utils/Pretty.hs

    {-# LANGUAGE BangPatterns #-}
    {-
    *********************************************************************************
    *                                                                               *
    *       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 FastString

        The Chr and Str constructors are obvious enough.  The PStr constructor has a packed
        string (FastString) 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 occasionally 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  is behaves like sep,  but it uses <> for horizontal conposition
            fcat is 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, quote, 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 "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
    -}

    {-# LANGUAGE BangPatterns, CPP, MagicHash #-}

    module Pretty (
            -- * The document type
            Doc, TextDetails(..),

            -- * Constructing documents

            -- ** Converting values into documents
            char, text, ftext, ptext, ztext, sizedText, 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, quote, doubleQuotes,
            maybeParens,

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

            -- * Predicates on documents
            isEmpty,

            -- * Rendering documents

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

            -- ** General rendering
            fullRender,

            -- ** GHC-specific rendering
            printDoc, printDoc_,
            bufLeftRender -- performance hack

      ) where

    import BufWrite
    import FastString
    import Panic
    import System.IO
    import Prelude hiding (error)

    --for a RULES
    import GHC.Base ( unpackCString# )
    import GHC.Ptr  ( Ptr(..) )

    -- Don't import Util( assertPanic ) because it makes a loop in the module structure

    -- ---------------------------------------------------------------------------
    -- 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 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
    -}

    -- ---------------------------------------------------------------------------
    -- 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 #-} !LitString {-#UNPACK #-} !Int
                       -- a '\0'-terminated array of bytes

    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 = case length s of {sl -> textBeside_ (Str s)  sl 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 (Ptr a)
     #-}

    ftext :: FastString -> Doc
    ftext s = case lengthFS s of {sl -> textBeside_ (PStr s) sl Empty}

    ptext :: LitString -> Doc
    ptext s = case lengthLS s of {sl -> textBeside_ (LStr s sl) sl Empty}

    ztext :: FastZString -> Doc
    ztext s = case lengthFZS s of {sl -> textBeside_ (ZStr s) sl 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

    -- | 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

    -- | Produce spacing for indenting the amount specified.
    --
    -- an old version inserted tabs being 8 columns apart in the output.
    spaces :: Int -> String
    spaces !n = replicate n ' '

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

    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)@
    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)

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

    -- | 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) = beside p g (reduceDoc q)
    reduceDoc (Above  p g q) = 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)

    -- Specfication: 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_ (Str (spaces 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, 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

    -- | 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 = unpackLitString s1 ++ 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"

            -- optimise long indentations using LitString chunks of 8 spaces
            indent !n r | n >= 8    = LStr (sLit "        ") 8 `txt`
                                      indent (n - 8) r
                        | otherwise = Str (spaces 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 "")

    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 l) next = hPutLitString hdl s l >> next

        done = return () -- hPutChar hdl '\n'

      -- some versions of hPutBuf will barf if the length is zero
    hPutLitString :: Handle -> Ptr a -> Int -> IO ()
    hPutLitString handle a l = if l == 0
                                then return ()
                                else 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.
    --
    -- (3) a few hacks in layLeft below to convince GHC to generate the right
    --     code.

    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)

    -- HACK ALERT!  the "return () >>" below convinces GHC to eta-expand
    -- this function with the IO state lambda.  Otherwise we end up with
    -- closures in all the case branches.
    layLeft :: BufHandle -> Doc -> IO ()
    layLeft b _ | b `seq` False  = undefined -- make it strict in b
    layLeft _ NoDoc              = error "layLeft: NoDoc"
    layLeft b (Union p q)        = return () >> layLeft b (first p q)
    layLeft b (Nest _ p)         = return () >> 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 _ | b `seq` False = undefined
        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 l) = bPutLitString b s l
    layLeft _ _                  = panic "layLeft: Unhandled case"

    -- Define error=panic, for easier comparison with libraries/pretty.
    error :: String -> a
    error = panic