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Text File | 2000-09-21 | 31.0 KB | 909 lines

********************************************************************************* * * * 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 $$ = Above Then, where I need to get to a "TextBeside" or "NilAbove" form I "force" the Doc to squeeze out these suspended calls to Beside and Above; but in so doing I re-associate. It's quite simple, but I'm not satisfied that I've done the best possible job. I'll send you the code if you are interested. * Added new exports: punctuate, hang int, integer, float, double, rational, lparen, rparen, lbrack, rbrack, lbrace, rbrace, * fullRender's type signature has changed. Rather than producing a string it now takes an extra couple of arguments that tells it how to glue fragments of output together: fullRender :: Mode -> Int -- Line length -> Float -- Ribbons per line -> (TextDetails -> a -> a) -- What to do with text -> a -- What to do at the end -> Doc -> a -- Result The "fragments" are encapsulated in the TextDetails data type: data TextDetails = Chr Char | Str String | PStr FAST_STRING The Chr and Str constructors are obvious enough. The PStr constructor has a packed string (FAST_STRING) inside it. It's generated by using the new "ptext" export. An advantage of this new setup is that you can get the renderer to do output directly (by passing in a function of type (TextDetails -> IO () -> IO ()), rather than producing a string that you then print. Version 2.0 24 April 1997 * Made empty into a left unit for <> as well as a right unit; it is also now true that nest k empty = empty which wasn't true before. * Fixed an obscure bug in sep that occassionally gave very wierd behaviour * Added $+$ * Corrected and tidied up the laws and invariants ====================================================================== Relative to John's original paper, there are the following new features: 1. There's an empty document, "empty". It's a left and right unit for both <> and $$, 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 itc 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 $$ sep (separate) is either like hsep or like vcat, depending on what fits cat is behaves like sep, but it uses <> for horizontal conposition fcat is behaves like fsep, but it uses <> for horizontal conposition These new ones do the obvious things: char, semi, comma, colon, space, parens, brackets, braces, quotes, doubleQuotes 4. The "above" combinator, $$, 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 \begin{code} module Pretty ( Doc, -- Abstract Mode(..), TextDetails(..), empty, isEmpty, nest, text, char, ptext, int, integer, float, double, rational, parens, brackets, braces, quotes, doubleQuotes, semi, comma, colon, space, equals, lparen, rparen, lbrack, rbrack, lbrace, rbrace, (<>), (<+>), hcat, hsep, ($$), ($+$), vcat, sep, cat, fsep, fcat, hang, punctuate, -- renderStyle, -- Haskell 1.3 only render, fullRender ) where -- Don't import Util( assertPanic ) because it makes a loop in the module structure infixl 6 <> infixl 6 <+> infixl 5 $$, $+$ \end{code} ********************************************************* * * \subsection{CPP magic so that we can compile with both GHC and Hugs} * * ********************************************************* The library uses unboxed types to get a bit more speed, but these CPP macros allow you to use either GHC or Hugs. To get GHC, just set the CPP variable __GLASGOW_HASKELL__ ********************************************************* * * \subsection{The interface} * * ********************************************************* The primitive @Doc@ values \begin{code} empty :: Doc isEmpty :: Doc -> Bool 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, doubleQuotes :: Doc -> Doc int :: Int -> Doc integer :: Integer -> Doc float :: Float -> Doc double :: Double -> Doc rational :: Rational -> Doc \end{code} Combining @Doc@ values \begin{code} (<>) :: 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 \end{code} GHC-specific ones. \begin{code} hang :: Doc -> Int -> Doc -> Doc punctuate :: Doc -> [Doc] -> [Doc] -- punctuate p [d1, ... dn] = [d1 <> p, d2 <> p, ... dn-1 <> p, dn] \end{code} Displaying @Doc@ values. \begin{code} instance Show Doc where showsPrec prec doc cont = showDoc doc cont render :: Doc -> String -- Uses default style 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 {- When we start using 1.3 renderStyle :: Style -> Doc -> String data Style = Style { lineLength :: Int, -- In chars ribbonsPerLine :: Float, -- Ratio of ribbon length to line length mode :: Mode } style :: Style -- The default style style = Style { lineLength = 100, ribbonsPerLine = 2.5, mode = PageMode } -} data Mode = PageMode -- Normal | ZigZagMode -- With zig-zag cuts | LeftMode -- No indentation, infinitely long lines | OneLineMode -- All on one line \end{code} ********************************************************* * * \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 $$ nest (-length s) y) = text s <> nest (-length s) y \end{verbatim} ********************************************************* * * \subsection{Simple derived definitions} * * ********************************************************* \begin{code} semi = char ';' colon = char ':' comma = char ',' space = char ' ' equals = char '=' lparen = char '(' rparen = char ')' lbrack = char '[' rbrack = char ']' lbrace = char '{' rbrace = char '}' int n = text (show n) integer n = text (show n) float n = text (show n) double n = text (show n) rational n = text (show n) -- SIGBJORN wrote instead: -- rational n = text (show (fromRationalX n)) quotes p = char '`' <> p <> char '\'' doubleQuotes p = char '"' <> p <> char '"' parens p = char '(' <> p <> char ')' brackets p = char '[' <> p <> char ']' braces p = char '{' <> p <> char '}' hcat = foldr (<>) empty hsep = foldr (<+>) empty vcat = foldr ($$) empty hang d1 n d2 = sep [d1, nest n d2] punctuate p [] = [] punctuate p (d:ds) = go d ds where go d [] = [d] go d (e:es) = (d <> p) : go e es \end{code} ********************************************************* * * \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. \begin{code} 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 = Chr ' ' nl_text = Chr '\n' \end{code} 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} \begin{code} -- Arg of a NilAbove is always an RDoc nilAbove_ p = NilAbove p -- Arg of a TextBeside is always an RDoc textBeside_ s sl p = TextBeside s sl p -- Arg of Nest is always an RDoc nest_ k p = Nest k p -- Args of union are always RDocs union_ p q = Union p q \end{code} 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@} * * ********************************************************* \begin{code} 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} 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 k (Nest k1 p) = mkNest (k + k1) p mkNest k NoDoc = NoDoc mkNest k Empty = Empty mkNest 0 p = p -- Worth a try! mkNest k p = nest_ k p -- mkUnion checks for an empty document mkUnion Empty q = Empty mkUnion p q = p `union_` q \end{code} ********************************************************* * * \subsection{Vertical composition @$$@} * * ********************************************************* \begin{code} 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 NoDoc g k q = NoDoc aboveNest (p1 `Union` p2) g k q = aboveNest p1 g k q `union_` aboveNest p2 g k q aboveNest Empty g 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 other -> aboveNest p g k1 q \end{code} \begin{code} nilAboveNest :: Bool -> Int -> RDoc -> RDoc -- Specification: text s <> nilaboveNest g k q -- = text s <> (text "" $g$ nest k q) nilAboveNest g k 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) \end{code} ********************************************************* * * \subsection{Horizontal composition @<>@} * * ********************************************************* \begin{code} 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 g q = NoDoc beside (p1 `Union` p2) g q = (beside p1 g q) `union_` (beside p2 g q) beside Empty g 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 other -> beside p g q \end{code} \begin{code} nilBeside :: Bool -> RDoc -> RDoc -- Specification: text "" <> nilBeside g p -- = text "" <g> p nilBeside g 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 \end{code} ********************************************************* * * \subsection{Separate, @sep@, Hughes version} * * ********************************************************* \begin{code} -- Specification: sep ps = oneLiner (hsep ps) -- `union` -- vcat ps sep = sepX True -- Separate with spaces cat = sepX False -- Don't sepX x [] = 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 g NoDoc k ys = 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 g (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) -- 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 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 \end{code} ********************************************************* * * \subsection{@fill@} * * ********************************************************* \begin{code} 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 g [] = empty fill g (p:ps) = fill1 g (reduceDoc p) 0 ps fill1 :: Bool -> RDoc -> Int -> [Doc] -> Doc fill1 g NoDoc k ys = 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) fillNB g (Nest _ p) k ys = fillNB g p k ys fillNB g Empty k [] = 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 - 1 | otherwise = k fillNB g p k ys = fill1 g p k ys \end{code} ********************************************************* * * \subsection{Selecting the best layout} * * ********************************************************* \begin{code} best :: Mode -> Int -- Line length -> Int -- Ribbon length -> RDoc -> RDoc -- No unions in here! best OneLineMode w r p = get p 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 k p) = get p -- Elide nest get (p `Union` q) = first (get p) (get q) best mode w r p = get w p where get :: Int -- (Remaining) width of line -> Doc -> Doc get w Empty = Empty get w 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) 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 sl Empty = Empty get1 w sl 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 k p) = get1 w sl p get1 w sl (p `Union` q) = nicest1 w r sl (get1 w sl p) (get1 w sl q) nicest w r p q = nicest1 w r 0 p q 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 p | n < 0 = False fits n NoDoc = False fits n Empty = True fits n (NilAbove _) = True fits n (TextBeside _ sl p) = fits (n - sl) p minn x y | x < y = x | otherwise = y \end{code} @first@ and @nonEmptySet@ are similar to @nicest@ and @fits@, only simpler. @first@ returns its first argument if it is non-empty, otherwise its second. \begin{code} first p q | nonEmptySet p = p | otherwise = q nonEmptySet NoDoc = False nonEmptySet (p `Union` q) = True nonEmptySet Empty = True nonEmptySet (NilAbove p) = True -- NoDoc always in first line nonEmptySet (TextBeside _ _ p) = nonEmptySet p nonEmptySet (Nest _ p) = nonEmptySet p \end{code} @oneLiner@ returns the one-line members of the given set of @Doc@s. \begin{code} oneLiner :: Doc -> Doc oneLiner NoDoc = NoDoc oneLiner Empty = Empty oneLiner (NilAbove p) = NoDoc oneLiner (TextBeside s sl p) = textBeside_ s sl (oneLiner p) oneLiner (Nest k p) = nest_ k (oneLiner p) oneLiner (p `Union` q) = oneLiner p \end{code} ********************************************************* * * \subsection{Displaying the best layout} * * ********************************************************* \begin{code} {- renderStyle Style{mode, lineLength, ribbonsPerLine} doc = fullRender mode lineLength ribbonsPerLine doc "" -} render doc = showDoc doc "" showDoc doc rest = fullRender PageMode 100 1.5 string_txt rest doc string_txt (Chr c) s = c:s string_txt (Str s1) s2 = s1 ++ s2 string_txt (PStr s1) s2 = s1 ++ s2 \end{code} \begin{code} 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 mode line_length ribbons_per_line txt end doc = display mode line_length ribbon_length txt end best_doc where best_doc = best 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 mode of { ZigZagMode -> maxBound; other -> line_length } display 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 (Nest k1 p) = lay (k + k1) p lay k 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 < 0 -> nl_text `txt` ( Str (multi_ch shift '\\') `txt` ( nl_text `txt` ( lay1 (k + shift) s sl p ))) other -> lay1 k s sl p lay1 k s sl p = Str (indent k) `txt` (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 k Empty = end in lay 0 doc }} cant_fail = error "easy_display: NoDoc" easy_display nl_text txt end doc = lay doc cant_fail where lay NoDoc no_doc = no_doc lay (Union p q) no_doc = {- lay p -} (lay q cant_fail) -- Second arg can't be NoDoc lay (Nest k p) no_doc = lay p no_doc lay Empty no_doc = end lay (NilAbove p) no_doc = nl_text `txt` lay p cant_fail -- NoDoc always on first line lay (TextBeside s sl p) no_doc = s `txt` lay p no_doc indent n | n >= 8 = '\t' : indent (n - 8) | otherwise = spaces n multi_ch 0 ch = "" multi_ch n ch = ch : multi_ch (n - 1) ch spaces 0 = "" spaces n = ' ' : spaces (n - 1) \end{code}