> x
          x@Empty  -> x
          x@Char{} -> x
          x@Text{} -> x
  
  
  
  -- | By default, @('flatAlt' x y)@ renders as @x@. However when 'group'ed,
  -- @y@ will be preferred, with @x@ as the fallback for the case when @y@
  -- doesn't fit.
  --
  -- >>> let doc = flatAlt "a" "b"
  -- >>> putDoc doc
  -- a
  -- >>> putDoc (group doc)
  -- b
  -- >>> putDocW 0 (group doc)
  -- a
  --
  -- 'flatAlt' is particularly useful for defining conditional separators such as
  --
  -- @
  -- softline = 'group' ('flatAlt' 'hardline' " ")
  -- @
  --
  -- >>> let hello = "Hello" <> softline <> "world!"
  -- >>> putDocW 12 hello
  -- Hello world!
  -- >>> putDocW 11 hello
  -- Hello
  -- world!
  --
  -- === __Example: Haskell's do-notation__
  --
  -- We can use this to render Haskell's do-notation nicely:
  --
  -- >>> let open        = flatAlt "" "{ "
  -- >>> let close       = flatAlt "" " }"
  -- >>> let separator   = flatAlt "" "; "
  -- >>> let prettyDo xs = group ("do" <+> align (encloseSep open close separator xs))
  -- >>> let statements  = ["name:_ <- getArgs", "let greet = \"Hello, \" <> name", "putStrLn greet"]
  --
  -- This is put into a single line with @{;}@ style if it fits:
  --
  -- >>> putDocW 80 (prettyDo statements)
  -- do { name:_ <- getArgs; let greet = "Hello, " <> name; putStrLn greet }
  --
  -- When there is not enough space the statements are broken up into lines
  -- nicely:
  --
  -- >>> putDocW 10 (prettyDo statements)
  -- do name:_ <- getArgs
  --    let greet = "Hello, " <> name
  --    putStrLn greet
  --
  -- === Notes
  --
  -- Users should be careful to choose @x@ to be less wide than @y@.
  -- Otherwise, if @y@ turns out not to fit the page, we fall back on an even
  -- wider layout:
  --
  -- >>> let ugly = group (flatAlt "even wider" "too wide")
  -- >>> putDocW 7 ugly
  -- even wider
  --
  -- Also note that 'group' will flatten @y@:
  --
  -- >>> putDoc (group (flatAlt "x" ("y" <> line <> "y")))
  -- y y
  --
  -- This also means that an "unflattenable" @y@ which contains a hard linebreak
  -- will /never/ be rendered:
  --
  -- >>> putDoc (group (flatAlt "x" ("y" <> hardline <> "y")))
  -- x
  flatAlt
      :: Doc ann -- ^ Default
      -> Doc ann -- ^ Preferred when 'group'ed
      -> Doc ann
  flatAlt = FlatAlt
  
  
  
  -- | @('align' x)@ lays out the document @x@ with the nesting level set to the
  -- current column. It is used for example to implement 'hang'.
  --
  -- As an example, we will put a document right above another one, regardless of
  -- the current nesting level. Without 'align'ment, the second line is put simply
  -- below everything we've had so far:
  --
  -- >>> "lorem" <+> vsep ["ipsum", "dolor"]
  -- lorem ipsum
  -- dolor
  --
  -- If we add an 'align' to the mix, the @'vsep'@'s contents all start in the
  -- same column:
  --
  -- >>> "lorem" <+> align (vsep ["ipsum", "dolor"])
  -- lorem ipsum
  --       dolor
  align :: Doc ann -> Doc ann
  align d = column (\k -> nesting (\i -> nest (k - i) d)) -- nesting might be negative!
  
  -- | @('hang' i x)@ lays out the document @x@ with a nesting level set to the
  -- /current column/ plus @i@. Negative values are allowed, and decrease the
  -- nesting level accordingly.
  --
  -- >>> let doc = reflow "Indenting these words with hang"
  -- >>> putDocW 24 ("prefix" <+> hang 4 doc)
  -- prefix Indenting these
  --            words with
  --            hang
  --
  -- This differs from 'nest', which is based on the /current nesting level/ plus
  -- @i@. When you're not sure, try the more efficient 'nest' first. In our
  -- example, this would yield
  --
  -- >>> let doc = reflow "Indenting these words with nest"
  -- >>> putDocW 24 ("prefix" <+> nest 4 doc)
  -- prefix Indenting these
  --     words with nest
  --
  -- @
  -- 'hang' i doc = 'align' ('nest' i doc)
  -- @
  hang
      :: Int -- ^ Change of nesting level, relative to the start of the first line
      -> Doc ann
      -> Doc ann
  hang i d = align (nest i d)
  
  -- | @('indent' i x)@ indents document @x@ by @i@ columns, starting from the
  -- current cursor position.
  --
  -- >>> let doc = reflow "The indent function indents these words!"
  -- >>> putDocW 24 ("prefix" <> indent 4 doc)
  -- prefix    The indent
  --           function
  --           indents these
  --           words!
  --
  -- @
  -- 'indent' i d = 'hang' i ({i spaces} <> d)
  -- @
  indent
      :: Int -- ^ Number of spaces to increase indentation by
      -> Doc ann
      -> Doc ann
  indent i d = hang i (spaces i <> d)
  
  -- | @('encloseSep' l r sep xs)@ concatenates the documents @xs@ separated by
  -- @sep@, and encloses the resulting document by @l@ and @r@.
  --
  -- The documents are laid out horizontally if that fits the page:
  --
  -- >>> let doc = "list" <+> align (encloseSep lbracket rbracket comma (map pretty [1,20,300,4000]))
  -- >>> putDocW 80 doc
  -- list [1,20,300,4000]
  --
  -- If there is not enough space, then the input is split into lines entry-wise
  -- therwise they are laid out vertically, with separators put in the front:
  --
  -- >>> putDocW 10 doc
  -- list [1
  --      ,20
  --      ,300
  --      ,4000]
  --
  -- Note that @doc@ contains an explicit call to 'align' so that the list items
  -- are aligned vertically.
  --
  -- For putting separators at the end of entries instead, have a look at
  -- 'punctuate'.
  encloseSep
      :: Doc ann   -- ^ left delimiter
      -> Doc ann   -- ^ right delimiter
      -> Doc ann   -- ^ separator
      -> [Doc ann] -- ^ input documents
      -> Doc ann
  encloseSep l r s ds = case ds of
      []  -> l <> r
      [d] -> l <> d <> r
      _   -> cat (zipWith (<>) (l : repeat s) ds) <> r
  
  -- | Haskell-inspired variant of 'encloseSep' with braces and comma as
  -- separator.
  --
  -- >>> let doc = list (map pretty [1,20,300,4000])
  --
  -- >>> putDocW 80 doc
  -- [1, 20, 300, 4000]
  --
  -- >>> putDocW 10 doc
  -- [ 1
  -- , 20
  -- , 300
  -- , 4000 ]
  list :: [Doc ann] -> Doc ann
  list = group . encloseSep (flatAlt "[ " "[")
                            (flatAlt " ]" "]")
                            ", "
  
  -- | Haskell-inspired variant of 'encloseSep' with parentheses and comma as
  -- separator.
  --
  -- >>> let doc = tupled (map pretty [1,20,300,4000])
  --
  -- >>> putDocW 80 doc
  -- (1, 20, 300, 4000)
  --
  -- >>> putDocW 10 doc
  -- ( 1
  -- , 20
  -- , 300
  -- , 4000 )
  tupled :: [Doc ann] -> Doc ann
  tupled = group . encloseSep (flatAlt "( " "(")
                              (flatAlt " )" ")")
                              ", "
  
  
  
  -- | @(x '<+>' y)@ concatenates document @x@ and @y@ with a @'space'@ in
  -- between.
  --
  -- >>> "hello" <+> "world"
  -- hello world
  --
  -- @
  -- x '<+>' y = x '<>' 'space' '<>' y
  -- @
  (<+>) :: Doc ann -> Doc ann -> Doc ann
  x <+> y = x <> Char ' ' <> y
  infixr 6 <+> -- like <>
  
  
  
  -- | Concatenate all documents element-wise with a binary function.
  --
  -- @
  -- 'concatWith' _ [] = 'mempty'
  -- 'concatWith' (**) [x,y,z] = x ** y ** z
  -- @
  --
  -- Multiple convenience definitions based on 'concatWith' are already predefined,
  -- for example:
  --
  -- @
  -- 'hsep'    = 'concatWith' ('<+>')
  -- 'fillSep' = 'concatWith' (\\x y -> x '<>' 'softline' '<>' y)
  -- @
  --
  -- This is also useful to define customized joiners:
  --
  -- >>> concatWith (surround dot) ["Prettyprinter", "Render", "Text"]
  -- Prettyprinter.Render.Text
  concatWith :: Foldable t => (Doc ann -> Doc ann -> Doc ann) -> t (Doc ann) -> Doc ann
  concatWith f ds
  #if !(FOLDABLE_TRAVERSABLE_IN_PRELUDE)
      | foldr (\_ _ -> False) True ds = mempty
  #else
      | null ds = mempty
  #endif
      | otherwise = foldr1 f ds
  {-# INLINE concatWith #-}
  {-# SPECIALIZE concatWith :: (Doc ann -> Doc ann -> Doc ann) -> [Doc ann] -> Doc ann #-}
  
  -- | @('hsep' xs)@ concatenates all documents @xs@ horizontally with @'<+>'@,
  -- i.e. it puts a space between all entries.
  --
  -- >>> let docs = Util.words "lorem ipsum dolor sit amet"
  --
  -- >>> hsep docs
  -- lorem ipsum dolor sit amet
  --
  -- @'hsep'@ does not introduce line breaks on its own, even when the page is too
  -- narrow:
  --
  -- >>> putDocW 5 (hsep docs)
  -- lorem ipsum dolor sit amet
  --
  -- For automatic line breaks, consider using 'fillSep' instead.
  hsep :: [Doc ann] -> Doc ann
  hsep = concatWith (<+>)
  
  -- | @('vsep' xs)@ concatenates all documents @xs@ above each other. If a
  -- 'group' undoes the line breaks inserted by @vsep@, the documents are
  -- separated with a 'space' instead.
  --
  -- Using 'vsep' alone yields
  --
  -- >>> "prefix" <+> vsep ["text", "to", "lay", "out"]
  -- prefix text
  -- to
  -- lay
  -- out
  --
  -- 'group'ing a 'vsep' separates the documents with a 'space' if it fits the
  -- page (and does nothing otherwise). See the @'sep'@ convenience function for
  -- this use case.
  --
  -- The 'align' function can be used to align the documents under their first
  -- element:
  --
  -- >>> "prefix" <+> align (vsep ["text", "to", "lay", "out"])
  -- prefix text
  --        to
  --        lay
  --        out
  --
  -- Since 'group'ing a 'vsep' is rather common, 'sep' is a built-in for doing
  -- that.
  vsep :: [Doc ann] -> Doc ann
  vsep = concatWith (\x y -> x <> line <> y)
  
  -- | @('fillSep' xs)@ concatenates the documents @xs@ horizontally with @'<+>'@
  -- as long as it fits the page, then inserts a @'line'@ and continues doing that
  -- for all documents in @xs@. (@'line'@ means that if 'group'ed, the documents
  -- are separated with a 'space' instead of newlines. Use 'fillCat' if you do not
  -- want a 'space'.)
  --
  -- Let's print some words to fill the line:
  --
  -- >>> let docs = take 20 (cycle ["lorem", "ipsum", "dolor", "sit", "amet"])
  -- >>> putDocW 80 ("Docs:" <+> fillSep docs)
  -- Docs: lorem ipsum dolor sit amet lorem ipsum dolor sit amet lorem ipsum dolor
  -- sit amet lorem ipsum dolor sit amet
  --
  -- The same document, printed at a width of only 40, yields
  --
  -- >>> putDocW 40 ("Docs:" <+> fillSep docs)
  -- Docs: lorem ipsum dolor sit amet lorem
  -- ipsum dolor sit amet lorem ipsum dolor
  -- sit amet lorem ipsum dolor sit amet
  fillSep :: [Doc ann] -> Doc ann
  fillSep = concatWith (\x y -> x <> softline <> y)
  
  -- | @('sep' xs)@ tries laying out the documents @xs@ separated with 'space's,
  -- and if this does not fit the page, separates them with newlines. This is what
  -- differentiates it from 'vsep', which always lays out its contents beneath
  -- each other.
  --
  -- >>> let doc = "prefix" <+> sep ["text", "to", "lay", "out"]
  -- >>> putDocW 80 doc
  -- prefix text to lay out
  --
  -- With a narrower layout, the entries are separated by newlines:
  --
  -- >>> putDocW 20 doc
  -- prefix text
  -- to
  -- lay
  -- out
  --
  -- @
  -- 'sep' = 'group' . 'vsep'
  -- @
  sep :: [Doc ann] -> Doc ann
  sep = group . vsep
  
  
  
  -- | @('hcat' xs)@ concatenates all documents @xs@ horizontally with @'<>'@
  -- (i.e. without any spacing).
  --
  -- It is provided only for consistency, since it is identical to 'mconcat'.
  --
  -- >>> let docs = Util.words "lorem ipsum dolor"
  -- >>> hcat docs
  -- loremipsumdolor
  hcat :: [Doc ann] -> Doc ann
  hcat = concatWith (<>)
  
  -- | @('vcat' xs)@ vertically concatenates the documents @xs@. If it is
  -- 'group'ed, the line breaks are removed.
  --
  -- In other words @'vcat'@ is like @'vsep'@, with newlines removed instead of
  -- replaced by 'space's.
  --
  -- >>> let docs = Util.words "lorem ipsum dolor"
  -- >>> vcat docs
  -- lorem
  -- ipsum
  -- dolor
  -- >>> group (vcat docs)
  -- loremipsumdolor
  --
  -- Since 'group'ing a 'vcat' is rather common, 'cat' is a built-in shortcut for
  -- it.
  vcat :: [Doc ann] -> Doc ann
  vcat = concatWith (\x y -> x <> line' <> y)
  
  -- | @('fillCat' xs)@ concatenates documents @xs@ horizontally with @'<>'@ as
  -- long as it fits the page, then inserts a @'line''@ and continues doing that
  -- for all documents in @xs@. This is similar to how an ordinary word processor
  -- lays out the text if you just keep typing after you hit the maximum line
  -- length.
  --
  -- (@'line''@ means that if 'group'ed, the documents are separated with nothing
  -- instead of newlines. See 'fillSep' if you want a 'space' instead.)
  --
  -- Observe the difference between 'fillSep' and 'fillCat'. 'fillSep'
  -- concatenates the entries 'space'd when 'group'ed:
  --
  -- >>> let docs = take 20 (cycle (["lorem", "ipsum", "dolor", "sit", "amet"]))
  -- >>> putDocW 40 ("Grouped:" <+> group (fillSep docs))
  -- Grouped: lorem ipsum dolor sit amet
  -- lorem ipsum dolor sit amet lorem ipsum
  -- dolor sit amet lorem ipsum dolor sit
  -- amet
  --
  -- On the other hand, 'fillCat' concatenates the entries directly when
  -- 'group'ed:
  --
  -- >>> putDocW 40 ("Grouped:" <+> group (fillCat docs))
  -- Grouped: loremipsumdolorsitametlorem
  -- ipsumdolorsitametloremipsumdolorsitamet
  -- loremipsumdolorsitamet
  fillCat :: [Doc ann] -> Doc ann
  fillCat = concatWith (\x y -> x <> softline' <> y)
  
  -- | @('cat' xs)@ tries laying out the documents @xs@ separated with nothing,
  -- and if this does not fit the page, separates them with newlines. This is what
  -- differentiates it from 'vcat', which always lays out its contents beneath
  -- each other.
  --
  -- >>> let docs = Util.words "lorem ipsum dolor"
  -- >>> putDocW 80 ("Docs:" <+> cat docs)
  -- Docs: loremipsumdolor
  --
  -- When there is enough space, the documents are put above one another:
  --
  -- >>> putDocW 10 ("Docs:" <+> cat docs)
  -- Docs: lorem
  -- ipsum
  -- dolor
  --
  -- @
  -- 'cat' = 'group' . 'vcat'
  -- @
  cat :: [Doc ann] -> Doc ann
  cat = group . vcat
  
  
  
  -- | @('punctuate' p xs)@ appends @p@ to all but the last document in @xs@.
  --
  -- >>> let docs = punctuate comma (Util.words "lorem ipsum dolor sit amet")
  -- >>> putDocW 80 (hsep docs)
  -- lorem, ipsum, dolor, sit, amet
  --
  -- The separators are put at the end of the entries, which we can see if we
  -- position the result vertically:
  --
  -- >>> putDocW 20 (vsep docs)
  -- lorem,
  -- ipsum,
  -- dolor,
  -- sit,
  -- amet
  --
  -- If you want put the commas in front of their elements instead of at the end,
  -- you should use 'tupled' or, in general, 'encloseSep'.
  punctuate
      :: Doc ann -- ^ Punctuation, e.g. 'comma'
      -> [Doc ann]
      -> [Doc ann]
  punctuate p = go
    where
      go []     = []
      go [d]    = [d]
      go (d:ds) = (d <> p) : go ds
  
  
  
  -- | Layout a document depending on which column it starts at. 'align' is
  -- implemented in terms of 'column'.
  --
  -- >>> column (\l -> "Columns are" <+> pretty l <> "-based.")
  -- Columns are 0-based.
  --
  -- >>> let doc = "prefix" <+> column (\l -> "| <- column" <+> pretty l)
  -- >>> vsep [indent n doc | n <- [0,4,8]]
  -- prefix | <- column 7
  --     prefix | <- column 11
  --         prefix | <- column 15
  column :: (Int -> Doc ann) -> Doc ann
  column = Column
  
  -- | Layout a document depending on the current 'nest'ing level. 'align' is
  -- implemented in terms of 'nesting'.
  --
  -- >>> let doc = "prefix" <+> nesting (\l -> brackets ("Nested:" <+> pretty l))
  -- >>> vsep [indent n doc | n <- [0,4,8]]
  -- prefix [Nested: 0]
  --     prefix [Nested: 4]
  --         prefix [Nested: 8]
  nesting :: (Int -> Doc ann) -> Doc ann
  nesting = Nesting
  
  -- | @('width' doc f)@ lays out the document 'doc', and makes the column width
  -- of it available to a function.
  --
  -- >>> let annotate doc = width (brackets doc) (\w -> " <- width:" <+> pretty w)
  -- >>> align (vsep (map annotate ["---", "------", indent 3 "---", vsep ["---", indent 4 "---"]]))
  -- [---] <- width: 5
  -- [------] <- width: 8
  -- [   ---] <- width: 8
  -- [---
  --     ---] <- width: 8
  width :: Doc ann -> (Int -> Doc ann) -> Doc ann
  width doc f
    = column (\colStart ->
          doc <> column (\colEnd ->
              f (colEnd - colStart)))
  
  -- | Layout a document depending on the page width, if one has been specified.
  --
  -- >>> let prettyPageWidth (AvailablePerLine l r) = "Width:" <+> pretty l <> ", ribbon fraction:" <+> pretty r
  -- >>> let doc = "prefix" <+> pageWidth (brackets . prettyPageWidth)
  -- >>> putDocW 32 (vsep [indent n doc | n <- [0,4,8]])
  -- prefix [Width: 32, ribbon fraction: 1.0]
  --     prefix [Width: 32, ribbon fraction: 1.0]
  --         prefix [Width: 32, ribbon fraction: 1.0]
  pageWidth :: (PageWidth -> Doc ann) -> Doc ann
  pageWidth = WithPageWidth
  
  
  
  -- | @('fill' i x)@ lays out the document @x@. It then appends @space@s until
  -- the width is equal to @i@. If the width of @x@ is already larger, nothing is
  -- appended.
  --
  -- This function is quite useful in practice to output a list of bindings:
  --
  -- >>> let types = [("empty","Doc"), ("nest","Int -> Doc -> Doc"), ("fillSep","[Doc] -> Doc")]
  -- >>> let ptype (name, tp) = fill 5 (pretty name) <+> "::" <+> pretty tp
  -- >>> "let" <+> align (vcat (map ptype types))
  -- let empty :: Doc
  --     nest  :: Int -> Doc -> Doc
  --     fillSep :: [Doc] -> Doc
  fill
      :: Int -- ^ Append spaces until the document is at least this wide
      -> Doc ann
      -> Doc ann
  fill n doc = width doc (\w -> spaces (n - w))
  
  -- | @('fillBreak' i x)@ first lays out the document @x@. It then appends @space@s
  -- until the width is equal to @i@. If the width of @x@ is already larger than
  -- @i@, the nesting level is increased by @i@ and a @line@ is appended. When we
  -- redefine @ptype@ in the example given in 'fill' to use @'fillBreak'@, we get
  -- a useful variation of the output:
  --
  -- >>> let types = [("empty","Doc"), ("nest","Int -> Doc -> Doc"), ("fillSep","[Doc] -> Doc")]
  -- >>> let ptype (name, tp) = fillBreak 5 (pretty name) <+> "::" <+> pretty tp
  -- >>> "let" <+> align (vcat (map ptype types))
  -- let empty :: Doc
  --     nest  :: Int -> Doc -> Doc
  --     fillSep
  --           :: [Doc] -> Doc
  fillBreak
      :: Int -- ^ Append spaces until the document is at least this wide
      -> Doc ann
      -> Doc ann
  fillBreak f x = width x (\w ->
      if w > f
          then nest f line'
          else spaces (f - w))
  
  -- | Insert a number of spaces. Negative values count as 0.
  spaces :: Int -> Doc ann
  spaces n
    | n <= 0    = Empty
    | n == 1    = Char ' '
    | otherwise = Text n (textSpaces n)
  
  -- $
  -- prop> \(NonNegative n) -> length (show (spaces n)) == n
  --
  -- >>> case spaces 1 of Char ' ' -> True; _ -> False
  -- True
  --
  -- >>> case spaces 0 of Empty -> True; _ -> False
  -- True
  --
  -- prop> \(Positive n) -> case (spaces (-n)) of Empty -> True; _ -> False
  
  
  
  -- | @('plural' n one many)@ is @one@ if @n@ is @1@, and @many@ otherwise. A
  -- typical use case is  adding a plural "s".
  --
  -- >>> let things = [True]
  -- >>> let amount = length things
  -- >>> pretty things <+> "has" <+> pretty amount <+> plural "entry" "entries" amount
  -- [True] has 1 entry
  plural
      :: (Num amount, Eq amount)
      => doc -- ^ @1@ case
      -> doc -- ^ other cases
      -> amount
      -> doc
  plural one multiple n
      | n == 1    = one
      | otherwise = multiple
  
  -- | @('enclose' l r x)@ encloses document @x@ between documents @l@ and @r@
  -- using @'<>'@.
  --
  -- >>> enclose "A" "Z" "·"
  -- A·Z
  --
  -- @
  -- 'enclose' l r x = l '<>' x '<>' r
  -- @
  enclose
      :: Doc ann -- ^ L
      -> Doc ann -- ^ R
      -> Doc ann -- ^ x
      -> Doc ann -- ^ LxR
  enclose l r x = l <> x <> r
  
  -- | @('surround' x l r)@ surrounds document @x@ with @l@ and @r@.
  --
  -- >>> surround "·" "A" "Z"
  -- A·Z
  --
  -- This is merely an argument reordering of @'enclose'@, but allows for
  -- definitions like
  --
  -- >>> concatWith (surround dot) ["Prettyprinter", "Render", "Text"]
  -- Prettyprinter.Render.Text
  surround
      :: Doc ann
      -> Doc ann
      -> Doc ann
      -> Doc ann
  surround x l r = l <> x <> r
  
  
  
  
  
  
  -- | Add an annotation to a @'Doc'@. This annotation can then be used by the
  -- renderer to e.g. add color to certain parts of the output. For a full
  -- tutorial example on how to use it, see the
  -- "Prettyprinter.Render.Tutorials.StackMachineTutorial" or
  -- "Prettyprinter.Render.Tutorials.TreeRenderingTutorial" modules.
  --
  -- This function is only relevant for custom formats with their own annotations,
  -- and not relevant for basic prettyprinting. The predefined renderers, e.g.
  -- "Prettyprinter.Render.Text", should be enough for the most common
  -- needs.
  annotate :: ann -> Doc ann -> Doc ann
  annotate = Annotated
  
  -- | Remove all annotations.
  --
  -- Although 'unAnnotate' is idempotent with respect to rendering,
  --
  -- @
  -- 'unAnnotate' . 'unAnnotate' = 'unAnnotate'
  -- @
  --
  -- it should not be used without caution, for each invocation traverses the
  -- entire contained document. If possible, it is preferrable to unannotate after
  -- producing the layout by using 'unAnnotateS'.
  unAnnotate :: Doc ann -> Doc xxx
  unAnnotate = alterAnnotations (const [])
  
  -- | Change the annotation of a 'Doc'ument.
  --
  -- Useful in particular to embed documents with one form of annotation in a more
  -- generally annotated document.
  --
  -- Since this traverses the entire @'Doc'@ tree, including parts that are not
  -- rendered due to other layouts fitting better, it is preferrable to reannotate
  -- after producing the layout by using @'reAnnotateS'@.
  --
  -- Since @'reAnnotate'@ has the right type and satisfies @'reAnnotate id = id'@,
  -- it is used to define the @'Functor'@ instance of @'Doc'@.
  reAnnotate :: (ann -> ann') -> Doc ann -> Doc ann'
  reAnnotate re = alterAnnotations (pure . re)
  
  -- | Change the annotations of a 'Doc'ument. Individual annotations can be
  -- removed, changed, or replaced by multiple ones.
  --
  -- This is a general function that combines 'unAnnotate' and 'reAnnotate', and
  -- it is useful for mapping semantic annotations (such as »this is a keyword«)
  -- to display annotations (such as »this is red and underlined«), because some
  -- backends may not care about certain annotations, while others may.
  --
  -- Annotations earlier in the new list will be applied earlier, i.e. returning
  -- @[Bold, Green]@ will result in a bold document that contains green text, and
  -- not vice-versa.
  --
  -- Since this traverses the entire @'Doc'@ tree, including parts that are not
  -- rendered due to other layouts fitting better, it is preferrable to reannotate
  -- after producing the layout by using @'alterAnnotationsS'@.
  alterAnnotations :: (ann -> [ann']) -> Doc ann -> Doc ann'
  alterAnnotations re = go
    where
      go = \doc -> case doc of
          Fail     -> Fail
          Empty    -> Empty
          Char c   -> Char c
          Text l t -> Text l t
          Line     -> Line
  
          FlatAlt x y     -> FlatAlt (go x) (go y)
          Cat x y         -> Cat (go x) (go y)
          Nest i x        -> Nest i (go x)
          Union x y       -> Union (go x) (go y)
          Column f        -> Column (go . f)
          WithPageWidth f -> WithPageWidth (go . f)
          Nesting f       -> Nesting (go . f)
          Annotated ann x -> foldr Annotated (go x) (re ann)
  
  -- $
  -- >>> let doc = "lorem" <+> annotate () "ipsum" <+> "dolor"
  -- >>> let re () = ["FOO", "BAR"]
  -- >>> layoutPretty defaultLayoutOptions (alterAnnotations re doc)
  -- SText 5 "lorem" (SChar ' ' (SAnnPush "FOO" (SAnnPush "BAR" (SText 5 "ipsum" (SAnnPop (SAnnPop (SChar ' ' (SText 5 "dolor" SEmpty))))))))
  
  -- | Remove all annotations. 'unAnnotate' for 'SimpleDocStream'.
  unAnnotateS :: SimpleDocStream ann -> SimpleDocStream xxx
  unAnnotateS = go
    where
      go = \doc -> case doc of
          SFail              -> SFail
          SEmpty             -> SEmpty
          SChar c rest       -> SChar c (go rest)
          SText l t rest     -> SText l t (go rest)
          SLine l rest       -> SLine l (go rest)
          SAnnPop rest       -> go rest
          SAnnPush _ann rest -> go rest
  
  -- | Change the annotation of a document. 'reAnnotate' for 'SimpleDocStream'.
  reAnnotateS :: (ann -> ann') -> SimpleDocStream ann -> SimpleDocStream ann'
  reAnnotateS re = go
    where
      go = \doc -> case doc of
          SFail             -> SFail
          SEmpty            -> SEmpty
          SChar c rest      -> SChar c (go rest)
          SText l t rest    -> SText l t (go rest)
          SLine l rest      -> SLine l (go rest)
          SAnnPop rest      -> SAnnPop (go rest)
          SAnnPush ann rest -> SAnnPush (re ann) (go rest)
  
  data AnnotationRemoval = Remove | DontRemove
    deriving Typeable
  
  -- | Change the annotation of a document to a different annotation, or none at
  -- all. 'alterAnnotations' for 'SimpleDocStream'.
  --
  -- Note that the 'Doc' version is more flexible, since it allows changing a
  -- single annotation to multiple ones.
  -- ('Prettyprinter.Render.Util.SimpleDocTree.SimpleDocTree' restores
  -- this flexibility again.)
  alterAnnotationsS :: (ann -> Maybe ann') -> SimpleDocStream ann -> SimpleDocStream ann'
  alterAnnotationsS re = go []
    where
      -- We keep a stack of whether to remove a pop so that we can remove exactly
      -- the pops corresponding to annotations that mapped to Nothing.
      go stack = \sds -> case sds of
          SFail             -> SFail
          SEmpty            -> SEmpty
          SChar c rest      -> SChar c (go stack rest)
          SText l t rest    -> SText l t (go stack rest)
          SLine l rest      -> SLine l (go stack rest)
          SAnnPush ann rest -> case re ann of
              Nothing   -> go (Remove:stack) rest
              Just ann' -> SAnnPush ann' (go (DontRemove:stack) rest)
          SAnnPop rest      -> case stack of
              []                -> panicPeekedEmpty
              DontRemove:stack' -> SAnnPop (go stack' rest)
              Remove:stack'     -> go stack' rest
  
  -- | Fusion depth parameter, used by 'fuse'.
  data FusionDepth =
  
      -- | Do not dive deep into nested documents, fusing mostly concatenations of
      -- text nodes together.
      Shallow
  
      -- | Recurse into all parts of the 'Doc', including different layout
      -- alternatives, and location-sensitive values such as created by 'nesting'
      -- which cannot be fused before, but only during, the layout process. As a
      -- result, the performance cost of using deep fusion is often hard to
      -- predict, and depends on the interplay between page layout and document to
      -- prettyprint.
      --
      -- This value should only be used if profiling shows it is significantly
      -- faster than using 'Shallow'.
      | Deep
      deriving (Eq, Ord, Show, Typeable)
  
  -- | @('fuse' depth doc)@ combines text nodes so they can be rendered more
  -- efficiently. A fused document is always laid out identical to its unfused
  -- version.
  --
  -- When laying a 'Doc'ument out to a 'SimpleDocStream', every component of the
  -- input is translated directly to the simpler output format. This sometimes
  -- yields undesirable chunking when many pieces have been concatenated together.
  --
  -- For example
  --
  -- >>> "a" <> "b" <> pretty 'c' <> "d"
  -- abcd
  --
  -- results in a chain of four entries in a 'SimpleDocStream', although this is fully
  -- equivalent to the tightly packed
  --
  -- >>> "abcd" :: Doc ann
  -- abcd
  --
  -- which is only a single 'SimpleDocStream' entry, and can be processed faster.
  --
  -- It is therefore a good idea to run 'fuse' on concatenations of lots of small
  -- strings that are used many times:
  --
  -- >>> let oftenUsed = fuse Shallow ("a" <> "b" <> pretty 'c' <> "d")
  -- >>> hsep (replicate 5 oftenUsed)
  -- abcd abcd abcd abcd abcd
  fuse :: FusionDepth -> Doc ann -> Doc ann
  fuse depth = go
    where
      go = \doc -> case doc of
          Cat Empty x                   -> go x
          Cat x Empty                   -> go x
          Cat (Char c1) (Char c2)       -> Text 2 (T.singleton c1 <> T.singleton c2)
          Cat (Text lt t) (Char c)      -> Text (lt+1) (T.snoc t c)
          Cat (Char c) (Text lt t)      -> Text (1+lt) (T.cons c t)
          Cat (Text l1 t1) (Text l2 t2) -> Text (l1+l2) (t1 <> t2)
  
          Cat x@Char{} (Cat y@Char{} z) -> go (Cat (go (Cat x y)) z)
          Cat x@Text{} (Cat y@Char{} z) -> go (Cat (go (Cat x y)) z)
          Cat x@Char{} (Cat y@Text{} z) -> go (Cat (go (Cat x y)) z)
          Cat x@Text{} (Cat y@Text{} z) -> go (Cat (go (Cat x y)) z)
  
          Cat (Cat x y@Char{}) z -> go (Cat x (go (Cat y z)))
          Cat (Cat x y@Text{}) z -> go (Cat x (go (Cat y z)))
  
          Cat x y -> Cat (go x) (go y)
  
          Nest i (Nest j x) -> let !fused = Nest (i+j) x
                               in go fused
          Nest _ x@Empty{} -> x
          Nest _ x@Text{}  -> x
          Nest _ x@Char{}  -> x
          Nest 0 x         -> go x
          Nest i x         -> Nest i (go x)
  
          Annotated ann x -> Annotated ann (go x)
  
          FlatAlt x1 x2 -> FlatAlt (go x1) (go x2)
          Union x1 x2   -> Union (go x1) (go x2)
  
          other | depth == Shallow -> other
  
          Column f        -> Column (go . f)
          WithPageWidth f -> WithPageWidth (go . f)
          Nesting f       -> Nesting (go . f)
  
          other -> other
  
  
  
  -- | The data type @SimpleDocStream@ represents laid out documents and is used
  -- by the display functions.
  --
  -- A simplified view is that @'Doc' = ['SimpleDocStream']@, and the layout
  -- functions pick one of the 'SimpleDocStream's based on which one fits the
  -- layout constraints best. This means that 'SimpleDocStream' has all complexity
  -- contained in 'Doc' resolved, making it very easy to convert it to other
  -- formats, such as plain text or terminal output.
  --
  -- To write your own @'Doc'@ to X converter, it is therefore sufficient to
  -- convert from @'SimpleDocStream'@. The »Render« submodules provide some
  -- built-in converters to do so, and helpers to create own ones.
  data SimpleDocStream ann =
        SFail
      | SEmpty
      | SChar !Char (SimpleDocStream ann)
  
      -- | 'T.length' is /O(n)/, so we cache it in the 'Int' field.
      | SText !Int !Text (SimpleDocStream ann)
  
      -- | @Int@ = indentation level for the (next) line
      | SLine !Int (SimpleDocStream ann)
  
      -- | Add an annotation to the remaining document.
      | SAnnPush ann (SimpleDocStream ann)
  
      -- | Remove a previously pushed annotation.
      | SAnnPop (SimpleDocStream ann)
      deriving (Eq, Ord, Show, Generic, Typeable)
  
  -- | Remove all trailing space characters.
  --
  -- This has some performance impact, because it does an entire additional pass
  -- over the 'SimpleDocStream'.
  --
  -- No trimming will be done inside annotations, which are considered to contain
  -- no (trimmable) whitespace, since the annotation might actually be /about/ the
  -- whitespace, for example a renderer that colors the background of trailing
  -- whitespace, as e.g. @git diff@ can be configured to do.
  --
  -- /Historical note:/ Since v1.7.0, 'layoutPretty' and 'layoutSmart' avoid
  -- producing the trailing whitespace that was the original motivation for
  -- creating 'removeTrailingWhitespace'.
  -- See <https://github.com/quchen/prettyprinter/pull/139> for some background
  -- info.
  removeTrailingWhitespace :: SimpleDocStream ann -> SimpleDocStream ann
  removeTrailingWhitespace = go (RecordedWhitespace [] 0)
    where
      commitWhitespace
          :: [Int] -- Withheld lines
          -> Int -- Withheld spaces
          -> SimpleDocStream ann
          -> SimpleDocStream ann
      commitWhitespace is !n sds = case is of
          []      -> case n of
                         0 -> sds
                         1 -> SChar ' ' sds
                         _ -> SText n (textSpaces n) sds
          (i:is') -> let !end = SLine (i + n) sds
                     in prependEmptyLines is' end
  
      prependEmptyLines :: [Int] -> SimpleDocStream ann -> SimpleDocStream ann
      prependEmptyLines is sds0 = foldr (\_ sds -> SLine 0 sds) sds0 is
  
      go :: WhitespaceStrippingState -> SimpleDocStream ann -> SimpleDocStream ann
      -- We do not strip whitespace inside annotated documents, since it might
      -- actually be relevant there.
      go annLevel@(AnnotationLevel annLvl) = \sds -> case sds of
          SFail             -> SFail
          SEmpty            -> SEmpty
          SChar c rest      -> SChar c (go annLevel rest)
          SText l text rest -> SText l text (go annLevel rest)
          SLine i rest      -> SLine i (go annLevel rest)
          SAnnPush ann rest -> let !annLvl' = annLvl+1
                               in SAnnPush ann (go (AnnotationLevel annLvl') rest)
          SAnnPop rest
              | annLvl > 1  -> let !annLvl' = annLvl-1
                               in SAnnPop (go (AnnotationLevel annLvl') rest)
              | otherwise   -> SAnnPop (go (RecordedWhitespace [] 0) rest)
      -- Record all spaces/lines encountered, and once proper text starts again,
      -- release only the necessary ones.
      go (RecordedWhitespace withheldLines withheldSpaces) = \sds -> case sds of
          SFail -> SFail
          SEmpty -> prependEmptyLines withheldLines SEmpty
          SChar c rest
              | c == ' ' -> go (RecordedWhitespace withheldLines (withheldSpaces+1)) rest
              | otherwise -> commitWhitespace
                                 withheldLines
                                 withheldSpaces
                                 (SChar c (go (RecordedWhitespace [] 0) rest))
          SText textLength text rest ->
              let stripped = T.dropWhileEnd (== ' ') text
                  strippedLength = T.length stripped
                  trailingLength = textLength - strippedLength
                  isOnlySpace = strippedLength == 0
              in if isOnlySpace
                  then go (RecordedWhitespace withheldLines (withheldSpaces + textLength)) rest
                  else commitWhitespace
                          withheldLines
                          withheldSpaces
                          (SText strippedLength
                                 stripped
                                 (go (RecordedWhitespace [] trailingLength) rest))
          SLine i rest -> go (RecordedWhitespace (i:withheldLines) 0) rest
          SAnnPush ann rest -> commitWhitespace
                                   withheldLines
                                   withheldSpaces
                                   (SAnnPush ann (go (AnnotationLevel 1) rest))
          SAnnPop _ -> error "Tried skipping spaces in unannotated data! Please report this as a bug in 'prettyprinter'."
  
  data WhitespaceStrippingState
      = AnnotationLevel !Int
      | RecordedWhitespace [Int] !Int
        -- ^ [Newline with indentation i] Spaces
    deriving Typeable
  
  
  
  -- $
  -- >>> import qualified Data.Text.IO as T
  -- >>> doc = "lorem" <> hardline <> hardline <> pretty "ipsum"
  -- >>> go = T.putStrLn . renderStrict . removeTrailingWhitespace . layoutPretty defaultLayoutOptions
  -- >>> go doc
  -- lorem
  -- <BLANKLINE>
  -- ipsum
  
  
  
  -- | Alter the document’s annotations.
  --
  -- This instance makes 'SimpleDocStream' more flexible (because it can be used in
  -- 'Functor'-polymorphic values), but @'fmap'@ is much less readable compared to
  -- using @'reAnnotateST'@ in code that only works for @'SimpleDocStream'@ anyway.
  -- Consider using the latter when the type does not matter.
  instance Functor SimpleDocStream where
      fmap = reAnnotateS
  
  -- | Collect all annotations from a document.
  instance Foldable SimpleDocStream where
      foldMap f = go
        where
          go = \sds -> case sds of
              SFail             -> mempty
              SEmpty            -> mempty
              SChar _ rest      -> go rest
              SText _ _ rest    -> go rest
              SLine _ rest      -> go rest
              SAnnPush ann rest -> f ann `mappend` go rest
              SAnnPop rest      -> go rest
  
  -- | Transform a document based on its annotations, possibly leveraging
  -- 'Applicative' effects.
  instance Traversable SimpleDocStream where
      traverse f = go
        where
          go = \sds -> case sds of
              SFail             -> pure SFail
              SEmpty            -> pure SEmpty
              SChar c rest      -> SChar c   <$> go rest
              SText l t rest    -> SText l t <$> go rest
              SLine i rest      -> SLine i   <$> go rest
              SAnnPush ann rest -> SAnnPush  <$> f ann <*> go rest
              SAnnPop rest      -> SAnnPop   <$> go rest
  
  -- | Decide whether a 'SimpleDocStream' fits the constraints given, namely
  --
  --   - original indentation of the current line
  --   - current column
  --   - initial indentation of the alternative 'SimpleDocStream' if it
  --     starts with a line break (used by 'layoutSmart')
  --   - width in which to fit the first line
  newtype FittingPredicate ann
    = FittingPredicate (Int
                     -> Int
                     -> Maybe Int
                     -> SimpleDocStream ann
                     -> Bool)
    deriving Typeable
  
  -- | List of nesting level/document pairs yet to be laid out.
  data LayoutPipeline ann =
        Nil
      | Cons !Int (Doc ann) (LayoutPipeline ann)
      | UndoAnn (LayoutPipeline ann)
    deriving Typeable
  
  -- | Maximum number of characters that fit in one line. The layout algorithms
  -- will try not to exceed the set limit by inserting line breaks when applicable
  -- (e.g. via 'softline'').
  data PageWidth
  
      = AvailablePerLine !Int !Double
      -- ^ Layouters should not exceed the specified space per line.
      --
      --   - The 'Int' is the number of characters, including whitespace, that
      --     fit in a line. A typical value is 80.
      --
      --   - The 'Double' is the ribbon with, i.e. the fraction of the total
      --     page width that can be printed on. This allows limiting the length
      --     of printable text per line. Values must be between 0 and 1, and
      --     0.4 to 1 is typical.
  
      | Unbounded
      -- ^ Layouters should not introduce line breaks on their own.
  
      deriving (Eq, Ord, Show, Typeable)
  
  defaultPageWidth :: PageWidth
  defaultPageWidth = AvailablePerLine 80 1
  
  -- | The remaining width on the current line.
  remainingWidth :: Int -> Double -> Int -> Int -> Int
  remainingWidth lineLength ribbonFraction lineIndent currentColumn =
      min columnsLeftInLine columnsLeftInRibbon
    where
      columnsLeftInLine = lineLength - currentColumn
      columnsLeftInRibbon = lineIndent + ribbonWidth - currentColumn
      ribbonWidth =
          (max 0 . min lineLength . floor)
              (fromIntegral lineLength * ribbonFraction)
  
  -- $ Test to avoid surprising behaviour
  -- >>> Unbounded > AvailablePerLine maxBound 1
  -- True
  
  -- | Options to influence the layout algorithms.
  newtype LayoutOptions = LayoutOptions { layoutPageWidth :: PageWidth }
      deriving (Eq, Ord, Show, Typeable)
  
  -- | The default layout options, suitable when you just want some output, and
  -- don’t particularly care about the details. Used by the 'Show' instance, for
  -- example.
  --
  -- >>> defaultLayoutOptions
  -- LayoutOptions {layoutPageWidth = AvailablePerLine 80 1.0}
  defaultLayoutOptions :: LayoutOptions
  defaultLayoutOptions = LayoutOptions { layoutPageWidth = defaultPageWidth }
  
  -- | This is the default layout algorithm, and it is used by 'show', 'putDoc'
  -- and 'hPutDoc'.
  --
  -- @'layoutPretty'@ commits to rendering something in a certain way if the 
  -- remainder of the current line fits the layout constraints; in other words, 
  -- it has up to one line of lookahead when rendering. Consider using the
  -- smarter, but a bit less performant, @'layoutSmart'@ algorithm if the results
  -- seem to run off to the right before having lots of line breaks.
  layoutPretty
      :: LayoutOptions
      -> Doc ann
      -> SimpleDocStream ann
  layoutPretty (LayoutOptions pageWidth_@(AvailablePerLine lineLength ribbonFraction)) =
      layoutWadlerLeijen
          (FittingPredicate
               (\lineIndent currentColumn _initialIndentY sdoc ->
                   fits
                       (remainingWidth lineLength ribbonFraction lineIndent currentColumn)
                       sdoc))
          pageWidth_
    where
      fits :: Int -- ^ Width in which to fit the first line
           -> SimpleDocStream ann
           -> Bool
      fits w _ | w < 0      = False
      fits _ SFail          = False
      fits _ SEmpty         = True
      fits w (SChar _ x)    = fits (w - 1) x
      fits w (SText l _t x) = fits (w - l) x
      fits _ SLine{}        = True
      fits w (SAnnPush _ x) = fits w x
      fits w (SAnnPop x)    = fits w x
  
  layoutPretty (LayoutOptions Unbounded) = layoutUnbounded
  
  -- | A layout algorithm with more lookahead than 'layoutPretty', that introduces
  -- line breaks earlier if the content does not (or will not, rather) fit within
  -- the page width.
  --
  -- Consider the following python-ish document,
  --
  -- >>> let fun x = hang 2 ("fun(" <> softline' <> x) <> ")"
  -- >>> let doc = (fun . fun . fun . fun . fun) (align (list ["abcdef", "ghijklm"]))
  --
  -- which we’ll be rendering using the following pipeline (where the layout
  -- algorithm has been left open):
  --
  -- >>> import Data.Text.IO as T
  -- >>> import Prettyprinter.Render.Text
  -- >>> let hr = pipe <> pretty (replicate (26-2) '-') <> pipe
  -- >>> let go layouter x = (T.putStrLn . renderStrict . layouter (LayoutOptions (AvailablePerLine 26 1))) (vsep [hr, x, hr])
  --
  -- If we render this using 'layoutPretty' with a page width of 26 characters
  -- per line, all the @fun@ calls fit into the first line so they will be put
  -- there:
  --
  -- >>> go layoutPretty doc
  -- |------------------------|
  -- fun(fun(fun(fun(fun(
  --                   [ abcdef
  --                   , ghijklm ])))))
  -- |------------------------|
  --
  -- Note that this exceeds the desired 26 character page width. The same
  -- document, rendered with @'layoutSmart'@, fits the layout contstraints:
  --
  -- >>> go layoutSmart doc
  -- |------------------------|
  -- fun(
  --   fun(
  --     fun(
  --       fun(
  --         fun(
  --           [ abcdef
  --           , ghijklm ])))))
  -- |------------------------|
  --
  -- The key difference between 'layoutPretty' and 'layoutSmart' is that the
  -- latter will check the potential document until it encounters a line with the
  -- same indentation or less than the start of the document. Any line encountered
  -- earlier is assumed to belong to the same syntactic structure.
  -- 'layoutPretty' checks only the first line.
  --
  -- Consider for example the question of whether the @A@s fit into the document
  -- below:
  --
  -- > 1 A
  -- > 2   A
  -- > 3  A
  -- > 4 B
  -- > 5   B
  --
  -- 'layoutPretty' will check only line 1, ignoring whether e.g. line 2 might
  -- already be too wide.
  -- By contrast, 'layoutSmart' stops only once it reaches line 4, where the @B@
  -- has the same indentation as the first @A@.
  layoutSmart
      :: LayoutOptions
      -> Doc ann
      -> SimpleDocStream ann
  layoutSmart (LayoutOptions pageWidth_@(AvailablePerLine lineLength ribbonFraction)) =
      layoutWadlerLeijen (FittingPredicate fits) pageWidth_
    where
      -- Why doesn't layoutSmart simply check the entire document?
      --
      -- 1. That would be very expensive.
      -- 2. In that case the layout of a particular part of a document would
      --    depend on the fit of completely unrelated parts of the same document.
      --    See https://github.com/quchen/prettyprinter/issues/83 for a related
      --    bug.
  
      fits :: Int -> Int -> Maybe Int -> SimpleDocStream ann -> Bool
      fits lineIndent currentColumn initialIndentY = go availableWidth
        where
          go w _ | w < 0          = False
          go _ SFail              = False
          go _ SEmpty             = True
          go w (SChar _ x)        = go (w - 1) x
          go w (SText l _t x)     = go (w - l) x
          go _ (SLine i x)
            | minNestingLevel < i = go (lineLength - i) x -- TODO: Take ribbon width into account?! (#142)
            | otherwise           = True
          go w (SAnnPush _ x)     = go w x
          go w (SAnnPop x)        = go w x
  
          availableWidth = remainingWidth lineLength ribbonFraction lineIndent currentColumn
  
          minNestingLevel =
              -- See the Note
              -- [Choosing the right minNestingLevel for consistent smart layouts]
              case initialIndentY of
                  Just i ->
                      -- y could be a (less wide) hanging layout. If so, let's
                      -- check x a bit more thoroughly so we don't miss a potentially
                      -- better fitting y.
                      min i currentColumn
                  Nothing ->
                      -- y definitely isn't a hanging layout. Let's check x with the
                      -- same minNestingLevel that any subsequent lines with the same
                      -- indentation use.
                      currentColumn
  
  layoutSmart (LayoutOptions Unbounded) = layoutUnbounded
  
  -- | Layout a document with @Unbounded@ page width.
  layoutUnbounded :: Doc ann -> SimpleDocStream ann
  layoutUnbounded =
      layoutWadlerLeijen
          (FittingPredicate
              (\_lineIndent _currentColumn _initialIndentY sdoc -> not (failsOnFirstLine sdoc)))
          Unbounded
    where
      -- See the Note [Detecting failure with Unbounded page width].
      failsOnFirstLine :: SimpleDocStream ann -> Bool
      failsOnFirstLine = go
        where
          go sds = case sds of
              SFail        -> True
              SEmpty       -> False
              SChar _ s    -> go s
              SText _ _ s  -> go s
              SLine _ _    -> False
              SAnnPush _ s -> go s
              SAnnPop s    -> go s
  
  -- | The Wadler/Leijen layout algorithm
  layoutWadlerLeijen
      :: forall ann. FittingPredicate ann
      -> PageWidth
      -> Doc ann
      -> SimpleDocStream ann
  layoutWadlerLeijen
      (FittingPredicate fits)
      pageWidth_
      doc
    = best 0 0 (Cons 0 doc Nil)
    where
  
      -- * current column >= current nesting level
      -- * current column - current indentaion = number of chars inserted in line
      best
          :: Int -- Current nesting level
          -> Int -- Current column, i.e. "where the cursor is"
          -> LayoutPipeline ann -- Documents remaining to be handled (in order)
          -> SimpleDocStream ann
      best !_ !_ Nil           = SEmpty
      best nl cc (UndoAnn ds)  = SAnnPop (best nl cc ds)
      best nl cc (Cons i d ds) = case d of
          Fail            -> SFail
          Empty           -> best nl cc ds
          Char c          -> let !cc' = cc+1 in SChar c (best nl cc' ds)
          Text l t        -> let !cc' = cc+l in SText l t (best nl cc' ds)
          Line            -> let x = best i i ds
                                 -- Don't produce indentation if there's no
                                 -- following text on the same line.
                                 -- This prevents trailing whitespace.
                                 i' = case x of
                                     SEmpty  -> 0
                                     SLine{} -> 0
                                     _       -> i
                             in SLine i' x
          FlatAlt x _     -> best nl cc (Cons i x ds)
          Cat x y         -> best nl cc (Cons i x (Cons i y ds))
          Nest j x        -> let !ij = i+j in best nl cc (Cons ij x ds)
          Union x y       -> let x' = best nl cc (Cons i x ds)
                                 y' = best nl cc (Cons i y ds)
                             in selectNicer nl cc x' y'
          Column f        -> best nl cc (Cons i (f cc) ds)
          WithPageWidth f -> best nl cc (Cons i (f pageWidth_) ds)
          Nesting f       -> best nl cc (Cons i (f i) ds)
          Annotated ann x -> SAnnPush ann (best nl cc (Cons i x (UndoAnn ds)))
  
      -- Select the better fitting of two documents:
      -- Choice A if it fits, otherwise choice B.
      --
      -- The fit of choice B is /not/ checked! It is ultimately the user's
      -- responsibility to provide an alternative that can fit the page even when
      -- choice A doesn't.
      selectNicer
          :: Int           -- ^ Current nesting level
          -> Int           -- ^ Current column
          -> SimpleDocStream ann -- ^ Choice A.
          -> SimpleDocStream ann -- ^ Choice B. Should fit more easily
                                 --   (== be less wide) than choice A.
          -> SimpleDocStream ann -- ^ Choice A if it fits, otherwise B.
      selectNicer lineIndent currentColumn x y
          | fits lineIndent currentColumn (initialIndentation y) x = x
          | otherwise = y
  
      initialIndentation :: SimpleDocStream ann -> Maybe Int
      initialIndentation sds = case sds of
          SLine i _    -> Just i
          SAnnPush _ s -> initialIndentation s
          SAnnPop s    -> initialIndentation s
          _            -> Nothing
  
  
  {- Note [Choosing the right minNestingLevel for consistent smart layouts]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Consider this document:
  
      doc =
              "Groceries: "
          <>  align
                  (cat
                      [ sep ["pommes", "de", "terre"]
                      , "apples"
                      , "Donaudampfschifffahrtskapitänskajütenmülleimer"
                      ]
                  )
  
  ... and assume we want to fit it into 40 columns as nicely as possible:
  
      opts = LayoutOptions (AvailablePerLine 40 1)
  
  We already have bad luck with the last item – it's longer than 40 characters
  on its own!
  
  We'd still like the first item, pommes de terre, to be laid out nicely, that is,
  on one line, since it's not too wide. This is what we'd like to see:
  
      Groceries: pommes de terre
                 apples
                 Donaudampfschifffahrtskapitänskajütenmülleimer
  
  Before #83 was fixed, that wasn't what we got! Instead we got this:
  
  > renderIO stdout $ layoutSmart opts doc
  Groceries: pommes
             de
             terre
             apples
             Donaudampfschifffahrtskapitänskajütenmülleimer
  
  Why?
  
  minNestingLevel was effectively defined as
  
      minNestingLevel = lineIndent
  
  The lineIndent for "pommes de terre" is 0.
  
  The FittingPredicate for layoutSmart will continue to check the rest of the
  document until it finds a line where the indentation <= minNestingLevel.
  In this case this meant that layoutSmart would traverse all the items,
  and note that the last item, Donaudampfschifffahrtskapitänskajütenmülleimer,
  doesn't fit into the available space! The "flatter" version of the document
  has failed, so "pommes de terre" gets spread over several lines!
  
  Obviously this would be an inconsistency with the layout of the other items.
  Their lineIndent is 11 each, so for them, the FittingPredicate stops already
  on the next line.
  
  The obvious solution is to change the definition of minNestingLevel:
  
      minNestingLevel = currentColumn
  
  This however breaks the "python-ish" document from the documentation for
  layoutSmart:
  
      expected: |------------------------|
                fun(
                  fun(
                    fun(
                      fun(
                        fun(
                          [ abcdef
                          , ghijklm ])))))
                |------------------------|
  
       but got: |------------------------|
                fun(
                  fun(
                    fun(
                      fun(
                        fun([ abcdef
                            , ghijklm ])))))
                |------------------------|
  
  We now accept the worse layout because the problematic last line has
  the same indentation as the current column of "[ abcdef", so we don't check it!
  
  The solution we went with in the end is a bit of a hack:
  
  We check whether the alternative, "high" layout is a (potentially less wide)
  hanging layout, and in that case pick its indentation as the minNestingLevel.
  
  This way we achieve the optimal layout in both scenarios.
  
  See https://github.com/quchen/prettyprinter/issues/83 for the bug that lead
  to the current solution.
  
  
  Note [Detecting failure with Unbounded page width]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  To understand why it is sufficient to check the first line of the
  SimpleDocStream, trace how an SFail ends up there:
  
  1. We group a Doc containing a hard linebreak (hardline), producing a
     (Union x y) where x contains Fail.
  
  2. In layoutWadlerLeijen.best, any Unions are handled recursively, rejecting any
     alternatives that would result in SFail.
  
  So once a SimpleDocStream reaches selectNicer, any SFail in it must
  appear before the first linebreak – any other SFail would have been
  detected and rejected in a previous iteration.
  -}
  
  
  
  -- | @(layoutCompact x)@ lays out the document @x@ without adding any
  -- indentation and without preserving annotations.
  -- Since no \'pretty\' printing is involved, this layouter is very
  -- fast. The resulting output contains fewer characters than a prettyprinted
  -- version and can be used for output that is read by other programs.
  --
  -- >>> let doc = hang 4 (vsep ["lorem", "ipsum", hang 4 (vsep ["dolor", "sit"])])
  -- >>> doc
  -- lorem
  --     ipsum
  --     dolor
  --         sit
  --
  -- >>> let putDocCompact = renderIO System.IO.stdout . layoutCompact
  -- >>> putDocCompact doc
  -- lorem
  -- ipsum
  -- dolor
  -- sit
  layoutCompact :: Doc ann1 -> SimpleDocStream ann2
  layoutCompact doc = scan 0 [doc]
    where
      scan _ [] = SEmpty
      scan !col (d:ds) = case d of
          Fail            -> SFail
          Empty           -> scan col ds
          Char c          -> SChar c (scan (col+1) ds)
          Text l t        -> let !col' = col+l in SText l t (scan col' ds)
          FlatAlt x _     -> scan col (x:ds)
          Line            -> SLine 0 (scan 0 ds)
          Cat x y         -> scan col (x:y:ds)
          Nest _ x        -> scan col (x:ds)
          Union _ y       -> scan col (y:ds)
          Column f        -> scan col (f col:ds)
          WithPageWidth f -> scan col (f Unbounded : ds)
          Nesting f       -> scan col (f 0 : ds)
          Annotated _ x   -> scan col (x:ds)
  
  -- | @('show' doc)@ prettyprints document @doc@ with 'defaultLayoutOptions',
  -- ignoring all annotations.
  instance Show (Doc ann) where
      showsPrec _ doc = renderShowS (layoutPretty defaultLayoutOptions doc)
  
  -- | Render a 'SimpleDocStream' to a 'ShowS', useful to write 'Show' instances
  -- based on the prettyprinter.
  --
  -- @
  -- instance 'Show' MyType where
  --     'showsPrec' _ = 'renderShowS' . 'layoutPretty' 'defaultLayoutOptions' . 'pretty'
  -- @
  renderShowS :: SimpleDocStream ann -> ShowS
  renderShowS = \sds -> case sds of
      SFail        -> panicUncaughtFail
      SEmpty       -> id
      SChar c x    -> showChar c . renderShowS x
      SText _l t x -> showString (T.unpack t) . renderShowS x
      SLine i x    -> showString ('\n' : replicate i ' ') . renderShowS x
      SAnnPush _ x -> renderShowS x
      SAnnPop x    -> renderShowS x
  
  
  -- | A utility for producing indentation etc.
  --
  -- >>> textSpaces 3
  -- "   "
  --
  -- This produces much better Core than the equivalent
  --
  -- > T.replicate n " "
  --
  -- (See <https://github.com/quchen/prettyprinter/issues/131>.)
  textSpaces :: Int -> Text
  textSpaces n = T.replicate n (T.singleton ' ')
  
  
  -- $setup
  --
  -- (Definitions for the doctests)
  --
  -- >>> :set -XOverloadedStrings
  -- >>> import Prettyprinter.Render.Text
  -- >>> import Prettyprinter.Symbols.Ascii
  -- >>> import Prettyprinter.Util as Util
  -- >>> import Test.QuickCheck.Modifiers
  