utils/Pretty.hs

    {-
    *********************************************************************************
    *                                                                               *
    *       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 (
            Doc,            -- Abstract
            Mode(..), TextDetails(..),

            empty, isEmpty, nest,

            char, text, ftext, ptext, ztext, zeroWidthText,
            int, integer, float, double, rational,
            parens, brackets, braces, quotes, quote, doubleQuotes,
            semi, comma, colon, space, equals,
            lparen, rparen, lbrack, rbrack, lbrace, rbrace, cparen,

            (<>), (<+>), hcat, hsep,
            ($$), ($+$), vcat,
            sep, cat,
            fsep, fcat,

            hang, punctuate,

            fullRender, printDoc, printDoc_, showDoc,
            bufLeftRender -- performance hack
      ) where

    import BufWrite
    import FastString
    import FastTypes
    import Panic
    import Numeric (fromRat)
    import System.IO

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

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

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

    -- Disable ASSERT checks; they are expensive!
    #define LOCAL_ASSERT(x)

    {-
    *********************************************************
    *                                                       *
    \subsection{The interface}
    *                                                       *
    *********************************************************

    The primitive @Doc@ values
    -}

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

    text                      :: String -> Doc
    char                      :: Char -> Doc

    semi, comma, colon, space, equals              :: Doc
    lparen, rparen, lbrack, rbrack, lbrace, rbrace :: Doc

    parens, brackets, braces    :: Doc -> Doc
    quotes, quote, doubleQuotes :: Doc -> Doc

    int      :: Int -> Doc
    integer  :: Integer -> Doc
    float    :: Float -> Doc
    double   :: Double -> Doc
    rational :: Rational -> Doc

    -- Combining @Doc@ values

    (<>)   :: Doc -> Doc -> Doc     -- Beside
    hcat   :: [Doc] -> Doc          -- List version of <>
    (<+>)  :: Doc -> Doc -> Doc     -- Beside, separated by space
    hsep   :: [Doc] -> Doc          -- List version of <+>

    ($$)   :: Doc -> Doc -> Doc     -- Above; if there is no
                                    -- overlap it "dovetails" the two
    vcat   :: [Doc] -> Doc          -- List version of $$

    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   :: Int -> Doc -> Doc     -- Nested

    -- GHC-specific ones.

    hang :: Doc -> Int -> Doc -> Doc
    punctuate :: Doc -> [Doc] -> [Doc]      -- punctuate p [d1, ... dn] = [d1 <> p, d2 <> p, ... dn-1 <> p, dn]

    -- Displaying @Doc@ values.

    instance Show Doc where
      showsPrec _ doc cont = showDocPlus PageMode 100 doc cont

    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

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

    {-
    *********************************************************
    *                                                       *
    \subsection{The @Doc@ calculus}
    *                                                       *
    *********************************************************

    The @Doc@ combinators satisfy the following laws:
    \begin{verbatim}
    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

    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
    \end{verbatim}

    You might think that the following verion of <m1> would
    be neater:
    \begin{verbatim}
    <3 NO>  (text s <> x) $$ y = text s <> ((empty <> x)) $$
                                             nest (-length s) y)
    \end{verbatim}
    But it doesn't work, for if x=empty, we would have
    \begin{verbatim}
            text s $$ y = text s <> (empty $$ nest (-length s) y)
                        = text s <> nest (-length s) y
    \end{verbatim}

    *********************************************************
    *                                                       *
    \subsection{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 (fromRat n :: Double))
    --rational n = text (show (fromRationalX n)) -- _showRational 30 n)

    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 '}'

    cparen :: Bool -> Doc -> Doc
    cparen True  = parens
    cparen False = id

    hcat = foldr (<>)  empty
    hsep = foldr (<+>) empty
    vcat = foldr ($$)  empty

    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

    {-
    *********************************************************
    *                                                       *
    \subsection{The @Doc@ data type}
    *                                                       *
    *********************************************************

    A @Doc@ represents a {\em 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 FastInt Doc       -- text s <> x
     | Nest FastInt 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  {-#UNPACK#-}!Char
                     | Str  String
                     | PStr FastString                      -- a hashed string
                     | ZStr FastZString                     -- a z-encoded string
                     | LStr {-#UNPACK#-}!LitString FastInt  -- a '\0'-terminated
                                                            -- array of bytes

    space_text :: TextDetails
    space_text = Chr ' '
    nl_text :: TextDetails
    nl_text    = Chr '\n'

    {-
    Here are the invariants:
    \begin{itemize}
    \item
    The argument of @NilAbove@ is never @Empty@. Therefore
    a @NilAbove@ occupies at least two lines.

    \item
    The arugment of @TextBeside@ is never @Nest@.

    \item
    The layouts of the two arguments of @Union@ both flatten to the same string.

    \item
    The arguments of @Union@ are either @TextBeside@, or @NilAbove@.

    \item
    The right argument of a union cannot be equivalent to the empty set (@NoDoc@).
    If the left argument of a union is equivalent to the empty set (@NoDoc@),
    then the @NoDoc@ appears in the first line.

    \item
    An empty document is always represented by @Empty@.
    It can't be hidden inside a @Nest@, or a @Union@ of two @Empty@s.

    \item
    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.
    \end{itemize}
    -}

    -- Arg of a NilAbove is always an RDoc
    nilAbove_ :: Doc -> Doc
    nilAbove_ p = LOCAL_ASSERT( _ok p ) NilAbove p
                where
                  _ok Empty = False
                  _ok _     = True

    -- Arg of a TextBeside is always an RDoc
    textBeside_ :: TextDetails -> FastInt -> Doc -> Doc
    textBeside_ s sl p = TextBeside s sl (LOCAL_ASSERT( _ok p ) p)
                       where
                         _ok (Nest _ _) = False
                         _ok _          = True

    -- Arg of Nest is always an RDoc
    nest_ :: FastInt -> Doc -> Doc
    nest_ k p = Nest k (LOCAL_ASSERT( _ok p ) p)
              where
                _ok Empty = False
                _ok _     = True

    -- Args of union are always RDocs
    union_ :: Doc -> Doc -> Doc
    union_ p q = Union (LOCAL_ASSERT( _ok p ) p) (LOCAL_ASSERT( _ok q ) q)
               where
                 _ok (TextBeside _ _ _) = True
                 _ok (NilAbove _)       = True
                 _ok (Union _ _)        = True
                 _ok _                  = False

    {-
    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)

    *********************************************************
    *                                                       *
    \subsection{@empty@, @text@, @nest@, @union@}
    *                                                       *
    *********************************************************
    -}

    empty = Empty

    isEmpty Empty = True
    isEmpty _     = False

    char  c = textBeside_ (Chr c) (_ILIT(1)) Empty

    text  s = case iUnbox (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

    ftext :: FastString -> Doc
    ftext s = case iUnbox (lengthFS s) of {sl -> textBeside_ (PStr s) sl Empty}
    ptext :: LitString -> Doc
    ptext s = case iUnbox (lengthLS s) of {sl -> textBeside_ (LStr s sl) sl Empty}
    ztext :: FastZString -> Doc
    ztext s = case iUnbox (lengthFZS s) of {sl -> textBeside_ (ZStr s) sl Empty}
    zeroWidthText s = textBeside_ (Str s) (_ILIT(0)) Empty

    -- 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)
     #-}

    nest k  p = mkNest (iUnbox 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       (Nest k1 p) = mkNest (k +# k1) p
    mkNest _       NoDoc       = NoDoc
    mkNest _       Empty       = Empty
    mkNest k       p  | k ==# _ILIT(0)  = 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

    {-
    *********************************************************
    *                                                       *
    \subsection{Vertical composition @$$@}
    *                                                       *
    *********************************************************
    -}

    p $$  q = Above p False q
    ($+$) :: Doc -> Doc -> Doc
    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 (_ILIT(0)) (reduceDoc q)
    above p g q                  = aboveNest p             g (_ILIT(0)) (reduceDoc q)

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

    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 _                   _ _ _ = panic "aboveNest: Unhandled case"

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

    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 ># _ILIT(0))        -- No newline if no overlap
                                 = textBeside_ (Str (spaces k)) k q
                                 | otherwise                        -- Put them really above
                                 = nilAbove_ (mkNest k q)

    {-
    *********************************************************
    *                                                       *
    \subsection{Horizontal composition @<>@}
    *                                                       *
    *********************************************************
    -}

    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   = let d = reduceDoc p in d `seq` 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

    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 (_ILIT(1)) p
                           | otherwise = p

    {-
    *********************************************************
    *                                                       *
    \subsection{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) (_ILIT(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 -> FastInt -> [Doc] -> RDoc
    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 _ _                   _ _  = panic "sep1: Unhandled case"

    -- 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 -> FastInt -> [Doc] -> Doc
    sepNB g (Nest _ p)  k ys  = sepNB g p k ys

    sepNB g Empty k ys        = oneLiner (nilBeside g (reduceDoc rest))
                                    `mkUnion`
                                nilAboveNest False k (reduceDoc (vcat ys))
                              where
                                rest | g         = hsep ys
                                     | otherwise = hcat ys

    sepNB g p k ys            = sep1 g p k ys

    {-
    *********************************************************
    *                                                       *
    \subsection{@fill@}
    *                                                       *
    *********************************************************
    -}

    fsep = fill True
    fcat = fill False

    -- Specification:
    --   fill []  = empty
    --   fill [p] = p
    --   fill (p1:p2:ps) = oneLiner p1 <#> nest (length p1)
    --                                          (fill (oneLiner p2 : ps))
    --                     `union`
    --                      p1 $$ fill ps

    fill :: Bool -> [Doc] -> Doc
    fill _ []     = empty
    fill g (p:ps) = fill1 g (reduceDoc p) (_ILIT(0)) ps

    fill1 :: Bool -> RDoc -> FastInt -> [Doc] -> Doc
    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 _ _                   _ _  = panic "fill1: Unhandled case"

    fillNB :: Bool -> Doc -> Int# -> [Doc] -> Doc
    fillNB g (Nest _ p)  k ys  = fillNB g p k ys
    fillNB _ Empty _ []        = Empty
    fillNB g Empty k (y:ys)    = nilBeside g (fill1 g (oneLiner (reduceDoc y)) k1 ys)
                                 `mkUnion`
                                 nilAboveNest False k (fill g (y:ys))
                               where
                                 !k1 | g         = k -# _ILIT(1)
                                     | otherwise = k

    fillNB g p k ys            = fill1 g p k ys

    {-
    *********************************************************
    *                                                       *
    \subsection{Selecting the best layout}
    *                                                       *
    *********************************************************
    -}

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

    best w_ r_ p
      = get (iUnbox w_) p
      where
        !r = iUnbox r_
        get :: FastInt          -- (Remaining) width of line
            -> Doc -> Doc
        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 _ _                   = panic "best/get: Unhandled case"

        get1 :: FastInt         -- (Remaining) width of line
             -> FastInt         -- Amount of first line already eaten up
             -> Doc         -- This is an argument to TextBeside => eat Nests
             -> Doc         -- No unions in here!

        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 _ _  _                   = panic "best/get1: Unhandled case"

    nicest :: FastInt -> FastInt -> Doc -> Doc -> Doc
    nicest w r p q = nicest1 w r (_ILIT(0)) p q
    nicest1 :: FastInt -> FastInt -> Int# -> Doc -> Doc -> Doc
    nicest1 w r sl p q | fits ((w `minFastInt` r) -# sl) p = p
                       | otherwise                   = q

    fits :: FastInt     -- Space available
         -> Doc
         -> Bool    -- True if *first line* of Doc fits in space available

    fits n _   | n <# _ILIT(0) = False
    fits _ NoDoc               = False
    fits _ Empty               = True
    fits _ (NilAbove _)        = True
    fits n (TextBeside _ sl p) = fits (n -# sl) p
    fits _ _                   = panic "fits: Unhandled case"

    {-
    @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
              | 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 _                  = panic "nonEmptySet: Unhandled case"

    -- @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 _                   = panic "oneLiner: Unhandled case"

    {-
    *********************************************************
    *                                                       *
    \subsection{Displaying the best layout}
    *                                                       *
    *********************************************************
    -}

    showDocPlus :: Mode -> Int -> Doc -> String -> String
    showDocPlus mode cols doc rest = fullRender mode cols 1.5 string_txt rest doc

    showDoc :: Mode -> Int -> Doc -> String
    showDoc mode cols doc = showDocPlus mode cols 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 = unpackFS s1 ++ s2
    string_txt (ZStr s1) s2 = zString s1 ++ s2
    string_txt (LStr s1 _) s2 = unpackLitString s1 ++ s2

    fullRender OneLineMode _ _ txt end doc
      = lay (reduceDoc doc)
      where
        lay NoDoc              = cant_fail
        lay (Union _ q)        = lay q -- Second arg can't be NoDoc
        lay (Nest _ p)         = lay p
        lay Empty              = end
        lay (NilAbove p)       = space_text `txt` lay p -- NoDoc always on
                                                        -- first line
        lay (TextBeside s _ p) = s `txt` lay p
        lay _                  = panic "fullRender/OneLineMode/lay: Unhandled case"

    fullRender LeftMode    _ _ txt end doc
      = lay (reduceDoc doc)
      where
        lay NoDoc              = cant_fail
        lay (Union p q)        = lay (first p q)
        lay (Nest _ p)         = lay p
        lay Empty              = end
        lay (NilAbove p)       = nl_text `txt` lay p -- NoDoc always on first line
        lay (TextBeside s _ p) = s `txt` lay p
        lay _                  = panic "fullRender/LeftMode/lay: Unhandled case"

    fullRender mode line_length ribbons_per_line txt end doc
      = display mode line_length ribbon_length txt end best_doc
      where
        best_doc = best 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 mode of
                             ZigZagMode -> maxBound
                             _ -> line_length

    display :: Mode -> Int -> Int -> (TextDetails -> t -> t) -> t -> Doc -> t
    display mode page_width ribbon_width txt end doc
      = case (iUnbox page_width) -# (iUnbox ribbon_width) of { gap_width ->
        case gap_width `quotFastInt` _ILIT(2) of { shift ->
        let
            lay k (Nest k1 p)  = lay (k +# k1) p
            lay _ Empty        = end

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

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

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

                        _ -> lay1 k s sl p
            lay _ _            = panic "display/lay: Unhandled case"

            lay1 k s sl p = indent k (s `txt` lay2 (k +# sl) p)

            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 _ _                   = panic "display/lay2: Unhandled case"

            -- optimise long indentations using LitString chunks of 8 spaces
            indent n r | n >=# _ILIT(8) = LStr (sLit "        ") (_ILIT(8)) `txt`
                                          indent (n -# _ILIT(8)) r
                       | otherwise      = Str (spaces n) `txt` r
        in
        lay (_ILIT(0)) doc
        }}

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

    multi_ch :: Int# -> Char -> String
    multi_ch n ch | n <=# _ILIT(0) = ""
                  | otherwise      = ch : multi_ch (n -# _ILIT(1)) ch

    spaces :: Int# -> String
    spaces n | n <=# _ILIT(0) = ""
             | otherwise      = ' ' : spaces (n -# _ILIT(1))

    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 ==# _ILIT(0)
                                then return ()
                                else hPutBuf handle a (iBox 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              = cant_fail
    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"