content: -----------------------------------------------------------------------------
  -- |
  -- Module      :  Distribution.ParseUtils
  -- Copyright   :  (c) The University of Glasgow 2004
  -- License     :  BSD3
  --
  -- Maintainer  :  cabal-devel@haskell.org
  -- Portability :  portable
  --
  -- Utilities for parsing 'PackageDescription' and 'InstalledPackageInfo'.
  --
  -- The @.cabal@ file format is not trivial, especially with the introduction
  -- of configurations and the section syntax that goes with that. This module
  -- has a bunch of parsing functions that is used by the @.cabal@ parser and a
  -- couple others. It has the parsing framework code and also little parsers for
  -- many of the formats we get in various @.cabal@ file fields, like module
  -- names, comma separated lists etc.
  
  -- This module is meant to be local-only to Distribution...
  
  {-# OPTIONS_HADDOCK hide #-}
  {-# LANGUAGE Rank2Types #-}
  module Distribution.ParseUtils (
          LineNo, PError(..), PWarning(..), locatedErrorMsg, syntaxError, warning,
          runP, runE, ParseResult(..), catchParseError, parseFail, showPWarning,
          Field(..), fName, lineNo,
          FieldDescr(..), ppField, ppFields, readFields, readFieldsFlat,
          showFields, showSingleNamedField, showSimpleSingleNamedField,
          parseFields, parseFieldsFlat,
          parseFilePathQ, parseTokenQ, parseTokenQ',
          parseModuleNameQ,
          parseOptVersion, parsePackageName,
          parseTestedWithQ, parseLicenseQ, parseLanguageQ, parseExtensionQ,
          parseSepList, parseCommaList, parseOptCommaList,
          showFilePath, showToken, showTestedWith, showFreeText, parseFreeText,
          field, simpleField, listField, listFieldWithSep, spaceListField,
          commaListField, commaListFieldWithSep, commaNewLineListField,
          optsField, liftField, boolField, parseQuoted, parseMaybeQuoted, indentWith,
          readPToMaybe,
  
          UnrecFieldParser, warnUnrec, ignoreUnrec,
    ) where
  
  import Prelude ()
  import Distribution.Compat.Prelude hiding (get)
  
  import Distribution.Compiler
  import Distribution.License
  import Distribution.Version
  import Distribution.ModuleName
  import qualified Distribution.Compat.MonadFail as Fail
  import Distribution.Compat.ReadP as ReadP hiding (get)
  import Distribution.ReadE
  import Distribution.Text
  import Distribution.Utils.Generic
  import Distribution.PrettyUtils
  import Language.Haskell.Extension
  
  import Text.PrettyPrint
      ( Doc, render, style, renderStyle
      , text, colon, nest, punctuate, comma, sep
      , fsep, hsep, isEmpty, vcat, mode, Mode (..)
      , ($+$), (<+>)
      )
  import Data.Tree as Tree (Tree(..), flatten)
  import qualified Data.Map as Map
  import System.FilePath (normalise)
  
  -- -----------------------------------------------------------------------------
  
  type LineNo    = Int
  
  data PError = AmbiguousParse String LineNo
              | NoParse String LineNo
              | TabsError LineNo
              | FromString String (Maybe LineNo)
          deriving (Eq, Show)
  
  data PWarning = PWarning String
                | UTFWarning LineNo String
          deriving (Eq, Show)
  
  showPWarning :: FilePath -> PWarning -> String
  showPWarning fpath (PWarning msg) =
    normalise fpath ++ ": " ++ msg
  showPWarning fpath (UTFWarning line fname) =
    normalise fpath ++ ":" ++ show line
          ++ ": Invalid UTF-8 text in the '" ++ fname ++ "' field."
  
  data ParseResult a = ParseFailed PError | ParseOk [PWarning] a
          deriving Show
  
  instance Functor ParseResult where
          fmap _ (ParseFailed err) = ParseFailed err
          fmap f (ParseOk ws x) = ParseOk ws $ f x
  
  instance Applicative ParseResult where
          pure = ParseOk []
          (<*>) = ap
  
  
  instance Monad ParseResult where
          return = pure
          ParseFailed err >>= _ = ParseFailed err
          ParseOk ws x >>= f = case f x of
                                 ParseFailed err -> ParseFailed err
                                 ParseOk ws' x' -> ParseOk (ws'++ws) x'
          fail = Fail.fail
  
  instance Fail.MonadFail ParseResult where
          fail s = ParseFailed (FromString s Nothing)
  
  catchParseError :: ParseResult a -> (PError -> ParseResult a)
                  -> ParseResult a
  p@(ParseOk _ _) `catchParseError` _ = p
  ParseFailed e `catchParseError` k   = k e
  
  parseFail :: PError -> ParseResult a
  parseFail = ParseFailed
  
  runP :: LineNo -> String -> ReadP a a -> String -> ParseResult a
  runP line fieldname p s =
    case [ x | (x,"") <- results ] of
      [a] -> ParseOk (utf8Warnings line fieldname s) a
      --TODO: what is this double parse thing all about?
      --      Can't we just do the all isSpace test the first time?
      []  -> case [ x | (x,ys) <- results, all isSpace ys ] of
               [a] -> ParseOk (utf8Warnings line fieldname s) a
               []  -> ParseFailed (NoParse fieldname line)
               _   -> ParseFailed (AmbiguousParse fieldname line)
      _   -> ParseFailed (AmbiguousParse fieldname line)
    where results = readP_to_S p s
  
  runE :: LineNo -> String -> ReadE a -> String -> ParseResult a
  runE line fieldname p s =
      case runReadE p s of
        Right a -> ParseOk (utf8Warnings line fieldname s) a
        Left  e -> syntaxError line $
          "Parse of field '" ++ fieldname ++ "' failed (" ++ e ++ "): " ++ s
  
  utf8Warnings :: LineNo -> String -> String -> [PWarning]
  utf8Warnings line fieldname s =
    take 1 [ UTFWarning n fieldname
           | (n,l) <- zip [line..] (lines s)
           , '\xfffd' `elem` l ]
  
  locatedErrorMsg :: PError -> (Maybe LineNo, String)
  locatedErrorMsg (AmbiguousParse f n) = (Just n,
                                          "Ambiguous parse in field '"++f++"'.")
  locatedErrorMsg (NoParse f n)        = (Just n,
                                          "Parse of field '"++f++"' failed.")
  locatedErrorMsg (TabsError n)        = (Just n, "Tab used as indentation.")
  locatedErrorMsg (FromString s n)     = (n, s)
  
  syntaxError :: LineNo -> String -> ParseResult a
  syntaxError n s = ParseFailed $ FromString s (Just n)
  
  tabsError :: LineNo -> ParseResult a
  tabsError ln = ParseFailed $ TabsError ln
  
  warning :: String -> ParseResult ()
  warning s = ParseOk [PWarning s] ()
  
  -- | Field descriptor.  The parameter @a@ parameterizes over where the field's
  --   value is stored in.
  data FieldDescr a
    = FieldDescr
        { fieldName     :: String
        , fieldGet      :: a -> Doc
        , fieldSet      :: LineNo -> String -> a -> ParseResult a
          -- ^ @fieldSet n str x@ Parses the field value from the given input
          -- string @str@ and stores the result in @x@ if the parse was
          -- successful.  Otherwise, reports an error on line number @n@.
        }
  
  field :: String -> (a -> Doc) -> ReadP a a -> FieldDescr a
  field name showF readF =
    FieldDescr name showF (\line val _st -> runP line name readF val)
  
  -- Lift a field descriptor storing into an 'a' to a field descriptor storing
  -- into a 'b'.
  liftField :: (b -> a) -> (a -> b -> b) -> FieldDescr a -> FieldDescr b
  liftField get set (FieldDescr name showF parseF)
   = FieldDescr name (showF . get)
          (\line str b -> do
              a <- parseF line str (get b)
              return (set a b))
  
  -- Parser combinator for simple fields.  Takes a field name, a pretty printer,
  -- a parser function, an accessor, and a setter, returns a FieldDescr over the
  -- compoid structure.
  simpleField :: String -> (a -> Doc) -> ReadP a a
              -> (b -> a) -> (a -> b -> b) -> FieldDescr b
  simpleField name showF readF get set
    = liftField get set $ field name showF readF
  
  commaListFieldWithSep :: Separator -> String -> (a -> Doc) -> ReadP [a] a
                        -> (b -> [a]) -> ([a] -> b -> b) -> FieldDescr b
  commaListFieldWithSep separator name showF readF get set =
     liftField get set' $
       field name showF' (parseCommaList readF)
     where
       set' xs b = set (get b ++ xs) b
       showF'    = separator . punctuate comma . map showF
  
  commaListField :: String -> (a -> Doc) -> ReadP [a] a
                   -> (b -> [a]) -> ([a] -> b -> b) -> FieldDescr b
  commaListField = commaListFieldWithSep fsep
  
  commaNewLineListField :: String -> (a -> Doc) -> ReadP [a] a
                   -> (b -> [a]) -> ([a] -> b -> b) -> FieldDescr b
  commaNewLineListField = commaListFieldWithSep sep
  
  spaceListField :: String -> (a -> Doc) -> ReadP [a] a
                   -> (b -> [a]) -> ([a] -> b -> b) -> FieldDescr b
  spaceListField name showF readF get set =
    liftField get set' $
      field name showF' (parseSpaceList readF)
    where
      set' xs b = set (get b ++ xs) b
      showF'    = fsep . map showF
  
  listFieldWithSep :: Separator -> String -> (a -> Doc) -> ReadP [a] a
                   -> (b -> [a]) -> ([a] -> b -> b) -> FieldDescr b
  listFieldWithSep separator name showF readF get set =
    liftField get set' $
      field name showF' (parseOptCommaList readF)
    where
      set' xs b = set (get b ++ xs) b
      showF'    = separator . map showF
  
  listField :: String -> (a -> Doc) -> ReadP [a] a
            -> (b -> [a]) -> ([a] -> b -> b) -> FieldDescr b
  listField = listFieldWithSep fsep
  
  optsField :: String -> CompilerFlavor -> (b -> [(CompilerFlavor,[String])])
               -> ([(CompilerFlavor,[String])] -> b -> b) -> FieldDescr b
  optsField name flavor get set =
     liftField (fromMaybe [] . lookup flavor . get)
               (\opts b -> set (reorder (update flavor opts (get b))) b) $
          field name showF (sepBy parseTokenQ' (munch1 isSpace))
    where
          update _ opts l | all null opts = l  --empty opts as if no opts
          update f opts [] = [(f,opts)]
          update f opts ((f',opts'):rest)
             | f == f'   = (f, opts' ++ opts) : rest
             | otherwise = (f',opts') : update f opts rest
          reorder = sortBy (comparing fst)
          showF   = hsep . map text
  
  -- TODO: this is a bit smelly hack. It's because we want to parse bool fields
  --       liberally but not accept new parses. We cannot do that with ReadP
  --       because it does not support warnings. We need a new parser framework!
  boolField :: String -> (b -> Bool) -> (Bool -> b -> b) -> FieldDescr b
  boolField name get set = liftField get set (FieldDescr name showF readF)
    where
      showF = text . show
      readF line str _
        |  str == "True"  = ParseOk [] True
        |  str == "False" = ParseOk [] False
        | lstr == "true"  = ParseOk [caseWarning] True
        | lstr == "false" = ParseOk [caseWarning] False
        | otherwise       = ParseFailed (NoParse name line)
        where
          lstr = lowercase str
          caseWarning = PWarning $
            "The '" ++ name ++ "' field is case sensitive, use 'True' or 'False'."
  
  ppFields :: [FieldDescr a] -> a -> Doc
  ppFields fields x =
     vcat [ ppField name (getter x) | FieldDescr name getter _ <- fields ]
  
  ppField :: String -> Doc -> Doc
  ppField name fielddoc
     | isEmpty fielddoc         = mempty
     | name `elem` nestedFields = text name <<>> colon $+$ nest indentWith fielddoc
     | otherwise                = text name <<>> colon <+> fielddoc
     where
        nestedFields =
           [ "description"
           , "build-depends"
           , "maven-depends"
           , "maven-repos"
           , "data-files"
           , "extra-source-files"
           , "extra-tmp-files"
           , "exposed-modules"
           , "c-sources"
           , "js-sources"
           , "java-sources"
           , "extra-libraries"
           , "includes"
           , "install-includes"
           , "other-modules"
           , "autogen-modules"
           , "depends"
           ]
  
  showFields :: [FieldDescr a] -> a -> String
  showFields fields = render . ($+$ text "") . ppFields fields
  
  showSingleNamedField :: [FieldDescr a] -> String -> Maybe (a -> String)
  showSingleNamedField fields f =
    case [ get | (FieldDescr f' get _) <- fields, f' == f ] of
      []      -> Nothing
      (get:_) -> Just (render . ppField f . get)
  
  showSimpleSingleNamedField :: [FieldDescr a] -> String -> Maybe (a -> String)
  showSimpleSingleNamedField fields f =
    case [ get | (FieldDescr f' get _) <- fields, f' == f ] of
      []      -> Nothing
      (get:_) -> Just (renderStyle myStyle . get)
   where myStyle = style { mode = LeftMode }
  
  parseFields :: [FieldDescr a] -> a -> String -> ParseResult a
  parseFields fields initial str =
    readFields str >>= accumFields fields initial
  
  parseFieldsFlat :: [FieldDescr a] -> a -> String -> ParseResult a
  parseFieldsFlat fields initial str =
    readFieldsFlat str >>= accumFields fields initial
  
  accumFields :: [FieldDescr a] -> a -> [Field] -> ParseResult a
  accumFields fields = foldM setField
    where
      fieldMap = Map.fromList
        [ (name, f) | f@(FieldDescr name _ _) <- fields ]
      setField accum (F line name value) = case Map.lookup name fieldMap of
        Just (FieldDescr _ _ set) -> set line value accum
        Nothing -> do
          warning ("Unrecognized field " ++ name ++ " on line " ++ show line)
          return accum
      setField accum f = do
        warning ("Unrecognized stanza on line " ++ show (lineNo f))
        return accum
  
  -- | The type of a function which, given a name-value pair of an
  --   unrecognized field, and the current structure being built,
  --   decides whether to incorporate the unrecognized field
  --   (by returning  Just x, where x is a possibly modified version
  --   of the structure being built), or not (by returning Nothing).
  type UnrecFieldParser a = (String,String) -> a -> Maybe a
  
  -- | A default unrecognized field parser which simply returns Nothing,
  --   i.e. ignores all unrecognized fields, so warnings will be generated.
  warnUnrec :: UnrecFieldParser a
  warnUnrec _ _ = Nothing
  
  -- | A default unrecognized field parser which silently (i.e. no
  --   warnings will be generated) ignores unrecognized fields, by
  --   returning the structure being built unmodified.
  ignoreUnrec :: UnrecFieldParser a
  ignoreUnrec _ = Just
  
  ------------------------------------------------------------------------------
  
  -- The data type for our three syntactic categories
  data Field
      = F LineNo String String
        -- ^ A regular @<property>: <value>@ field
      | Section LineNo String String [Field]
        -- ^ A section with a name and possible parameter.  The syntactic
        -- structure is:
        --
        -- @
        --   <sectionname> <arg> {
        --     <field>*
        --   }
        -- @
      | IfBlock LineNo String [Field] [Field]
        -- ^ A conditional block with an optional else branch:
        --
        -- @
        --  if <condition> {
        --    <field>*
        --  } else {
        --    <field>*
        --  }
        -- @
        deriving (Show
                 ,Eq)   -- for testing
  
  lineNo :: Field -> LineNo
  lineNo (F n _ _) = n
  lineNo (Section n _ _ _) = n
  lineNo (IfBlock n _ _ _) = n
  
  fName :: Field -> String
  fName (F _ n _) = n
  fName (Section _ n _ _) = n
  fName _ = error "fname: not a field or section"
  
  readFields :: String -> ParseResult [Field]
  readFields input = ifelse
                 =<< traverse (mkField 0)
                 =<< mkTree tokens
  
    where ls = (lines . normaliseLineEndings) input
          tokens = (concatMap tokeniseLine . trimLines) ls
  
  readFieldsFlat :: String -> ParseResult [Field]
  readFieldsFlat input = traverse (mkField 0)
                     =<< mkTree tokens
    where ls = (lines . normaliseLineEndings) input
          tokens = (concatMap tokeniseLineFlat . trimLines) ls
  
  -- attach line number and determine indentation
  trimLines :: [String] -> [(LineNo, Indent, HasTabs, String)]
  trimLines ls = [ (lineno, indent, hastabs, trimTrailing l')
                 | (lineno, l) <- zip [1..] ls
                 , let (sps, l') = span isSpace l
                       indent    = length sps
                       hastabs   = '\t' `elem` sps
                 , validLine l' ]
    where validLine ('-':'-':_) = False      -- Comment
          validLine []          = False      -- blank line
          validLine _           = True
  
  -- | We parse generically based on indent level and braces '{' '}'. To do that
  -- we split into lines and then '{' '}' tokens and other spans within a line.
  data Token =
         -- | The 'Line' token is for bits that /start/ a line, eg:
         --
         -- > "\n  blah blah { blah"
         --
         -- tokenises to:
         --
         -- > [Line n 2 False "blah blah", OpenBracket, Span n "blah"]
         --
         -- so lines are the only ones that can have nested layout, since they
         -- have a known indentation level.
         --
         -- eg: we can't have this:
         --
         -- > if ... {
         -- > } else
         -- >     other
         --
         -- because other cannot nest under else, since else doesn't start a line
         -- so cannot have nested layout. It'd have to be:
         --
         -- > if ... {
         -- > }
         -- >   else
         -- >     other
         --
         -- but that's not so common, people would normally use layout or
         -- brackets not both in a single @if else@ construct.
         --
         -- > if ... { foo : bar }
         -- > else
         -- >    other
         --
         -- this is OK
         Line LineNo Indent HasTabs String
       | Span LineNo                String  -- ^ span in a line, following brackets
       | OpenBracket LineNo | CloseBracket LineNo
  
  type Indent = Int
  type HasTabs = Bool
  
  -- | Tokenise a single line, splitting on '{' '}' and the spans in between.
  -- Also trims leading & trailing space on those spans within the line.
  tokeniseLine :: (LineNo, Indent, HasTabs, String) -> [Token]
  tokeniseLine (n0, i, t, l) = case split n0 l of
                              (Span _ l':ss) -> Line n0 i t l' :ss
                              cs              -> cs
    where split _ "" = []
          split n s  = case span (\c -> c /='}' && c /= '{') s of
            ("", '{' : s') ->             OpenBracket  n : split n s'
            (w , '{' : s') -> mkspan n w (OpenBracket  n : split n s')
            ("", '}' : s') ->             CloseBracket n : split n s'
            (w , '}' : s') -> mkspan n w (CloseBracket n : split n s')
            (w ,        _) -> mkspan n w []
  
          mkspan n s ss | null s'   =             ss
                        | otherwise = Span n s' : ss
            where s' = trimTrailing (trimLeading s)
  
  tokeniseLineFlat :: (LineNo, Indent, HasTabs, String) -> [Token]
  tokeniseLineFlat (n0, i, t, l)
    | null l'   = []
    | otherwise = [Line n0 i t l']
    where
      l' = trimTrailing (trimLeading l)
  
  trimLeading, trimTrailing :: String -> String
  trimLeading  = dropWhile isSpace
  trimTrailing = dropWhileEndLE isSpace
  
  
  type SyntaxTree = Tree (LineNo, HasTabs, String)
  
  -- | Parse the stream of tokens into a tree of them, based on indent \/ layout
  mkTree :: [Token] -> ParseResult [SyntaxTree]
  mkTree toks =
    layout 0 [] toks >>= \(trees, trailing) -> case trailing of
      []               -> return trees
      OpenBracket  n:_ -> syntaxError n "mismatched brackets, unexpected {"
      CloseBracket n:_ -> syntaxError n "mismatched brackets, unexpected }"
      -- the following two should never happen:
      Span n     l  :_ -> syntaxError n $ "unexpected span: " ++ show l
      Line n _ _ l  :_ -> syntaxError n $ "unexpected line: " ++ show l
  
  
  -- | Parse the stream of tokens into a tree of them, based on indent
  -- This parse state expect to be in a layout context, though possibly
  -- nested within a braces context so we may still encounter closing braces.
  layout :: Indent       -- ^ indent level of the parent\/previous line
         -> [SyntaxTree] -- ^ accumulating param, trees in this level
         -> [Token]      -- ^ remaining tokens
         -> ParseResult ([SyntaxTree], [Token])
                         -- ^ collected trees on this level and trailing tokens
  layout _ a []                               = return (reverse a, [])
  layout i a (s@(Line _ i' _ _):ss) | i' < i  = return (reverse a, s:ss)
  layout i a (Line n _ t l:OpenBracket n':ss) = do
      (sub, ss') <- braces n' [] ss
      layout i (Node (n,t,l) sub:a) ss'
  
  layout i a (Span n     l:OpenBracket n':ss) = do
      (sub, ss') <- braces n' [] ss
      layout i (Node (n,False,l) sub:a) ss'
  
  -- look ahead to see if following lines are more indented, giving a sub-tree
  layout i a (Line n i' t l:ss) = do
      lookahead <- layout (i'+1) [] ss
      case lookahead of
          ([], _)   -> layout i (Node (n,t,l) [] :a) ss
          (ts, ss') -> layout i (Node (n,t,l) ts :a) ss'
  
  layout _ _ (   OpenBracket  n :_)  = syntaxError n "unexpected '{'"
  layout _ a (s@(CloseBracket _):ss) = return (reverse a, s:ss)
  layout _ _ (   Span n l       : _) = syntaxError n $ "unexpected span: "
                                                    ++ show l
  
  -- | Parse the stream of tokens into a tree of them, based on explicit braces
  -- This parse state expects to find a closing bracket.
  braces :: LineNo       -- ^ line of the '{', used for error messages
         -> [SyntaxTree] -- ^ accumulating param, trees in this level
         -> [Token]      -- ^ remaining tokens
         -> ParseResult ([SyntaxTree],[Token])
                         -- ^ collected trees on this level and trailing tokens
  braces m a (Line n _ t l:OpenBracket n':ss) = do
      (sub, ss') <- braces n' [] ss
      braces m (Node (n,t,l) sub:a) ss'
  
  braces m a (Span n     l:OpenBracket n':ss) = do
      (sub, ss') <- braces n' [] ss
      braces m (Node (n,False,l) sub:a) ss'
  
  braces m a (Line n i t l:ss) = do
      lookahead <- layout (i+1) [] ss
      case lookahead of
          ([], _)   -> braces m (Node (n,t,l) [] :a) ss
          (ts, ss') -> braces m (Node (n,t,l) ts :a) ss'
  
  braces m a (Span n       l:ss) = braces m (Node (n,False,l) []:a) ss
  braces _ a (CloseBracket _:ss) = return (reverse a, ss)
  braces n _ []                  = syntaxError n $ "opening brace '{'"
                                ++ "has no matching closing brace '}'"
  braces _ _ (OpenBracket  n:_)  = syntaxError n "unexpected '{'"
  
  -- | Convert the parse tree into the Field AST
  -- Also check for dodgy uses of tabs in indentation.
  mkField :: Int -> SyntaxTree -> ParseResult Field
  mkField d (Node (n,t,_) _) | d >= 1 && t = tabsError n
  mkField d (Node (n,_,l) ts) = case span (\c -> isAlphaNum c || c == '-') l of
    ([], _)       -> syntaxError n $ "unrecognised field or section: " ++ show l
    (name, rest)  -> case trimLeading rest of
      (':':rest') -> do let followingLines = concatMap Tree.flatten ts
                            tabs = not (null [()| (_,True,_) <- followingLines ])
                        if tabs && d >= 1
                          then tabsError n
                          else return $ F n (map toLower name)
                                            (fieldValue rest' followingLines)
      rest'       -> do ts' <- traverse (mkField (d+1)) ts
                        return (Section n (map toLower name) rest' ts')
   where    fieldValue firstLine followingLines =
              let firstLine' = trimLeading firstLine
                  followingLines' = map (\(_,_,s) -> stripDot s) followingLines
                  allLines | null firstLine' =              followingLines'
                           | otherwise       = firstLine' : followingLines'
               in intercalate "\n" allLines
            stripDot "." = ""
            stripDot s   = s
  
  -- | Convert if/then/else 'Section's to 'IfBlock's
  ifelse :: [Field] -> ParseResult [Field]
  ifelse [] = return []
  ifelse (Section n "if"   cond thenpart
         :Section _ "else" as   elsepart:fs)
         | null cond     = syntaxError n "'if' with missing condition"
         | null thenpart = syntaxError n "'then' branch of 'if' is empty"
         | not (null as) = syntaxError n "'else' takes no arguments"
         | null elsepart = syntaxError n "'else' branch of 'if' is empty"
         | otherwise     = do tp  <- ifelse thenpart
                              ep  <- ifelse elsepart
                              fs' <- ifelse fs
                              return (IfBlock n cond tp ep:fs')
  ifelse (Section n "if"   cond thenpart:fs)
         | null cond     = syntaxError n "'if' with missing condition"
         | null thenpart = syntaxError n "'then' branch of 'if' is empty"
         | otherwise     = do tp  <- ifelse thenpart
                              fs' <- ifelse fs
                              return (IfBlock n cond tp []:fs')
  ifelse (Section n "else" _ _:_) = syntaxError n
                                    "stray 'else' with no preceding 'if'"
  ifelse (Section n s a fs':fs) = do fs''  <- ifelse fs'
                                     fs''' <- ifelse fs
                                     return (Section n s a fs'' : fs''')
  ifelse (f:fs) = do fs' <- ifelse fs
                     return (f : fs')
  
  ------------------------------------------------------------------------------
  
  -- |parse a module name
  parseModuleNameQ :: ReadP r ModuleName
  parseModuleNameQ = parseMaybeQuoted parse
  
  parseFilePathQ :: ReadP r FilePath
  parseFilePathQ = parseTokenQ
    -- removed until normalise is no longer broken, was:
    --   liftM normalise parseTokenQ
  
  betweenSpaces :: ReadP r a -> ReadP r a
  betweenSpaces act = do skipSpaces
                         res <- act
                         skipSpaces
                         return res
  
  parsePackageName :: ReadP r String
  parsePackageName = do
    ns <- sepBy1 component (char '-')
    return $ intercalate "-" ns
    where
      component = do
        cs <- munch1 isAlphaNum
        if all isDigit cs then pfail else return cs
        -- each component must contain an alphabetic character, to avoid
        -- ambiguity in identifiers like foo-1 (the 1 is the version number).
  
  parseOptVersion :: ReadP r Version
  parseOptVersion = parseMaybeQuoted ver
    where ver :: ReadP r Version
          ver = parse <++ return nullVersion
  
  parseTestedWithQ :: ReadP r (CompilerFlavor,VersionRange)
  parseTestedWithQ = parseMaybeQuoted tw
    where
      tw :: ReadP r (CompilerFlavor,VersionRange)
      tw = do compiler <- parseCompilerFlavorCompat
              version <- betweenSpaces $ parse <++ return anyVersion
              return (compiler,version)
  
  parseLicenseQ :: ReadP r License
  parseLicenseQ = parseMaybeQuoted parse
  
  -- urgh, we can't define optQuotes :: ReadP r a -> ReadP r a
  -- because the "compat" version of ReadP isn't quite powerful enough.  In
  -- particular, the type of <++ is ReadP r r -> ReadP r a -> ReadP r a
  -- Hence the trick above to make 'lic' polymorphic.
  
  parseLanguageQ :: ReadP r Language
  parseLanguageQ = parseMaybeQuoted parse
  
  parseExtensionQ :: ReadP r Extension
  parseExtensionQ = parseMaybeQuoted parse
  
  parseHaskellString :: ReadP r String
  parseHaskellString = readS_to_P reads
  
  parseTokenQ :: ReadP r String
  parseTokenQ = parseHaskellString <++ munch1 (\x -> not (isSpace x) && x /= ',')
  
  parseTokenQ' :: ReadP r String
  parseTokenQ' = parseHaskellString <++ munch1 (not . isSpace)
  
  parseSepList :: ReadP r b
               -> ReadP r a -- ^The parser for the stuff between commas
               -> ReadP r [a]
  parseSepList sepr p = sepBy p separator
      where separator = betweenSpaces sepr
  
  parseSpaceList :: ReadP r a -- ^The parser for the stuff between commas
                 -> ReadP r [a]
  parseSpaceList p = sepBy p skipSpaces
  
  parseCommaList :: ReadP r a -- ^The parser for the stuff between commas
                 -> ReadP r [a]
  parseCommaList = parseSepList (ReadP.char ',')
  
  parseOptCommaList :: ReadP r a -- ^The parser for the stuff between commas
                    -> ReadP r [a]
  parseOptCommaList = parseSepList (optional (ReadP.char ','))
  
  parseQuoted :: ReadP r a -> ReadP r a
  parseQuoted = between (ReadP.char '"') (ReadP.char '"')
  
  parseMaybeQuoted :: (forall r. ReadP r a) -> ReadP r' a
  parseMaybeQuoted p = parseQuoted p <++ p
  
  parseFreeText :: ReadP.ReadP s String
  parseFreeText = ReadP.munch (const True)
  
  readPToMaybe :: ReadP a a -> String -> Maybe a
  readPToMaybe p str = listToMaybe [ r | (r,s) <- readP_to_S p str
                                       , all isSpace s ]
  