    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Text.PrettyPrint.HughesPJ
    -- Copyright   :  (c) The University of Glasgow 2001
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  provisional
    -- 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.cs.chalmers.se/~rjmh/Papers/pretty.ps>
    --
    -- Heavily modified by Simon Peyton Jones, Dec 96
    --
    -----------------------------------------------------------------------------

    {-
    Version 3.0     28 May 1997
      * Cured massive performance bug.  If you write

            foldl <> empty (map (text.show) [1..10000])

        you get quadratic behaviour with V2.0.  Why?  For just the same
        reason as you get quadratic behaviour with left-associated (++)
        chains.

        This is really bad news.  One thing a pretty-printer abstraction
        should certainly guarantee is insensivity to associativity.  It
        matters: suddenly GHC's compilation times went up by a factor of
        100 when I switched to the new pretty printer.

        I fixed it with a bit of a hack (because I wanted to get GHC back
        on the road).  I added two new constructors to the Doc type, Above
        and Beside:

             <> = Beside
             $$ = Above

        Then, where I need to get to a "TextBeside" or "NilAbove" form I
        "force" the Doc to squeeze out these suspended calls to Beside and
        Above; but in so doing I re-associate. It's quite simple, but I'm
        not satisfied that I've done the best possible job.  I'll send you
        the code if you are interested.

      * Added new exports:
            punctuate, hang
            int, integer, float, double, rational,
            lparen, rparen, lbrack, rbrack, lbrace, rbrace,

      * fullRender's type signature has changed.  Rather than producing a
        string it now takes an extra couple of arguments that tells it how
        to glue fragments of output together:

            fullRender :: Mode
                       -> Int                       -- Line length
                       -> Float                     -- Ribbons per line
                       -> (TextDetails -> a -> a)   -- What to do with text
                       -> a                         -- What to do at the end
                       -> Doc
                       -> a                         -- Result

        The "fragments" are encapsulated in the TextDetails data type:

            data TextDetails = Chr  Char
                             | Str  String
                             | PStr FAST_STRING

        The Chr and Str constructors are obvious enough.  The PStr
        constructor has a packed string (FAST_STRING) inside it.  It's
        generated by using the new "ptext" export.

        An advantage of this new setup is that you can get the renderer to
        do output directly (by passing in a function of type (TextDetails
        -> IO () -> IO ()), rather than producing a string that you then
        print.

    Version 2.0     24 April 1997
      * Made empty into a left unit for <> as well as a right unit;
        it is also now true that
            nest k empty = empty
        which wasn't true before.

      * Fixed an obscure bug in sep that occassionally gave very weird behaviour

      * Added $+$

      * Corrected and tidied up the laws and invariants

    ======================================================================
    Relative to John's original paper, there are the following new features:

    1.  There's an empty document, "empty".  It's a left and right unit for
        both <> and $$, and anywhere in the argument list for
        sep, hcat, hsep, vcat, fcat etc.

        It is Really Useful in practice.

    2.  There is a paragraph-fill combinator, fsep, that's much like sep,
        only it keeps fitting things on one line until it can't fit any more.

    3.  Some random useful extra combinators are provided.
            <+> puts its arguments beside each other with a space between them,
                unless either argument is empty in which case it returns the other

            hcat is a list version of <>
            hsep is a list version of <+>
            vcat is a list version of $$

            sep (separate) is either like hsep or like vcat, depending on what fits

            cat  behaves like sep,  but it uses <> for horizontal conposition
            fcat behaves like fsep, but it uses <> for horizontal conposition

            These new ones do the obvious things:
                    char, semi, comma, colon, space,
                    parens, brackets, braces,
                    quotes, doubleQuotes

    4.  The "above" combinator, $$, now overlaps its two arguments if the
        last line of the top argument stops before the first line of the
        second begins.

            For example:  text "hi" $$ nest 5 (text "there")
            lays out as
                            hi   there
            rather than
                            hi
                                 there

            There are two places this is really useful

            a) When making labelled blocks, like this:
                    Left ->   code for left
                    Right ->  code for right
                    LongLongLongLabel ->
                              code for longlonglonglabel
               The block is on the same line as the label if the label is
               short, but on the next line otherwise.

            b) When laying out lists like this:
                    [ first
                    , second
                    , third
                    ]
               which some people like.  But if the list fits on one line
               you want [first, second, third].  You can't do this with
               John's original combinators, but it's quite easy with the
               new $$.

            The combinator $+$ gives the original "never-overlap" behaviour.

    5.      Several different renderers are provided:
                    * a standard one
                    * one that uses cut-marks to avoid deeply-nested documents
                            simply piling up in the right-hand margin
                    * one that ignores indentation (fewer chars output; good for machines)
                    * one that ignores indentation and newlines (ditto, only more so)

    6.      Numerous implementation tidy-ups
            Use of unboxed data types to speed up the implementation
    -}

    module Text.PrettyPrint.HughesPJ (

    	-- * The document type
            Doc,            -- Abstract

    	-- * Constructing documents
    	-- ** Converting values into documents
            char, text, ptext, zeroWidthText,
            int, integer, float, double, rational,

    	-- ** Simple derived documents
            semi, comma, colon, space, equals,
            lparen, rparen, lbrack, rbrack, lbrace, rbrace,

    	-- ** Wrapping documents in delimiters
            parens, brackets, braces, quotes, doubleQuotes,

    	-- ** Combining documents
            empty,
            (<>), (<+>), hcat, hsep,
            ($$), ($+$), vcat,
            sep, cat,
            fsep, fcat,
    	nest,
            hang, punctuate,

    	-- * Predicates on documents
    	isEmpty,

    	-- * Rendering documents

    	-- ** Default rendering
    	render,

    	-- ** Rendering with a particular style
    	Style(..),
    	style,
            renderStyle,

    	-- ** General rendering
            fullRender,
            Mode(..), TextDetails(..),

      ) where

    import Prelude

    infixl 6 <>
    infixl 6 <+>
    infixl 5 $$, $+$

    -- ---------------------------------------------------------------------------
    -- The interface

    -- The primitive Doc values

    isEmpty :: Doc    -> Bool;  -- ^ Returns 'True' if the document is empty

    -- | 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

    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

    -- | A document of height and width 1, containing a literal character.
    char 	 :: Char     -> Doc

    -- | 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

    -- | An obsolete function, now identical to 'text'.
    ptext	 :: String   -> Doc

    -- | Some text, but without any width. Use for non-printing text
    -- such as a HTML or Latex tags
    zeroWidthText :: String   -> Doc

    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)@

    parens       :: Doc -> Doc; 	-- ^ Wrap document in @(...)@
    brackets     :: Doc -> Doc;  	-- ^ Wrap document in @[...]@
    braces	     :: Doc -> Doc;   	-- ^ Wrap document in @{...}@
    quotes	     :: Doc -> Doc;	-- ^ Wrap document in @\'...\'@
    doubleQuotes :: Doc -> Doc;	-- ^ Wrap document in @\"...\"@

    -- Combining @Doc@ values

    -- | Beside.
    -- '<>' is associative, with identity 'empty'.
    (<>)   :: Doc -> Doc -> Doc

    -- | Beside, separated by space, unless one of the arguments is 'empty'.
    -- '<+>' is associative, with identity 'empty'.
    (<+>)  :: Doc -> Doc -> Doc

    -- | 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

    -- | Above, with no overlapping.
    -- '$+$' is associative, with identity 'empty'.
    ($+$)   :: Doc -> Doc -> Doc

    hcat   :: [Doc] -> Doc;          -- ^List version of '<>'.
    hsep   :: [Doc] -> Doc;          -- ^List version of '<+>'.
    vcat   :: [Doc] -> Doc;          -- ^List version of '$$'.

    cat    :: [Doc] -> Doc;          -- ^ Either 'hcat' or 'vcat'.
    sep    :: [Doc] -> Doc;          -- ^ Either 'hsep' or 'vcat'.
    fcat   :: [Doc] -> Doc;          -- ^ \"Paragraph fill\" version of 'cat'.
    fsep   :: [Doc] -> Doc;          -- ^ \"Paragraph fill\" version of 'sep'.

    -- | 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

    -- GHC-specific ones.

    -- | @hang d1 n d2 = sep [d1, nest n d2]@
    hang :: Doc -> Int -> Doc -> Doc

    -- | @punctuate p [d1, ... dn] = [d1 \<> p, d2 \<> p, ... dn-1 \<> p, dn]@
    punctuate :: Doc -> [Doc] -> [Doc]

    -- Displaying @Doc@ values.

    instance Show Doc where
      showsPrec _ doc cont = showDoc doc cont

    -- | Renders the document as a string using the default 'style'.
    render     :: Doc -> String

    -- | 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

    -- | Render the document as a string using a specified style.
    renderStyle  :: Style -> Doc -> String

    -- | A rendering style.
    data Style
     = Style { mode           :: Mode     -- ^ The rendering mode
     	 , lineLength     :: Int      -- ^ Length of line, in chars
             , ribbonsPerLine :: Float    -- ^ Ratio of ribbon length to line length
             }

    -- | The default style (@mode=PageMode, lineLength=100, ribbonsPerLine=1.5@).
    style :: Style
    style = Style { lineLength = 100, ribbonsPerLine = 1.5, mode = PageMode }

    -- | Rendering mode.
    data Mode = PageMode            -- ^Normal
              | ZigZagMode          -- ^With zig-zag cuts
              | LeftMode            -- ^No indentation, infinitely long lines
              | OneLineMode         -- ^All on one line

    -- ---------------------------------------------------------------------------
    -- The Doc calculus

    -- The Doc combinators satisfy the following laws:

    {-
    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 z <> 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 verion 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
    -}

    -- ---------------------------------------------------------------------------
    -- Simple derived definitions

    semi  = char ';'
    colon = char ':'
    comma = char ','
    space = char ' '
    equals = char '='
    lparen = char '('
    rparen = char ')'
    lbrack = char '['
    rbrack = char ']'
    lbrace = char '{'
    rbrace = char '}'

    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)
    -- SIGBJORN wrote instead:
    -- rational n = text (show (fromRationalX n))

    quotes p        = char '\'' <> p <> char '\''
    doubleQuotes p  = char '"' <> p <> char '"'
    parens p        = char '(' <> p <> char ')'
    brackets p      = char '[' <> p <> char ']'
    braces p        = char '{' <> p <> char '}'

    -- lazy list versions
    hcat = reduceAB . foldr (beside_' False) empty
    hsep = reduceAB . foldr (beside_' True)  empty
    vcat = reduceAB . foldr (above_' True) empty

    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

    hang d1 n d2 = sep [d1, nest n d2]

    punctuate _ []     = []
    punctuate p (d:ds) = go d ds
                       where
                         go d' [] = [d']
                         go d' (e:es) = (d' <> p) : go e es

    -- ---------------------------------------------------------------------------
    -- The Doc data type

    -- A Doc represents a *set* of layouts.  A Doc with
    -- no occurrences of Union or NoDoc represents just one layout.

    -- | The abstract type of documents.
    -- The 'Show' instance is equivalent to using 'render'.
    data Doc
     = Empty                                -- empty
     | NilAbove Doc                         -- text "" $$ x
     | TextBeside TextDetails !Int Doc      -- text s <> x
     | Nest !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

    type RDoc = Doc         -- RDoc is a "reduced Doc", guaranteed not to have a top-level Above or Beside

    reduceDoc :: Doc -> RDoc
    reduceDoc (Beside p g q) = beside p g (reduceDoc q)
    reduceDoc (Above  p g q) = above  p g (reduceDoc q)
    reduceDoc p              = p

    data TextDetails = Chr  Char
                     | Str  String
                     | PStr String
    space_text, nl_text :: TextDetails
    space_text = Chr ' '
    nl_text    = Chr '\n'

    {-
      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.
    -}

    -- Invariant: Args to the 4 functions below are always RDocs
    nilAbove_ :: RDoc -> RDoc
    nilAbove_ p = NilAbove p

            -- Arg of a TextBeside is always an RDoc
    textBeside_ :: TextDetails -> Int -> RDoc -> RDoc
    textBeside_ s sl p = TextBeside s sl p

    nest_ :: Int -> RDoc -> RDoc
    nest_ k p = Nest k p

    union_ :: RDoc -> RDoc -> RDoc
    union_ p q = Union p q

    -- 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)

    -- ---------------------------------------------------------------------------
    -- @empty@, @text@, @nest@, @union@

    empty = Empty

    isEmpty Empty = True
    isEmpty _     = False

    char  c = textBeside_ (Chr c) 1 Empty
    text  s = case length s of {sl -> textBeside_ (Str s)  sl Empty}
    ptext s = case length s of {sl -> textBeside_ (PStr s) sl Empty}
    zeroWidthText s = textBeside_ (Str s) 0 Empty

    nest k  p = mkNest k (reduceDoc p)        -- Externally callable version

    -- 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                  -- Worth a try!
    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

    -- ---------------------------------------------------------------------------
    -- Vertical composition @$$@

    above_ :: Doc -> Bool -> Doc -> Doc
    above_ p _ Empty = p
    above_ Empty _ q = q
    above_ p g q = Above p g q

    p $$  q = above_ p False q
    p $+$ q = above_ p True 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)

    aboveNest :: RDoc -> Bool -> Int -> RDoc -> RDoc
    -- Specfication: aboveNest p g k q = p $g$ (nest k q)

    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 = k1 `seq` 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"

    nilAboveNest :: Bool -> Int -> RDoc -> RDoc
    -- Specification: text s <> nilaboveNest g k q
    --              = text s <> (text "" $g$ nest k q)

    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_ (Str (spaces k)) k q
                                 | otherwise                        -- Put them really above
                                 = nilAbove_ (mkNest k q)

    -- ---------------------------------------------------------------------------
    -- Horizontal composition @<>@

    beside_ :: Doc -> Bool -> Doc -> Doc
    beside_ p _ Empty = p
    beside_ Empty _ q = q
    beside_ p g q = Beside p g q

    p <>  q = beside_ p False q
    p <+> q = beside_ p True  q

    beside :: Doc -> Bool -> RDoc -> RDoc
    -- Specification: beside g p q = p <g> q

    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)       -- p non-empty
    beside p@(Beside p1 g1 q1) g2 q2
               {- (A `op1` B) `op2` C == A `op1` (B `op2` C)  iff op1 == op2
                                                     [ && (op1 == <> || op1 == <+>) ] -}
             | g1 == g2              = beside p1 g1 (beside q1 g2 q2)
             | otherwise             = beside (reduceDoc p) g2 q2
    beside p@(Above _ _ _)     g q   = beside (reduceDoc p) 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

    nilBeside :: Bool -> RDoc -> RDoc
    -- Specification: text "" <> nilBeside g p
    --              = text "" <g> p

    nilBeside _ Empty      = Empty  -- Hence the text "" in the spec
    nilBeside g (Nest _ p) = nilBeside g p
    nilBeside g p          | g         = textBeside_ space_text 1 p
                           | otherwise = p

    -- ---------------------------------------------------------------------------
    -- Separate, @sep@, Hughes version

    -- Specification: sep ps  = oneLiner (hsep ps)
    --                         `union`
    --                          vcat ps

    sep = sepX True         -- Separate with spaces
    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`
                                nilAboveNest True k (reduceDoc (vcat ys))
                              where
                                rest | g         = hsep ys
                                     | otherwise = hcat ys

    sepNB g p k ys            = sep1 g p k ys

    -- ---------------------------------------------------------------------------
    -- @fill@

    fsep = fill True
    fcat = fill False

    -- 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) k1 ys)
                                 `mkUnion`
                                 nilAboveNest True k (fill g (y:ys))
                               where
                                 k1 | g         = k - 1
                                    | otherwise = k

    elideNest :: Doc -> Doc
    elideNest (Nest _ d) = d
    elideNest d = d

    -- ---------------------------------------------------------------------------
    -- Selecting the best layout

    best :: Mode
         -> Int             -- Line length
         -> Int             -- Ribbon length
         -> RDoc
         -> RDoc            -- No unions in here!

    best OneLineMode _ _ p0
      = get p0 -- unused, due to the use of easy_display in full_render
      where
        get Empty               = Empty
        get NoDoc               = NoDoc
        get (NilAbove p)        = nilAbove_ (get p)
        get (TextBeside s sl p) = textBeside_ s sl (get p)
        get (Nest _ p)          = get p             -- Elide nest
        get (p `Union` q)       = first (get p) (get q)
        get (Above {})          = error "best OneLineMode get Above"
        get (Beside {})         = error "best OneLineMode get Beside"

    best _ w0 r p0
      = get w0 p0
      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 p q = nicest1 w r 0 p q

    nicest1 :: Int -> Int -> Int -> Doc -> Doc -> Doc
    nicest1 w r sl p q | fits ((w `minn` 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"

    minn :: Int -> Int -> Int
    minn x y | x < y    = x
             | otherwise = y

    -- @first@ and @nonEmptySet@ are similar to @nicest@ and @fits@, only simpler.
    -- @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           -- NoDoc always in first line
    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 @Doc@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"

    -- ---------------------------------------------------------------------------
    -- Displaying the best layout

    renderStyle the_style doc
      = fullRender (mode the_style)
                   (lineLength the_style)
                   (ribbonsPerLine the_style)
    	       string_txt
    	       ""
    	       doc

    render doc       = showDoc doc ""

    showDoc :: Doc -> String -> String
    showDoc doc rest = fullRender PageMode 100 1.5 string_txt rest doc

    string_txt :: TextDetails -> String -> String
    string_txt (Chr c)   s  = c:s
    string_txt (Str s1)  s2 = s1 ++ s2
    string_txt (PStr s1) s2 = s1 ++ s2

    fullRender OneLineMode _ _ txt end doc = easy_display space_text txt end (reduceDoc doc)
    fullRender LeftMode    _ _ txt end doc = easy_display nl_text    txt end (reduceDoc doc)

    fullRender the_mode line_length ribbons_per_line txt end doc
      = display the_mode line_length ribbon_length txt end best_doc
      where
        best_doc = best the_mode hacked_line_length ribbon_length (reduceDoc doc)

        hacked_line_length, ribbon_length :: Int
        ribbon_length = round (fromIntegral line_length / ribbons_per_line)
        hacked_line_length = case the_mode of
                             ZigZagMode -> maxBound
                             _ -> line_length

    display :: Mode -> Int -> Int -> (TextDetails -> a -> a) -> a -> Doc -> a
    display the_mode 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 _ (Above {})   = error "display lay Above"
            lay _ (Beside {})  = error "display lay Beside"
            lay _ NoDoc        = error "display lay NoDoc"
            lay _ (Union {})   = error "display lay Union"

            lay k (NilAbove p) = nl_text `txt` lay k p

            lay k (TextBeside s sl p)
                = case the_mode of
                        ZigZagMode |  k >= gap_width
                                   -> nl_text `txt` (
                                      Str (multi_ch shift '/') `txt` (
                                      nl_text `txt` (
                                      lay1 (k - shift) s sl p)))

                                   |  k < 0
                                   -> nl_text `txt` (
                                      Str (multi_ch shift '\\') `txt` (
                                      nl_text `txt` (
                                      lay1 (k + shift) s sl p )))

                        _ -> lay1 k s sl p

            lay1 k _ sl _ | k+sl `seq` False = undefined
            lay1 k s sl p = Str (indent k) `txt` (s `txt` lay2 (k + sl) p)

            lay2 k _ | k `seq` False = undefined
            lay2 k (NilAbove p)        = nl_text `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"
        in
        lay 0 doc
        }}

    cant_fail :: a
    cant_fail = error "easy_display: NoDoc"

    easy_display :: TextDetails -> (TextDetails -> a -> a) -> a -> Doc -> a
    easy_display nl_space_text txt end doc
      = lay doc cant_fail
      where
        lay NoDoc               no_doc = no_doc
        lay (Union _p q)        _      = {- lay p -} (lay q cant_fail)              -- Second arg can't be NoDoc
        lay (Nest _ p)          no_doc = lay p no_doc
        lay Empty               _      = end
        lay (NilAbove p)        _      = nl_space_text `txt` lay p cant_fail      -- NoDoc always on first line
        lay (TextBeside s _ p)  no_doc = s `txt` lay p no_doc
        lay (Above {}) _ = error "easy_display Above"
        lay (Beside {}) _ = error "easy_display Beside"

    -- OLD version: we shouldn't rely on tabs being 8 columns apart in the output.
    -- indent n | n >= 8 = '\t' : indent (n - 8)
    --          | otherwise      = spaces n
    indent :: Int -> String
    indent n = spaces n

    multi_ch :: Int -> Char -> String
    multi_ch 0 _ = ""
    multi_ch n       ch = ch : multi_ch (n - 1) ch

    -- (spaces n) generates a list of n spaces
    --
    -- returns the empty string on negative argument.
    --
    spaces :: Int -> String
    spaces n
     {-
     | n  < 0    = trace "Warning: negative indentation" ""
     -}
     | n <= 0    = ""
     | otherwise = ' ' : spaces (n - 1)

    {-
    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'.
    -}