content: {-# LANGUAGE MagicHash #-}
  
  -----------------------------------------------------------------------------
  -- |
  -- Module      :  GHC.Utils.Ppr
  -- Copyright   :  (c) The University of Glasgow 2001
  -- License     :  BSD-style (see the file LICENSE)
  --
  -- Maintainer  :  David Terei <code@davidterei.com>
  -- Stability   :  stable
  -- Portability :  portable
  --
  -- John Hughes's and Simon Peyton Jones's Pretty Printer Combinators
  --
  -- Based on /The Design of a Pretty-printing Library/
  -- in Advanced Functional Programming,
  -- Johan Jeuring and Erik Meijer (eds), LNCS 925
  -- <http://www.cse.chalmers.se/~rjmh/Papers/pretty.ps>
  --
  -----------------------------------------------------------------------------
  
  {-
  Note [Differences between libraries/pretty and compiler/GHC/Utils/Ppr.hs]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  For historical reasons, there are two different copies of `Pretty` in the GHC
  source tree:
   * `libraries/pretty` is a submodule containing
     https://github.com/haskell/pretty. This is the `pretty` library as released
     on hackage. It is used by several other libraries in the GHC source tree
     (e.g. template-haskell and Cabal).
   * `compiler/GHC/Utils/Ppr.hs` (this module). It is used by GHC only.
  
  There is an ongoing effort in https://github.com/haskell/pretty/issues/1 and
  https://gitlab.haskell.org/ghc/ghc/issues/10735 to try to get rid of GHC's copy
  of Pretty.
  
  Currently, GHC's copy of Pretty resembles pretty-1.1.2.0, with the following
  major differences:
   * GHC's copy uses `Faststring` for performance reasons.
   * GHC's copy has received a backported bugfix for #12227, which was
     released as pretty-1.1.3.4 ("Remove harmful $! forcing in beside",
     https://github.com/haskell/pretty/pull/35).
  
  Other differences are minor. Both copies define some extra functions and
  instances not defined in the other copy. To see all differences, do this in a
  ghc git tree:
  
      $ cd libraries/pretty
      $ git checkout v1.1.2.0
      $ cd -
      $ vimdiff compiler/GHC/Utils/Ppr.hs \
                libraries/pretty/src/Text/PrettyPrint/HughesPJ.hs
  
  For parity with `pretty-1.1.2.1`, the following two `pretty` commits would
  have to be backported:
    * "Resolve foldr-strictness stack overflow bug"
      (307b8173f41cd776eae8f547267df6d72bff2d68)
    * "Special-case reduce for horiz/vert"
      (c57c7a9dfc49617ba8d6e4fcdb019a3f29f1044c)
  This has not been done sofar, because these commits seem to cause more
  allocation in the compiler (see thomie's comments in
  https://github.com/haskell/pretty/pull/9).
  -}
  
  module GHC.Utils.Ppr (
  
          -- * The document type
          Doc, TextDetails(..),
  
          -- * Constructing documents
  
          -- ** Converting values into documents
          char, text, ftext, ptext, ztext, sizedText, zeroWidthText, emptyText,
          int, integer, float, double, rational, hex,
  
          -- ** Simple derived documents
          semi, comma, colon, space, equals, ellipsis,
          lparen, rparen, lbrack, rbrack, lbrace, rbrace,
  
          -- ** Wrapping documents in delimiters
          parens, brackets, braces, quotes, squotes, quote, doubleQuotes,
          maybeParens,
  
          -- ** Combining documents
          empty,
          (<>), (<+>), hcat, hsep,
          ($$), ($+$), vcat,
          sep, cat,
          fsep, fcat,
          nest,
          hang, hangNotEmpty, punctuate,
  
          -- * Predicates on documents
          isEmpty,
          docHead,
  
          -- * Rendering documents
  
          -- ** Rendering with a particular style
          Style(..),
          style,
          renderStyle,
          Mode(..),
  
          -- ** General rendering
          fullRender, txtPrinter,
  
          -- ** GHC-specific rendering
          printDoc, printDoc_,
          bufLeftRender, printLeftRender -- performance hack
  
    ) where
  
  import GHC.Prelude.Basic hiding (error)
  import Control.Applicative ((<|>))
  
  import GHC.Utils.BufHandle
  import GHC.Data.FastString
  import GHC.Utils.Panic.Plain
  import System.IO
  import Numeric (showHex)
  
  --for a RULES
  import GHC.Base ( unpackCString#, unpackNBytes#, Int(..) )
  import GHC.Ptr  ( Ptr(..) )
  
  -- ---------------------------------------------------------------------------
  -- The Doc calculus
  
  {-
  Laws for $$
  ~~~~~~~~~~~
  <a1>    (x $$ y) $$ z   = x $$ (y $$ z)
  <a2>    empty $$ x      = x
  <a3>    x $$ empty      = x
  
          ...ditto $+$...
  
  Laws for <>
  ~~~~~~~~~~~
  <b1>    (x <> y) <> z   = x <> (y <> z)
  <b2>    empty <> x      = empty
  <b3>    x <> empty      = x
  
          ...ditto <+>...
  
  Laws for text
  ~~~~~~~~~~~~~
  <t1>    text s <> text t        = text (s++t)
  <t2>    text "" <> x            = x, if x non-empty
  
  ** because of law n6, t2 only holds if x doesn't
  ** start with `nest'.
  
  
  Laws for nest
  ~~~~~~~~~~~~~
  <n1>    nest 0 x                = x
  <n2>    nest k (nest k' x)      = nest (k+k') x
  <n3>    nest k (x <> y)         = nest k x <> nest k y
  <n4>    nest k (x $$ y)         = nest k x $$ nest k y
  <n5>    nest k empty            = empty
  <n6>    x <> nest k y           = x <> y, if x non-empty
  
  ** Note the side condition on <n6>!  It is this that
  ** makes it OK for empty to be a left unit for <>.
  
  Miscellaneous
  ~~~~~~~~~~~~~
  <m1>    (text s <> x) $$ y = text s <> ((text "" <> x) $$
                                           nest (-length s) y)
  
  <m2>    (x $$ y) <> z = x $$ (y <> z)
          if y non-empty
  
  
  Laws for list versions
  ~~~~~~~~~~~~~~~~~~~~~~
  <l1>    sep (ps++[empty]++qs)   = sep (ps ++ qs)
          ...ditto hsep, hcat, vcat, fill...
  
  <l2>    nest k (sep ps) = sep (map (nest k) ps)
          ...ditto hsep, hcat, vcat, fill...
  
  Laws for oneLiner
  ~~~~~~~~~~~~~~~~~
  <o1>    oneLiner (nest k p) = nest k (oneLiner p)
  <o2>    oneLiner (x <> y)   = oneLiner x <> oneLiner y
  
  You might think that the following version of <m1> would
  be neater:
  
  <3 NO>  (text s <> x) $$ y = text s <> ((empty <> x)) $$
                                           nest (-length s) y)
  
  But it doesn't work, for if x=empty, we would have
  
          text s $$ y = text s <> (empty $$ nest (-length s) y)
                      = text s <> nest (-length s) y
  -}
  
  -- ---------------------------------------------------------------------------
  -- Operator fixity
  
  infixl 6 <>
  infixl 6 <+>
  infixl 5 $$, $+$
  
  
  -- ---------------------------------------------------------------------------
  -- The Doc data type
  
  -- | The abstract type of documents.
  -- A Doc represents a *set* of layouts. A Doc with
  -- no occurrences of Union or NoDoc represents just one layout.
  data Doc
    = Empty                                            -- empty
    | NilAbove Doc                                     -- text "" $$ x
    | TextBeside !TextDetails {-# UNPACK #-} !Int Doc  -- text s <> x
    | Nest {-# UNPACK #-} !Int Doc                     -- nest k x
    | Union Doc Doc                                    -- ul `union` ur
    | NoDoc                                            -- The empty set of documents
    | Beside Doc Bool Doc                              -- True <=> space between
    | Above Doc Bool Doc                               -- True <=> never overlap
  
  {-
  Here are the invariants:
  
  1) The argument of NilAbove is never Empty. Therefore
     a NilAbove occupies at least two lines.
  
  2) The argument of @TextBeside@ is never @Nest@.
  
  3) The layouts of the two arguments of @Union@ both flatten to the same
     string.
  
  4) The arguments of @Union@ are either @TextBeside@, or @NilAbove@.
  
  5) A @NoDoc@ may only appear on the first line of the left argument of an
     union. Therefore, the right argument of an union can never be equivalent
     to the empty set (@NoDoc@).
  
  6) An empty document is always represented by @Empty@.  It can't be
     hidden inside a @Nest@, or a @Union@ of two @Empty@s.
  
  7) The first line of every layout in the left argument of @Union@ is
     longer than the first line of any layout in the right argument.
     (1) ensures that the left argument has a first line.  In view of
     (3), this invariant means that the right argument must have at
     least two lines.
  
  Notice the difference between
     * NoDoc (no documents)
     * Empty (one empty document; no height and no width)
     * text "" (a document containing the empty string;
                one line high, but has no width)
  -}
  
  
  -- | RDoc is a "reduced GDoc", guaranteed not to have a top-level Above or Beside.
  type RDoc = Doc
  
  -- | The TextDetails data type
  --
  -- A TextDetails represents a fragment of text that will be
  -- output at some point.
  data TextDetails = Chr  {-# UNPACK #-} !Char -- ^ A single Char fragment
                   | Str  String -- ^ A whole String fragment
                   | PStr FastString                      -- a hashed string
                   | ZStr FastZString                     -- a z-encoded string
                   | LStr {-# UNPACK #-} !PtrString
                     -- a '\0'-terminated array of bytes
                   | RStr {-# UNPACK #-} !Int {-# UNPACK #-} !Char
                     -- a repeated character (e.g., ' ')
  
  instance Show Doc where
    showsPrec _ doc cont = fullRender (mode style) (lineLength style)
                                      (ribbonsPerLine style)
                                      txtPrinter cont doc
  
  
  -- ---------------------------------------------------------------------------
  -- Values and Predicates on GDocs and TextDetails
  
  -- | A document of height and width 1, containing a literal character.
  char :: Char -> Doc
  char c = textBeside_ (Chr c) 1 Empty
  
  -- | A document of height 1 containing a literal string.
  -- 'text' satisfies the following laws:
  --
  -- * @'text' s '<>' 'text' t = 'text' (s'++'t)@
  --
  -- * @'text' \"\" '<>' x = x@, if @x@ non-empty
  --
  -- The side condition on the last law is necessary because @'text' \"\"@
  -- has height 1, while 'empty' has no height.
  text :: String -> Doc
  text s = textBeside_ (Str s) (length s) Empty
  {-# NOINLINE [0] text #-}   -- Give the RULE a chance to fire
                              -- It must wait till after phase 1 when
                              -- the unpackCString first is manifested
  
  -- RULE that turns (text "abc") into (ptext (A# "abc"#)) to avoid the
  -- intermediate packing/unpacking of the string.
  {-# RULES "text/str"
      forall a. text (unpackCString# a)  = ptext (mkPtrString# a)
    #-}
  {-# RULES "text/unpackNBytes#"
      forall p n. text (unpackNBytes# p n) = ptext (PtrString (Ptr p) (I# n))
    #-}
  
  -- Empty strings are desugared into [] (not "unpackCString#..."), hence they are
  -- not matched by the text/str rule above.
  {-# RULES "text/[]"
      text [] = emptyText
    #-}
  
  ftext :: FastString -> Doc
  ftext s = textBeside_ (PStr s) (lengthFS s) Empty
  
  ptext :: PtrString -> Doc
  ptext s = textBeside_ (LStr s) (lengthPS s) Empty
  
  ztext :: FastZString -> Doc
  ztext s = textBeside_ (ZStr s) (lengthFZS s) Empty
  
  -- | Some text with any width. (@text s = sizedText (length s) s@)
  sizedText :: Int -> String -> Doc
  sizedText l s = textBeside_ (Str s) l Empty
  
  -- | Some text, but without any width. Use for non-printing text
  -- such as a HTML or Latex tags
  zeroWidthText :: String -> Doc
  zeroWidthText = sizedText 0
  
  -- | Empty text (one line high but no width). (@emptyText = text ""@)
  emptyText :: Doc
  emptyText = sizedText 0 []
    -- defined as a CAF. Sharing occurs especially via the text/[] rule above.
    -- Every use of `text ""` in user code should be replaced with this.
  
  -- | The empty document, with no height and no width.
  -- 'empty' is the identity for '<>', '<+>', '$$' and '$+$', and anywhere
  -- in the argument list for 'sep', 'hcat', 'hsep', 'vcat', 'fcat' etc.
  empty :: Doc
  empty = Empty
  
  -- | Returns 'True' if the document is empty
  isEmpty :: Doc -> Bool
  isEmpty Empty = True
  isEmpty _     = False
  
  -- | Get the first character of a document. We also return a new document,
  -- equivalent to the original one but faster to render. Use it to avoid work
  -- duplication.
  docHead :: Doc -> (Maybe Char, Doc)
  docHead d = (headChar, rdoc)
    where
      rdoc = reduceDoc d
      headChar = go rdoc
  
      go :: RDoc -> Maybe Char
      go (Union p q)  = go (first p q)
      go (Nest _ p)   = go p
      go Empty        = Nothing
      go (NilAbove _) = Just '\n'
      go (TextBeside td _ p) = go_td td <|> go p
      go NoDoc       = error "docHead: NoDoc"
      go (Above {})  = error "docHead: Above"
      go (Beside {}) = error "docHead: Beside"
  
      go_td :: TextDetails -> Maybe Char
      go_td (Chr c)  = Just c
      go_td (Str s)  = go_str s
      go_td (PStr s) = go_str (unpackFS s) -- O(1) because unpackFS is lazy
      go_td (ZStr s) = go_str (zStringTakeN 1 s)
      go_td (LStr s) = go_str (unpackPtrStringTakeN 1 s)
      go_td (RStr n c) = if n > 0 then Just c else Nothing
  
      go_str :: String -> Maybe Char
      go_str []    = Nothing
      go_str (c:_) = Just c
  
  {-
  Q: What is the reason for negative indentation (i.e. argument to indent
     is < 0) ?
  
  A:
  This indicates an error in the library client's code.
  If we compose a <> b, and the first line of b is more indented than some
  other lines of b, the law <n6> (<> eats nests) may cause the pretty
  printer to produce an invalid layout:
  
  doc       |0123345
  ------------------
  d1        |a...|
  d2        |...b|
            |c...|
  
  d1<>d2    |ab..|
           c|....|
  
  Consider a <> b, let `s' be the length of the last line of `a', `k' the
  indentation of the first line of b, and `k0' the indentation of the
  left-most line b_i of b.
  
  The produced layout will have negative indentation if `k - k0 > s', as
  the first line of b will be put on the (s+1)th column, effectively
  translating b horizontally by (k-s). Now if the i^th line of b has an
  indentation k0 < (k-s), it is translated out-of-page, causing
  `negative indentation'.
  -}
  
  
  semi   :: Doc -- ^ A ';' character
  comma  :: Doc -- ^ A ',' character
  colon  :: Doc -- ^ A ':' character
  space  :: Doc -- ^ A space character
  equals :: Doc -- ^ A '=' character
  lparen :: Doc -- ^ A '(' character
  rparen :: Doc -- ^ A ')' character
  lbrack :: Doc -- ^ A '[' character
  rbrack :: Doc -- ^ A ']' character
  lbrace :: Doc -- ^ A '{' character
  rbrace :: Doc -- ^ A '}' character
  ellipsis :: Doc -- ^ A '...' ellipsis
  semi   = char ';'
  comma  = char ','
  colon  = char ':'
  space  = char ' '
  equals = char '='
  lparen = char '('
  rparen = char ')'
  lbrack = char '['
  rbrack = char ']'
  lbrace = char '{'
  rbrace = char '}'
  ellipsis = text "..."
  
  spaceText, nlText :: TextDetails
  spaceText = Chr ' '
  nlText    = Chr '\n'
  
  int      :: Int      -> Doc -- ^ @int n = text (show n)@
  integer  :: Integer  -> Doc -- ^ @integer n = text (show n)@
  float    :: Float    -> Doc -- ^ @float n = text (show n)@
  double   :: Double   -> Doc -- ^ @double n = text (show n)@
  rational :: Rational -> Doc -- ^ @rational n = text (show n)@
  hex      :: Integer  -> Doc -- ^ See Note [Print Hexadecimal Literals]
  int      n = text (show n)
  integer  n = text (show n)
  float    n = text (show n)
  double   n = text (show n)
  rational n = text (show n)
  hex      n = text ('0' : 'x' : padded)
      where
      str = showHex n ""
      strLen = max 1 (length str)
      len = 2 ^ (ceiling (logBase 2 (fromIntegral strLen :: Double)) :: Int)
      padded = replicate (len - strLen) '0' ++ str
  
  parens       :: Doc -> Doc -- ^ Wrap document in @(...)@
  brackets     :: Doc -> Doc -- ^ Wrap document in @[...]@
  braces       :: Doc -> Doc -- ^ Wrap document in @{...}@
  quotes       :: Doc -> Doc -- ^ Wrap document in @\`...\'@
  squotes      :: Doc -> Doc -- ^ Wrap document in @\'...\'@
  quote        :: Doc -> Doc
  doubleQuotes :: Doc -> Doc -- ^ Wrap document in @\"...\"@
  quotes p       = char '`' <> p <> char '\''
  squotes p      = char '\'' <> p <> char '\''
  quote p        = char '\'' <> p
  doubleQuotes p = char '"' <> p <> char '"'
  parens p       = char '(' <> p <> char ')'
  brackets p     = char '[' <> p <> char ']'
  braces p       = char '{' <> p <> char '}'
  
  {-
  Note [Print Hexadecimal Literals]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Relevant discussions:
   * Phabricator: https://phabricator.haskell.org/D4465
   * GHC Trac: https://gitlab.haskell.org/ghc/ghc/issues/14872
  
  There is a flag `-dhex-word-literals` that causes literals of
  type `Word#` or `Word64#` to be displayed in hexadecimal instead
  of decimal when dumping GHC core. It also affects the presentation
  of these in GHC's error messages. Additionally, the hexadecimal
  encoding of these numbers is zero-padded so that its length is
  a power of two. As an example of what this does,
  consider the following haskell file `Literals.hs`:
  
      module Literals where
  
      alpha :: Int
      alpha = 100 + 200
  
      beta :: Word -> Word
      beta x = x + div maxBound 255 + div 0xFFFFFFFF 255 + 0x0202
  
  We get the following dumped core when we compile on a 64-bit
  machine with ghc -O2 -fforce-recomp -ddump-simpl -dsuppress-all
  -dhex-word-literals literals.hs:
  
      ==================== Tidy Core ====================
  
      ... omitted for brevity ...
  
      -- RHS size: {terms: 2, types: 0, coercions: 0, joins: 0/0}
      alpha
      alpha = I# 300#
  
      -- RHS size: {terms: 12, types: 3, coercions: 0, joins: 0/0}
      beta
      beta
        = \ x_aYE ->
            case x_aYE of { W# x#_a1v0 ->
            W#
              (plusWord#
                 (plusWord# (plusWord# x#_a1v0 0x0101010101010101##) 0x01010101##)
                 0x0202##)
            }
  
  Notice that the word literals are in hexadecimals and that they have
  been padded with zeroes so that their lengths are 16, 8, and 4, respectively.
  
  -}
  
  -- | Apply 'parens' to 'Doc' if boolean is true.
  maybeParens :: Bool -> Doc -> Doc
  maybeParens False = id
  maybeParens True = parens
  
  -- ---------------------------------------------------------------------------
  -- Structural operations on GDocs
  
  -- | Perform some simplification of a built up @GDoc@.
  reduceDoc :: Doc -> RDoc
  reduceDoc (Beside p g q) = p `seq` g `seq` (beside p g $! reduceDoc q)
  reduceDoc (Above  p g q) = p `seq` g `seq` (above  p g $! reduceDoc q)
  reduceDoc p              = p
  
  -- | List version of '<>'.
  hcat :: [Doc] -> Doc
  hcat = reduceAB . foldr (beside_' False) empty
  
  -- | List version of '<+>'.
  hsep :: [Doc] -> Doc
  hsep = reduceAB . foldr (beside_' True)  empty
  
  -- | List version of '$$'.
  vcat :: [Doc] -> Doc
  vcat = reduceAB . foldr (above_' False) empty
  
  -- | Nest (or indent) a document by a given number of positions
  -- (which may also be negative).  'nest' satisfies the laws:
  --
  -- * @'nest' 0 x = x@
  --
  -- * @'nest' k ('nest' k' x) = 'nest' (k+k') x@
  --
  -- * @'nest' k (x '<>' y) = 'nest' k z '<>' 'nest' k y@
  --
  -- * @'nest' k (x '$$' y) = 'nest' k x '$$' 'nest' k y@
  --
  -- * @'nest' k 'empty' = 'empty'@
  --
  -- * @x '<>' 'nest' k y = x '<>' y@, if @x@ non-empty
  --
  -- The side condition on the last law is needed because
  -- 'empty' is a left identity for '<>'.
  nest :: Int -> Doc -> Doc
  nest k p = mkNest k (reduceDoc p)
  
  -- | @hang d1 n d2 = sep [d1, nest n d2]@
  hang :: Doc -> Int -> Doc -> Doc
  hang d1 n d2 = sep [d1, nest n d2]
  
  -- | Apply 'hang' to the arguments if the first 'Doc' is not empty.
  hangNotEmpty :: Doc -> Int -> Doc -> Doc
  hangNotEmpty d1 n d2 = if isEmpty d1
                         then d2
                         else hang d1 n d2
  
  -- | @punctuate p [d1, ... dn] = [d1 \<> p, d2 \<> p, ... dn-1 \<> p, dn]@
  punctuate :: Doc -> [Doc] -> [Doc]
  punctuate _ []     = []
  punctuate p (x:xs) = go x xs
                     where go y []     = [y]
                           go y (z:zs) = (y <> p) : go z zs
  
  -- mkNest checks for Nest's invariant that it doesn't have an Empty inside it
  mkNest :: Int -> Doc -> Doc
  mkNest k _ | k `seq` False = undefined
  mkNest k (Nest k1 p)       = mkNest (k + k1) p
  mkNest _ NoDoc             = NoDoc
  mkNest _ Empty             = Empty
  mkNest 0 p                 = p
  mkNest k p                 = nest_ k p
  
  -- mkUnion checks for an empty document
  mkUnion :: Doc -> Doc -> Doc
  mkUnion Empty _ = Empty
  mkUnion p q     = p `union_` q
  
  beside_' :: Bool -> Doc -> Doc -> Doc
  beside_' _ p Empty = p
  beside_' g p q     = Beside p g q
  
  above_' :: Bool -> Doc -> Doc -> Doc
  above_' _ p Empty = p
  above_' g p q     = Above p g q
  
  reduceAB :: Doc -> Doc
  reduceAB (Above  Empty _ q) = q
  reduceAB (Beside Empty _ q) = q
  reduceAB doc                = doc
  
  nilAbove_ :: RDoc -> RDoc
  nilAbove_ = NilAbove
  
  -- Arg of a TextBeside is always an RDoc
  textBeside_ :: TextDetails -> Int -> RDoc -> RDoc
  textBeside_ = TextBeside
  
  nest_ :: Int -> RDoc -> RDoc
  nest_ = Nest
  
  union_ :: RDoc -> RDoc -> RDoc
  union_ = Union
  
  
  -- ---------------------------------------------------------------------------
  -- Vertical composition @$$@
  
  -- | Above, except that if the last line of the first argument stops
  -- at least one position before the first line of the second begins,
  -- these two lines are overlapped.  For example:
  --
  -- >    text "hi" $$ nest 5 (text "there")
  --
  -- lays out as
  --
  -- >    hi   there
  --
  -- rather than
  --
  -- >    hi
  -- >         there
  --
  -- '$$' is associative, with identity 'empty', and also satisfies
  --
  -- * @(x '$$' y) '<>' z = x '$$' (y '<>' z)@, if @y@ non-empty.
  --
  ($$) :: Doc -> Doc -> Doc
  p $$  q = above_ p False q
  
  -- | Above, with no overlapping.
  -- '$+$' is associative, with identity 'empty'.
  ($+$) :: Doc -> Doc -> Doc
  p $+$ q = above_ p True q
  
  above_ :: Doc -> Bool -> Doc -> Doc
  above_ p _ Empty = p
  above_ Empty _ q = q
  above_ p g q     = Above p g q
  
  above :: Doc -> Bool -> RDoc -> RDoc
  above (Above p g1 q1)  g2 q2 = above p g1 (above q1 g2 q2)
  above p@(Beside{})     g  q  = aboveNest (reduceDoc p) g 0 (reduceDoc q)
  above p g q                  = aboveNest p             g 0 (reduceDoc q)
  
  -- Specification: aboveNest p g k q = p $g$ (nest k q)
  aboveNest :: RDoc -> Bool -> Int -> RDoc -> RDoc
  aboveNest _                   _ k _ | k `seq` False = undefined
  aboveNest NoDoc               _ _ _ = NoDoc
  aboveNest (p1 `Union` p2)     g k q = aboveNest p1 g k q `union_`
                                        aboveNest p2 g k q
  
  aboveNest Empty               _ k q = mkNest k q
  aboveNest (Nest k1 p)         g k q = nest_ k1 (aboveNest p g (k - k1) q)
                                    -- p can't be Empty, so no need for mkNest
  
  aboveNest (NilAbove p)        g k q = nilAbove_ (aboveNest p g k q)
  aboveNest (TextBeside s sl p) g k q = textBeside_ s sl rest
                                      where
                                        !k1  = k - sl
                                        rest = case p of
                                                  Empty -> nilAboveNest g k1 q
                                                  _     -> aboveNest  p g k1 q
  aboveNest (Above {})          _ _ _ = error "aboveNest Above"
  aboveNest (Beside {})         _ _ _ = error "aboveNest Beside"
  
  -- Specification: text s <> nilaboveNest g k q
  --              = text s <> (text "" $g$ nest k q)
  nilAboveNest :: Bool -> Int -> RDoc -> RDoc
  nilAboveNest _ k _           | k `seq` False = undefined
  nilAboveNest _ _ Empty       = Empty
                                 -- Here's why the "text s <>" is in the spec!
  nilAboveNest g k (Nest k1 q) = nilAboveNest g (k + k1) q
  nilAboveNest g k q           | not g && k > 0      -- No newline if no overlap
                               = textBeside_ (RStr k ' ') k q
                               | otherwise           -- Put them really above
                               = nilAbove_ (mkNest k q)
  
  
  -- ---------------------------------------------------------------------------
  -- Horizontal composition @<>@
  
  -- We intentionally avoid Data.Monoid.(<>) here due to interactions of
  -- Data.Monoid.(<>) and (<+>).  See
  -- http://www.haskell.org/pipermail/libraries/2011-November/017066.html
  
  -- | Beside.
  -- '<>' is associative, with identity 'empty'.
  (<>) :: Doc -> Doc -> Doc
  p <>  q = beside_ p False q
  
  -- | Beside, separated by space, unless one of the arguments is 'empty'.
  -- '<+>' is associative, with identity 'empty'.
  (<+>) :: Doc -> Doc -> Doc
  p <+> q = beside_ p True  q
  
  beside_ :: Doc -> Bool -> Doc -> Doc
  beside_ p _ Empty = p
  beside_ Empty _ q = q
  beside_ p g q     = Beside p g q
  
  -- Specification: beside g p q = p <g> q
  beside :: Doc -> Bool -> RDoc -> RDoc
  beside NoDoc               _ _   = NoDoc
  beside (p1 `Union` p2)     g q   = beside p1 g q `union_` beside p2 g q
  beside Empty               _ q   = q
  beside (Nest k p)          g q   = nest_ k $! beside p g q
  beside p@(Beside p1 g1 q1) g2 q2
           | g1 == g2              = beside p1 g1 $! beside q1 g2 q2
           | otherwise             = beside (reduceDoc p) g2 q2
  beside p@(Above{})         g q   = let !d = reduceDoc p in beside d g q
  beside (NilAbove p)        g q   = nilAbove_ $! beside p g q
  beside (TextBeside s sl p) g q   = textBeside_ s sl rest
                                 where
                                    rest = case p of
                                             Empty -> nilBeside g q
                                             _     -> beside p g q
  
  -- Specification: text "" <> nilBeside g p
  --              = text "" <g> p
  nilBeside :: Bool -> RDoc -> RDoc
  nilBeside _ Empty         = Empty -- Hence the text "" in the spec
  nilBeside g (Nest _ p)    = nilBeside g p
  nilBeside g p | g         = textBeside_ spaceText 1 p
                | otherwise = p
  
  
  -- ---------------------------------------------------------------------------
  -- Separate, @sep@
  
  -- Specification: sep ps  = oneLiner (hsep ps)
  --                         `union`
  --                          vcat ps
  
  -- | Either 'hsep' or 'vcat'.
  sep  :: [Doc] -> Doc
  sep = sepX True   -- Separate with spaces
  
  -- | Either 'hcat' or 'vcat'.
  cat :: [Doc] -> Doc
  cat = sepX False  -- Don't
  
  sepX :: Bool -> [Doc] -> Doc
  sepX _ []     = empty
  sepX x (p:ps) = sep1 x (reduceDoc p) 0 ps
  
  
  -- Specification: sep1 g k ys = sep (x : map (nest k) ys)
  --                            = oneLiner (x <g> nest k (hsep ys))
  --                              `union` x $$ nest k (vcat ys)
  sep1 :: Bool -> RDoc -> Int -> [Doc] -> RDoc
  sep1 _ _                   k _  | k `seq` False = undefined
  sep1 _ NoDoc               _ _  = NoDoc
  sep1 g (p `Union` q)       k ys = sep1 g p k ys `union_`
                                    aboveNest q False k (reduceDoc (vcat ys))
  
  sep1 g Empty               k ys = mkNest k (sepX g ys)
  sep1 g (Nest n p)          k ys = nest_ n (sep1 g p (k - n) ys)
  
  sep1 _ (NilAbove p)        k ys = nilAbove_
                                    (aboveNest p False k (reduceDoc (vcat ys)))
  sep1 g (TextBeside s sl p) k ys = textBeside_ s sl (sepNB g p (k - sl) ys)
  sep1 _ (Above {})          _ _  = error "sep1 Above"
  sep1 _ (Beside {})         _ _  = error "sep1 Beside"
  
  -- Specification: sepNB p k ys = sep1 (text "" <> p) k ys
  -- Called when we have already found some text in the first item
  -- We have to eat up nests
  sepNB :: Bool -> Doc -> Int -> [Doc] -> Doc
  sepNB g (Nest _ p) k ys
    = sepNB g p k ys -- Never triggered, because of invariant (2)
  sepNB g Empty k ys
    = oneLiner (nilBeside g (reduceDoc rest)) `mkUnion`
      -- XXX: TODO: PRETTY: Used to use True here (but GHC used False...)
      nilAboveNest False k (reduceDoc (vcat ys))
    where
      rest | g         = hsep ys
           | otherwise = hcat ys
  sepNB g p k ys
    = sep1 g p k ys
  
  
  -- ---------------------------------------------------------------------------
  -- @fill@
  
  -- | \"Paragraph fill\" version of 'cat'.
  fcat :: [Doc] -> Doc
  fcat = fill False
  
  -- | \"Paragraph fill\" version of 'sep'.
  fsep :: [Doc] -> Doc
  fsep = fill True
  
  -- Specification:
  --
  -- fill g docs = fillIndent 0 docs
  --
  -- fillIndent k [] = []
  -- fillIndent k [p] = p
  -- fillIndent k (p1:p2:ps) =
  --    oneLiner p1 <g> fillIndent (k + length p1 + g ? 1 : 0)
  --                               (remove_nests (oneLiner p2) : ps)
  --     `Union`
  --    (p1 $*$ nest (-k) (fillIndent 0 ps))
  --
  -- $*$ is defined for layouts (not Docs) as
  -- layout1 $*$ layout2 | hasMoreThanOneLine layout1 = layout1 $$ layout2
  --                     | otherwise                  = layout1 $+$ layout2
  
  fill :: Bool -> [Doc] -> RDoc
  fill _ []     = empty
  fill g (p:ps) = fill1 g (reduceDoc p) 0 ps
  
  fill1 :: Bool -> RDoc -> Int -> [Doc] -> Doc
  fill1 _ _                   k _  | k `seq` False = undefined
  fill1 _ NoDoc               _ _  = NoDoc
  fill1 g (p `Union` q)       k ys = fill1 g p k ys `union_`
                                     aboveNest q False k (fill g ys)
  fill1 g Empty               k ys = mkNest k (fill g ys)
  fill1 g (Nest n p)          k ys = nest_ n (fill1 g p (k - n) ys)
  fill1 g (NilAbove p)        k ys = nilAbove_ (aboveNest p False k (fill g ys))
  fill1 g (TextBeside s sl p) k ys = textBeside_ s sl (fillNB g p (k - sl) ys)
  fill1 _ (Above {})          _ _  = error "fill1 Above"
  fill1 _ (Beside {})         _ _  = error "fill1 Beside"
  
  fillNB :: Bool -> Doc -> Int -> [Doc] -> Doc
  fillNB _ _           k _  | k `seq` False = undefined
  fillNB g (Nest _ p)  k ys   = fillNB g p k ys
                                -- Never triggered, because of invariant (2)
  fillNB _ Empty _ []         = Empty
  fillNB g Empty k (Empty:ys) = fillNB g Empty k ys
  fillNB g Empty k (y:ys)     = fillNBE g k y ys
  fillNB g p k ys             = fill1 g p k ys
  
  
  fillNBE :: Bool -> Int -> Doc -> [Doc] -> Doc
  fillNBE g k y ys
    = nilBeside g (fill1 g ((elideNest . oneLiner . reduceDoc) y) k' ys)
      -- XXX: TODO: PRETTY: Used to use True here (but GHC used False...)
      `mkUnion` nilAboveNest False k (fill g (y:ys))
    where k' = if g then k - 1 else k
  
  elideNest :: Doc -> Doc
  elideNest (Nest _ d) = d
  elideNest d          = d
  
  -- ---------------------------------------------------------------------------
  -- Selecting the best layout
  
  best :: Int   -- Line length
       -> Int   -- Ribbon length
       -> RDoc
       -> RDoc  -- No unions in here!
  best w0 r = get w0
    where
      get :: Int          -- (Remaining) width of line
          -> Doc -> Doc
      get w _ | w == 0 && False = undefined
      get _ Empty               = Empty
      get _ NoDoc               = NoDoc
      get w (NilAbove p)        = nilAbove_ (get w p)
      get w (TextBeside s sl p) = textBeside_ s sl (get1 w sl p)
      get w (Nest k p)          = nest_ k (get (w - k) p)
      get w (p `Union` q)       = nicest w r (get w p) (get w q)
      get _ (Above {})          = error "best get Above"
      get _ (Beside {})         = error "best get Beside"
  
      get1 :: Int         -- (Remaining) width of line
           -> Int         -- Amount of first line already eaten up
           -> Doc         -- This is an argument to TextBeside => eat Nests
           -> Doc         -- No unions in here!
  
      get1 w _ _ | w == 0 && False  = undefined
      get1 _ _  Empty               = Empty
      get1 _ _  NoDoc               = NoDoc
      get1 w sl (NilAbove p)        = nilAbove_ (get (w - sl) p)
      get1 w sl (TextBeside t tl p) = textBeside_ t tl (get1 w (sl + tl) p)
      get1 w sl (Nest _ p)          = get1 w sl p
      get1 w sl (p `Union` q)       = nicest1 w r sl (get1 w sl p)
                                                     (get1 w sl q)
      get1 _ _  (Above {})          = error "best get1 Above"
      get1 _ _  (Beside {})         = error "best get1 Beside"
  
  nicest :: Int -> Int -> Doc -> Doc -> Doc
  nicest !w !r = nicest1 w r 0
  
  nicest1 :: Int -> Int -> Int -> Doc -> Doc -> Doc
  nicest1 !w !r !sl p q | fits ((w `min` r) - sl) p = p
                        | otherwise                 = q
  
  fits :: Int  -- Space available
       -> Doc
       -> Bool -- True if *first line* of Doc fits in space available
  fits n _ | n < 0           = False
  fits _ NoDoc               = False
  fits _ Empty               = True
  fits _ (NilAbove _)        = True
  fits n (TextBeside _ sl p) = fits (n - sl) p
  fits _ (Above {})          = error "fits Above"
  fits _ (Beside {})         = error "fits Beside"
  fits _ (Union {})          = error "fits Union"
  fits _ (Nest {})           = error "fits Nest"
  
  -- | @first@ returns its first argument if it is non-empty, otherwise its second.
  first :: Doc -> Doc -> Doc
  first p q | nonEmptySet p = p -- unused, because (get OneLineMode) is unused
            | otherwise     = q
  
  nonEmptySet :: Doc -> Bool
  nonEmptySet NoDoc              = False
  nonEmptySet (_ `Union` _)      = True
  nonEmptySet Empty              = True
  nonEmptySet (NilAbove _)       = True
  nonEmptySet (TextBeside _ _ p) = nonEmptySet p
  nonEmptySet (Nest _ p)         = nonEmptySet p
  nonEmptySet (Above {})         = error "nonEmptySet Above"
  nonEmptySet (Beside {})        = error "nonEmptySet Beside"
  
  -- @oneLiner@ returns the one-line members of the given set of @GDoc@s.
  oneLiner :: Doc -> Doc
  oneLiner NoDoc               = NoDoc
  oneLiner Empty               = Empty
  oneLiner (NilAbove _)        = NoDoc
  oneLiner (TextBeside s sl p) = textBeside_ s sl (oneLiner p)
  oneLiner (Nest k p)          = nest_ k (oneLiner p)
  oneLiner (p `Union` _)       = oneLiner p
  oneLiner (Above {})          = error "oneLiner Above"
  oneLiner (Beside {})         = error "oneLiner Beside"
  
  
  -- ---------------------------------------------------------------------------
  -- Rendering
  
  -- | A rendering style.
  data Style
    = Style { mode           :: Mode  -- ^ The rendering mode
            , lineLength     :: Int   -- ^ Length of line, in chars
            , ribbonsPerLine :: Float -- ^ Ratio of line length to ribbon length
            }
  
  -- | The default style (@mode=PageMode False, lineLength=100, ribbonsPerLine=1.5@).
  style :: Style
  style = Style { lineLength = 100, ribbonsPerLine = 1.5, mode = PageMode False }
  
  -- | Rendering mode.
  data Mode = PageMode { asciiSpace :: Bool }    -- ^ Normal
            | ZigZagMode   -- ^ With zig-zag cuts
            | LeftMode     -- ^ No indentation, infinitely long lines
            | OneLineMode  -- ^ All on one line
  
  -- | Can we output an ascii space character for spaces?
  --   Mostly true, but not for e.g. UTF16
  --   See Note [putSpaces optimizations] for why we bother
  --   to track this.
  hasAsciiSpace :: Mode -> Bool
  hasAsciiSpace mode =
    case mode of
      PageMode asciiSpace -> asciiSpace
      _ -> False
  
  -- | Render the @Doc@ to a String using the given @Style@.
  renderStyle :: Style -> Doc -> String
  renderStyle s = fullRender (mode s) (lineLength s) (ribbonsPerLine s)
                  txtPrinter ""
  
  -- | Default TextDetails printer
  txtPrinter :: TextDetails -> String -> String
  txtPrinter (Chr c)    s  = c:s
  txtPrinter (Str s1)   s2 = s1 ++ s2
  txtPrinter (PStr s1)  s2 = unpackFS s1 ++ s2
  txtPrinter (ZStr s1)  s2 = zString s1 ++ s2
  txtPrinter (LStr s1)  s2 = unpackPtrString s1 ++ s2
  txtPrinter (RStr n c) s2 = replicate n c ++ s2
  
  -- | The general rendering interface.
  fullRender :: Mode                     -- ^ Rendering mode
             -> Int                      -- ^ Line length
             -> Float                    -- ^ Ribbons per line
             -> (TextDetails -> a -> a)  -- ^ What to do with text
             -> a                        -- ^ What to do at the end
             -> Doc                      -- ^ The document
             -> a                        -- ^ Result
  fullRender OneLineMode _ _ txt end doc
    = easyDisplay spaceText (\_ y -> y) txt end (reduceDoc doc)
  fullRender LeftMode    _ _ txt end doc
    = easyDisplay nlText first txt end (reduceDoc doc)
  
  fullRender m lineLen ribbons txt rest doc
    = display m lineLen ribbonLen txt rest doc'
    where
      doc' = best bestLineLen ribbonLen (reduceDoc doc)
  
      bestLineLen, ribbonLen :: Int
      ribbonLen   = round (fromIntegral lineLen / ribbons)
      bestLineLen = case m of
                        ZigZagMode -> maxBound
                        _          -> lineLen
  
  easyDisplay :: TextDetails
               -> (Doc -> Doc -> Doc)
               -> (TextDetails -> a -> a)
               -> a
               -> Doc
               -> a
  easyDisplay nlSpaceText choose txt end
    = lay
    where
      lay NoDoc              = error "easyDisplay: NoDoc"
      lay (Union p q)        = lay (choose p q)
      lay (Nest _ p)         = lay p
      lay Empty              = end
      lay (NilAbove p)       = nlSpaceText `txt` lay p
      lay (TextBeside s _ p) = s `txt` lay p
      lay (Above {})         = error "easyDisplay Above"
      lay (Beside {})        = error "easyDisplay Beside"
  
  display :: Mode -> Int -> Int -> (TextDetails -> a -> a) -> a -> Doc -> a
  display m !page_width !ribbon_width txt end doc
    = case page_width - ribbon_width of { gap_width ->
      case gap_width `quot` 2 of { shift ->
      let
          lay k _            | k `seq` False = undefined
          lay k (Nest k1 p)  = lay (k + k1) p
          lay _ Empty        = end
          lay k (NilAbove p) = nlText `txt` lay k p
          lay k (TextBeside s sl p)
              = case m of
                      ZigZagMode |  k >= gap_width
                                 -> nlText `txt` (
                                    Str (replicate shift '/') `txt` (
                                    nlText `txt`
                                    lay1 (k - shift) s sl p ))
  
                                 |  k < 0
                                 -> nlText `txt` (
                                    Str (replicate shift '\\') `txt` (
                                    nlText `txt`
                                    lay1 (k + shift) s sl p ))
  
                      _ -> lay1 k s sl p
          lay _ (Above {})   = error "display lay Above"
          lay _ (Beside {})  = error "display lay Beside"
          lay _ NoDoc        = error "display lay NoDoc"
          lay _ (Union {})   = error "display lay Union"
  
          lay1 !k s !sl p    = let !r = k + sl
                               in indent k (s `txt` lay2 r p)
  
          lay2 k _ | k `seq` False   = undefined
          lay2 k (NilAbove p)        = nlText `txt` lay k p
          lay2 k (TextBeside s sl p) = s `txt` lay2 (k + sl) p
          lay2 k (Nest _ p)          = lay2 k p
          lay2 _ Empty               = end
          lay2 _ (Above {})          = error "display lay2 Above"
          lay2 _ (Beside {})         = error "display lay2 Beside"
          lay2 _ NoDoc               = error "display lay2 NoDoc"
          lay2 _ (Union {})          = error "display lay2 Union"
  
          indent !n r                = RStr n ' ' `txt` r
      in
      lay 0 doc
      }}
  
  printDoc :: Mode -> Int -> Handle -> Doc -> IO ()
  -- printDoc adds a newline to the end
  printDoc mode cols hdl doc = printDoc_ mode cols hdl (doc $$ text "")
  
  {- Note [putSpaces optimizations]
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
  When using dump flags a lot of what we are dumping ends up being whitespace.
  This is especially true for Core/Stg dumps. Enough so that it's worth optimizing.
  
  Especially in the common case of writing to an UTF8 or similarly encoded file
  where space is equal to ascii space we use hPutBuf to write a preallocated
  buffer to the file. This avoids a fair bit of allocation.
  
  For other cases we fall back to the old and slow path for simplicity.
  
  -}
  
  printDoc_ :: Mode -> Int -> Handle -> Doc -> IO ()
  -- printDoc_ does not add a newline at the end, so that
  -- successive calls can output stuff on the same line
  -- Rather like putStr vs putStrLn
  printDoc_ LeftMode _ hdl doc
    = do { printLeftRender hdl doc; hFlush hdl }
  printDoc_ mode pprCols hdl doc
    = do { fullRender mode pprCols 1.5 put done doc ;
           hFlush hdl }
    where
      put (Chr c)    next = hPutChar hdl c >> next
      put (Str s)    next = hPutStr  hdl s >> next
      put (PStr s)   next = hPutStr  hdl (unpackFS s) >> next
                            -- NB. not hPutFS, we want this to go through
                            -- the I/O library's encoding layer. (#3398)
      put (ZStr s)   next = hPutFZS  hdl s >> next
      put (LStr s)   next = hPutPtrString hdl s >> next
      put (RStr n c) next
        | c == ' '
        = putSpaces n >> next
        | otherwise
        = hPutStr hdl (replicate n c) >> next
      putSpaces n
        -- If we use ascii spaces we are allowed to use hPutBuf
        -- See Note [putSpaces optimizations]
        | hasAsciiSpace mode
        , n <= 100
        = hPutBuf hdl (Ptr spaces') n
        | hasAsciiSpace mode
        , n > 100
        = hPutBuf hdl (Ptr spaces') 100 >> putSpaces (n-100)
  
        | otherwise = hPutStr hdl (replicate n ' ')
  
      done = return () -- hPutChar hdl '\n'
      -- 100 spaces, so we avoid the allocation of replicate n ' '
      spaces' = "                                                                                                    "#
  
  
    -- some versions of hPutBuf will barf if the length is zero
  hPutPtrString :: Handle -> PtrString -> IO ()
  hPutPtrString _handle (PtrString _ 0) = return ()
  hPutPtrString handle  (PtrString a l) = hPutBuf handle a l
  
  -- Printing output in LeftMode is performance critical: it's used when
  -- dumping C and assembly output, so we allow ourselves a few dirty
  -- hacks:
  --
  -- (1) we specialise fullRender for LeftMode with IO output.
  --
  -- (2) we add a layer of buffering on top of Handles.  Handles
  --     don't perform well with lots of hPutChars, which is mostly
  --     what we're doing here, because Handles have to be thread-safe
  --     and async exception-safe.  We only have a single thread and don't
  --     care about exceptions, so we add a layer of fast buffering
  --     over the Handle interface.
  
  printLeftRender :: Handle -> Doc -> IO ()
  printLeftRender hdl doc = do
    b <- newBufHandle hdl
    bufLeftRender b doc
    bFlush b
  
  bufLeftRender :: BufHandle -> Doc -> IO ()
  bufLeftRender b doc = layLeft b (reduceDoc doc)
  
  layLeft :: BufHandle -> Doc -> IO ()
  layLeft !_ NoDoc             = error "layLeft: NoDoc"
  layLeft b (Union p q)        = layLeft b $! first p q
  layLeft b (Nest _ p)         = layLeft b $! p
  layLeft b Empty              = bPutChar b '\n'
  layLeft b (NilAbove p)       = bPutChar b '\n' >> layLeft b p
  layLeft b (TextBeside s _ p) = put b s >> layLeft b p
   where
      put !b (Chr c)   = bPutChar b c
      put b (Str s)    = bPutStr  b s
      put b (PStr s)   = bPutFS   b s
      put b (ZStr s)   = bPutFZS  b s
      put b (LStr s)   = bPutPtrString b s
      put b (RStr n c) = bPutReplicate b n c
  layLeft _ _                  = panic "layLeft: Unhandled case"
  
  -- Define error=panic, for easier comparison with libraries/pretty.
  error :: String -> a
  error = panic
  