1 -----------------------------------------------------------------------------
3 -- Module : Text.PrettyPrint.HughesPJ
4 -- Copyright : (c) The University of Glasgow 2001
5 -- License : BSD-style (see the file libraries/base/LICENSE)
7 -- Maintainer : libraries@haskell.org
8 -- Stability : provisional
9 -- Portability : portable
11 -- John Hughes's and Simon Peyton Jones's Pretty Printer Combinators
13 -- Based on /The Design of a Pretty-printing Library/
14 -- in Advanced Functional Programming,
15 -- Johan Jeuring and Erik Meijer (eds), LNCS 925
16 -- <http://www.cs.chalmers.se/~rjmh/Papers/pretty.ps>
18 -- Heavily modified by Simon Peyton Jones, Dec 96
20 -----------------------------------------------------------------------------
23 Version 3.0 28 May 1997
24 * Cured massive performance bug. If you write
26 foldl <> empty (map (text.show) [1..10000])
28 you get quadratic behaviour with V2.0. Why? For just the same
29 reason as you get quadratic behaviour with left-associated (++)
32 This is really bad news. One thing a pretty-printer abstraction
33 should certainly guarantee is insensivity to associativity. It
34 matters: suddenly GHC's compilation times went up by a factor of
35 100 when I switched to the new pretty printer.
37 I fixed it with a bit of a hack (because I wanted to get GHC back
38 on the road). I added two new constructors to the Doc type, Above
44 Then, where I need to get to a "TextBeside" or "NilAbove" form I
45 "force" the Doc to squeeze out these suspended calls to Beside and
46 Above; but in so doing I re-associate. It's quite simple, but I'm
47 not satisfied that I've done the best possible job. I'll send you
48 the code if you are interested.
52 int, integer, float, double, rational,
53 lparen, rparen, lbrack, rbrack, lbrace, rbrace,
55 * fullRender's type signature has changed. Rather than producing a
56 string it now takes an extra couple of arguments that tells it how
57 to glue fragments of output together:
61 -> Float -- Ribbons per line
62 -> (TextDetails -> a -> a) -- What to do with text
63 -> a -- What to do at the end
67 The "fragments" are encapsulated in the TextDetails data type:
69 data TextDetails = Chr Char
73 The Chr and Str constructors are obvious enough. The PStr
74 constructor has a packed string (FAST_STRING) inside it. It's
75 generated by using the new "ptext" export.
77 An advantage of this new setup is that you can get the renderer to
78 do output directly (by passing in a function of type (TextDetails
79 -> IO () -> IO ()), rather than producing a string that you then
83 Version 2.0 24 April 1997
84 * Made empty into a left unit for <> as well as a right unit;
85 it is also now true that
87 which wasn't true before.
89 * Fixed an obscure bug in sep that occassionally gave very weird behaviour
93 * Corrected and tidied up the laws and invariants
95 ======================================================================
96 Relative to John's original paper, there are the following new features:
98 1. There's an empty document, "empty". It's a left and right unit for
99 both <> and $$, and anywhere in the argument list for
100 sep, hcat, hsep, vcat, fcat etc.
102 It is Really Useful in practice.
104 2. There is a paragraph-fill combinator, fsep, that's much like sep,
105 only it keeps fitting things on one line until it can't fit any more.
107 3. Some random useful extra combinators are provided.
108 <+> puts its arguments beside each other with a space between them,
109 unless either argument is empty in which case it returns the other
112 hcat is a list version of <>
113 hsep is a list version of <+>
114 vcat is a list version of $$
116 sep (separate) is either like hsep or like vcat, depending on what fits
118 cat behaves like sep, but it uses <> for horizontal conposition
119 fcat behaves like fsep, but it uses <> for horizontal conposition
121 These new ones do the obvious things:
122 char, semi, comma, colon, space,
123 parens, brackets, braces,
126 4. The "above" combinator, $$, now overlaps its two arguments if the
127 last line of the top argument stops before the first line of the
130 For example: text "hi" $$ nest 5 (text "there")
137 There are two places this is really useful
139 a) When making labelled blocks, like this:
140 Left -> code for left
141 Right -> code for right
143 code for longlonglonglabel
144 The block is on the same line as the label if the label is
145 short, but on the next line otherwise.
147 b) When laying out lists like this:
152 which some people like. But if the list fits on one line
153 you want [first, second, third]. You can't do this with
154 John's original combinators, but it's quite easy with the
157 The combinator $+$ gives the original "never-overlap" behaviour.
159 5. Several different renderers are provided:
161 * one that uses cut-marks to avoid deeply-nested documents
162 simply piling up in the right-hand margin
163 * one that ignores indentation (fewer chars output; good for machines)
164 * one that ignores indentation and newlines (ditto, only more so)
166 6. Numerous implementation tidy-ups
167 Use of unboxed data types to speed up the implementation
170 module Text.PrettyPrint.HughesPJ (
172 -- * The document type
175 -- * Constructing documents
176 -- ** Converting values into documents
178 int, integer, float, double, rational,
180 -- ** Simple derived documents
181 semi, comma, colon, space, equals,
182 lparen, rparen, lbrack, rbrack, lbrace, rbrace,
184 -- ** Wrapping documents in delimiters
185 parens, brackets, braces, quotes, doubleQuotes,
187 -- ** Combining documents
189 (<>), (<+>), hcat, hsep,
196 -- * Predicates on documents
199 -- * Rendering documents
201 -- ** Default rendering
204 -- ** Rendering with a particular style
209 -- ** General rendering
211 Mode(..), TextDetails(..),
222 -- ---------------------------------------------------------------------------
225 -- The primitive Doc values
227 isEmpty :: Doc -> Bool; -- ^ Returns 'True' if the document is empty
229 -- | The empty document, with no height and no width.
230 -- 'empty' is the identity for '<>', '<+>', '$$' and '$+$', and anywhere
231 -- in the argument list for 'sep', 'hcat', 'hsep', 'vcat', 'fcat' etc.
234 semi :: Doc; -- ^ A ';' character
235 comma :: Doc; -- ^ A ',' character
236 colon :: Doc; -- ^ A ':' character
237 space :: Doc; -- ^ A space character
238 equals :: Doc; -- ^ A '=' character
239 lparen :: Doc; -- ^ A '(' character
240 rparen :: Doc; -- ^ A ')' character
241 lbrack :: Doc; -- ^ A '[' character
242 rbrack :: Doc; -- ^ A ']' character
243 lbrace :: Doc; -- ^ A '{' character
244 rbrace :: Doc; -- ^ A '}' character
246 -- | A document of height and width 1, containing a literal character.
249 -- | A document of height 1 containing a literal string.
250 -- 'text' satisfies the following laws:
252 -- * @'text' s '<>' 'text' t = 'text' (s'++'t)@
254 -- * @'text' \"\" '<>' x = x@, if @x@ non-empty
256 -- The side condition on the last law is necessary because @'text' \"\"@
257 -- has height 1, while 'empty' has no height.
258 text :: String -> Doc
260 -- | An obsolete function, now identical to 'text'.
261 ptext :: String -> Doc
263 int :: Int -> Doc; -- ^ @int n = text (show n)@
264 integer :: Integer -> Doc; -- ^ @integer n = text (show n)@
265 float :: Float -> Doc; -- ^ @float n = text (show n)@
266 double :: Double -> Doc; -- ^ @double n = text (show n)@
267 rational :: Rational -> Doc; -- ^ @rational n = text (show n)@
269 parens :: Doc -> Doc; -- ^ Wrap document in @(...)@
270 brackets :: Doc -> Doc; -- ^ Wrap document in @[...]@
271 braces :: Doc -> Doc; -- ^ Wrap document in @{...}@
272 quotes :: Doc -> Doc; -- ^ Wrap document in @\'...\'@
273 doubleQuotes :: Doc -> Doc; -- ^ Wrap document in @\"...\"@
275 -- Combining @Doc@ values
278 -- '<>' is associative, with identity 'empty'.
279 (<>) :: Doc -> Doc -> Doc
281 -- | Beside, separated by space, unless one of the arguments is 'empty'.
282 -- '<+>' is associative, with identity 'empty'.
283 (<+>) :: Doc -> Doc -> Doc
285 -- | Above, except that if the last line of the first argument stops
286 -- at least one position before the first line of the second begins,
287 -- these two lines are overlapped. For example:
289 -- > text "hi" $$ nest 5 (text "there")
300 -- '$$' is associative, with identity 'empty', and also satisfies
302 -- * @(x '$$' y) '<>' z = x '$$' (y '<>' z)@, if @y@ non-empty.
304 ($$) :: Doc -> Doc -> Doc
306 -- | Above, with no overlapping.
307 -- '$+$' is associative, with identity 'empty'.
308 ($+$) :: Doc -> Doc -> Doc
310 hcat :: [Doc] -> Doc; -- ^List version of '<>'.
311 hsep :: [Doc] -> Doc; -- ^List version of '<+>'.
312 vcat :: [Doc] -> Doc; -- ^List version of '$$'.
314 cat :: [Doc] -> Doc; -- ^ Either 'hcat' or 'vcat'.
315 sep :: [Doc] -> Doc; -- ^ Either 'hsep' or 'vcat'.
316 fcat :: [Doc] -> Doc; -- ^ \"Paragraph fill\" version of 'cat'.
317 fsep :: [Doc] -> Doc; -- ^ \"Paragraph fill\" version of 'sep'.
319 -- | Nest (or indent) a document by a given number of positions
320 -- (which may also be negative). 'nest' satisfies the laws:
322 -- * @'nest' 0 x = x@
324 -- * @'nest' k ('nest' k' x) = 'nest' (k+k') x@
326 -- * @'nest' k (x '<>' y) = 'nest' k z '<>' 'nest' k y@
328 -- * @'nest' k (x '$$' y) = 'nest' k x '$$' 'nest' k y@
330 -- * @'nest' k 'empty' = 'empty'@
332 -- * @x '<>' 'nest' k y = x '<>' y@, if @x@ non-empty
334 -- The side condition on the last law is needed because
335 -- 'empty' is a left identity for '<>'.
336 nest :: Int -> Doc -> Doc
338 -- GHC-specific ones.
340 -- | @hang d1 n d2 = sep [d1, nest n d2]@
341 hang :: Doc -> Int -> Doc -> Doc
343 -- | @punctuate p [d1, ... dn] = [d1 \<> p, d2 \<> p, ... dn-1 \<> p, dn]@
344 punctuate :: Doc -> [Doc] -> [Doc]
347 -- Displaying @Doc@ values.
349 instance Show Doc where
350 showsPrec prec doc cont = showDoc doc cont
352 -- | Renders the document as a string using the default 'style'.
353 render :: Doc -> String
355 -- | The general rendering interface.
356 fullRender :: Mode -- ^Rendering mode
357 -> Int -- ^Line length
358 -> Float -- ^Ribbons per line
359 -> (TextDetails -> a -> a) -- ^What to do with text
360 -> a -- ^What to do at the end
361 -> Doc -- ^The document
364 -- | Render the document as a string using a specified style.
365 renderStyle :: Style -> Doc -> String
367 -- | A rendering style.
369 = Style { mode :: Mode -- ^ The rendering mode
370 , lineLength :: Int -- ^ Length of line, in chars
371 , ribbonsPerLine :: Float -- ^ Ratio of ribbon length to line length
374 -- | The default style (@mode=PageMode, lineLength=100, ribbonsPerLine=1.5@).
376 style = Style { lineLength = 100, ribbonsPerLine = 1.5, mode = PageMode }
379 data Mode = PageMode -- ^Normal
380 | ZigZagMode -- ^With zig-zag cuts
381 | LeftMode -- ^No indentation, infinitely long lines
382 | OneLineMode -- ^All on one line
384 -- ---------------------------------------------------------------------------
387 -- The Doc combinators satisfy the following laws:
392 <a1> (x $$ y) $$ z = x $$ (y $$ z)
400 <b1> (x <> y) <> z = x <> (y <> z)
401 <b2> empty <> x = empty
408 <t1> text s <> text t = text (s++t)
409 <t2> text "" <> x = x, if x non-empty
414 <n2> nest k (nest k' x) = nest (k+k') x
415 <n3> nest k (x <> y) = nest k z <> nest k y
416 <n4> nest k (x $$ y) = nest k x $$ nest k y
417 <n5> nest k empty = empty
418 <n6> x <> nest k y = x <> y, if x non-empty
420 ** Note the side condition on <n6>! It is this that
421 ** makes it OK for empty to be a left unit for <>.
425 <m1> (text s <> x) $$ y = text s <> ((text "" <> x)) $$
428 <m2> (x $$ y) <> z = x $$ (y <> z)
432 Laws for list versions
433 ~~~~~~~~~~~~~~~~~~~~~~
434 <l1> sep (ps++[empty]++qs) = sep (ps ++ qs)
435 ...ditto hsep, hcat, vcat, fill...
437 <l2> nest k (sep ps) = sep (map (nest k) ps)
438 ...ditto hsep, hcat, vcat, fill...
442 <o1> oneLiner (nest k p) = nest k (oneLiner p)
443 <o2> oneLiner (x <> y) = oneLiner x <> oneLiner y
445 You might think that the following verion of <m1> would
448 <3 NO> (text s <> x) $$ y = text s <> ((empty <> x)) $$
451 But it doesn't work, for if x=empty, we would have
453 text s $$ y = text s <> (empty $$ nest (-length s) y)
454 = text s <> nest (-length s) y
457 -- ---------------------------------------------------------------------------
458 -- Simple derived definitions
472 int n = text (show n)
473 integer n = text (show n)
474 float n = text (show n)
475 double n = text (show n)
476 rational n = text (show n)
477 -- SIGBJORN wrote instead:
478 -- rational n = text (show (fromRationalX n))
480 quotes p = char '\'' <> p <> char '\''
481 doubleQuotes p = char '"' <> p <> char '"'
482 parens p = char '(' <> p <> char ')'
483 brackets p = char '[' <> p <> char ']'
484 braces p = char '{' <> p <> char '}'
487 hcat = foldr (<>) empty
488 hsep = foldr (<+>) empty
489 vcat = foldr ($$) empty
491 hang d1 n d2 = sep [d1, nest n d2]
494 punctuate p (d:ds) = go d ds
497 go d (e:es) = (d <> p) : go e es
499 -- ---------------------------------------------------------------------------
502 -- A Doc represents a *set* of layouts. A Doc with
503 -- no occurrences of Union or NoDoc represents just one layout.
505 -- | The abstract type of documents.
506 -- The 'Show' instance is equivalent to using 'render'.
509 | NilAbove Doc -- text "" $$ x
510 | TextBeside TextDetails !Int Doc -- text s <> x
511 | Nest !Int Doc -- nest k x
512 | Union Doc Doc -- ul `union` ur
513 | NoDoc -- The empty set of documents
514 | Beside Doc Bool Doc -- True <=> space between
515 | Above Doc Bool Doc -- True <=> never overlap
517 type RDoc = Doc -- RDoc is a "reduced Doc", guaranteed not to have a top-level Above or Beside
520 reduceDoc :: Doc -> RDoc
521 reduceDoc (Beside p g q) = beside p g (reduceDoc q)
522 reduceDoc (Above p g q) = above p g (reduceDoc q)
526 data TextDetails = Chr Char
533 Here are the invariants:
535 * The argument of NilAbove is never Empty. Therefore
536 a NilAbove occupies at least two lines.
538 * The arugment of @TextBeside@ is never @Nest@.
541 * The layouts of the two arguments of @Union@ both flatten to the same
544 * The arguments of @Union@ are either @TextBeside@, or @NilAbove@.
546 * The right argument of a union cannot be equivalent to the empty set
547 (@NoDoc@). If the left argument of a union is equivalent to the
548 empty set (@NoDoc@), then the @NoDoc@ appears in the first line.
550 * An empty document is always represented by @Empty@. It can't be
551 hidden inside a @Nest@, or a @Union@ of two @Empty@s.
553 * The first line of every layout in the left argument of @Union@ is
554 longer than the first line of any layout in the right argument.
555 (1) ensures that the left argument has a first line. In view of
556 (3), this invariant means that the right argument must have at
560 -- Arg of a NilAbove is always an RDoc
561 nilAbove_ p = NilAbove p
563 -- Arg of a TextBeside is always an RDoc
564 textBeside_ s sl p = TextBeside s sl p
566 -- Arg of Nest is always an RDoc
569 -- Args of union are always RDocs
570 union_ p q = Union p q
573 -- Notice the difference between
574 -- * NoDoc (no documents)
575 -- * Empty (one empty document; no height and no width)
576 -- * text "" (a document containing the empty string;
577 -- one line high, but has no width)
580 -- ---------------------------------------------------------------------------
581 -- @empty@, @text@, @nest@, @union@
588 char c = textBeside_ (Chr c) 1 Empty
589 text s = case length s of {sl -> textBeside_ (Str s) sl Empty}
590 ptext s = case length s of {sl -> textBeside_ (PStr s) sl Empty}
592 nest k p = mkNest k (reduceDoc p) -- Externally callable version
594 -- mkNest checks for Nest's invariant that it doesn't have an Empty inside it
595 mkNest k _ | k `seq` False = undefined
596 mkNest k (Nest k1 p) = mkNest (k + k1) p
597 mkNest k NoDoc = NoDoc
598 mkNest k Empty = Empty
599 mkNest 0 p = p -- Worth a try!
600 mkNest k p = nest_ k p
602 -- mkUnion checks for an empty document
603 mkUnion Empty q = Empty
604 mkUnion p q = p `union_` q
606 -- ---------------------------------------------------------------------------
607 -- Vertical composition @$$@
609 above_ :: Doc -> Bool -> Doc -> Doc
612 above_ p g q = Above p g q
614 p $$ q = above_ p False q
615 p $+$ q = above_ p True q
617 above :: Doc -> Bool -> RDoc -> RDoc
618 above (Above p g1 q1) g2 q2 = above p g1 (above q1 g2 q2)
619 above p@(Beside _ _ _) g q = aboveNest (reduceDoc p) g 0 (reduceDoc q)
620 above p g q = aboveNest p g 0 (reduceDoc q)
622 aboveNest :: RDoc -> Bool -> Int -> RDoc -> RDoc
623 -- Specfication: aboveNest p g k q = p $g$ (nest k q)
625 aboveNest _ _ k _ | k `seq` False = undefined
626 aboveNest NoDoc g k q = NoDoc
627 aboveNest (p1 `Union` p2) g k q = aboveNest p1 g k q `union_`
630 aboveNest Empty g k q = mkNest k q
631 aboveNest (Nest k1 p) g k q = nest_ k1 (aboveNest p g (k - k1) q)
632 -- p can't be Empty, so no need for mkNest
634 aboveNest (NilAbove p) g k q = nilAbove_ (aboveNest p g k q)
635 aboveNest (TextBeside s sl p) g k q = k1 `seq` textBeside_ s sl rest
639 Empty -> nilAboveNest g k1 q
640 other -> aboveNest p g k1 q
643 nilAboveNest :: Bool -> Int -> RDoc -> RDoc
644 -- Specification: text s <> nilaboveNest g k q
645 -- = text s <> (text "" $g$ nest k q)
647 nilAboveNest _ k _ | k `seq` False = undefined
648 nilAboveNest g k Empty = Empty -- Here's why the "text s <>" is in the spec!
649 nilAboveNest g k (Nest k1 q) = nilAboveNest g (k + k1) q
651 nilAboveNest g k q | (not g) && (k > 0) -- No newline if no overlap
652 = textBeside_ (Str (spaces k)) k q
653 | otherwise -- Put them really above
654 = nilAbove_ (mkNest k q)
656 -- ---------------------------------------------------------------------------
657 -- Horizontal composition @<>@
659 beside_ :: Doc -> Bool -> Doc -> Doc
660 beside_ p _ Empty = p
661 beside_ Empty _ q = q
662 beside_ p g q = Beside p g q
664 p <> q = beside_ p False q
665 p <+> q = beside_ p True q
667 beside :: Doc -> Bool -> RDoc -> RDoc
668 -- Specification: beside g p q = p <g> q
670 beside NoDoc g q = NoDoc
671 beside (p1 `Union` p2) g q = (beside p1 g q) `union_` (beside p2 g q)
673 beside (Nest k p) g q = nest_ k (beside p g q) -- p non-empty
674 beside p@(Beside p1 g1 q1) g2 q2
675 {- (A `op1` B) `op2` C == A `op1` (B `op2` C) iff op1 == op2
676 [ && (op1 == <> || op1 == <+>) ] -}
677 | g1 == g2 = beside p1 g1 (beside q1 g2 q2)
678 | otherwise = beside (reduceDoc p) g2 q2
679 beside p@(Above _ _ _) g q = beside (reduceDoc p) g q
680 beside (NilAbove p) g q = nilAbove_ (beside p g q)
681 beside (TextBeside s sl p) g q = textBeside_ s sl rest
684 Empty -> nilBeside g q
685 other -> beside p g q
688 nilBeside :: Bool -> RDoc -> RDoc
689 -- Specification: text "" <> nilBeside g p
692 nilBeside g Empty = Empty -- Hence the text "" in the spec
693 nilBeside g (Nest _ p) = nilBeside g p
694 nilBeside g p | g = textBeside_ space_text 1 p
697 -- ---------------------------------------------------------------------------
698 -- Separate, @sep@, Hughes version
700 -- Specification: sep ps = oneLiner (hsep ps)
704 sep = sepX True -- Separate with spaces
705 cat = sepX False -- Don't
708 sepX x (p:ps) = sep1 x (reduceDoc p) 0 ps
711 -- Specification: sep1 g k ys = sep (x : map (nest k) ys)
712 -- = oneLiner (x <g> nest k (hsep ys))
713 -- `union` x $$ nest k (vcat ys)
715 sep1 :: Bool -> RDoc -> Int -> [Doc] -> RDoc
716 sep1 g _ k ys | k `seq` False = undefined
717 sep1 g NoDoc k ys = NoDoc
718 sep1 g (p `Union` q) k ys = sep1 g p k ys
720 (aboveNest q False k (reduceDoc (vcat ys)))
722 sep1 g Empty k ys = mkNest k (sepX g ys)
723 sep1 g (Nest n p) k ys = nest_ n (sep1 g p (k - n) ys)
725 sep1 g (NilAbove p) k ys = nilAbove_ (aboveNest p False k (reduceDoc (vcat ys)))
726 sep1 g (TextBeside s sl p) k ys = textBeside_ s sl (sepNB g p (k - sl) ys)
728 -- Specification: sepNB p k ys = sep1 (text "" <> p) k ys
729 -- Called when we have already found some text in the first item
730 -- We have to eat up nests
732 sepNB g (Nest _ p) k ys = sepNB g p k ys
734 sepNB g Empty k ys = oneLiner (nilBeside g (reduceDoc rest))
736 nilAboveNest False k (reduceDoc (vcat ys))
739 | otherwise = hcat ys
741 sepNB g p k ys = sep1 g p k ys
743 -- ---------------------------------------------------------------------------
752 -- fill (p1:p2:ps) = oneLiner p1 <#> nest (length p1)
753 -- (fill (oneLiner p2 : ps))
758 fill g (p:ps) = fill1 g (reduceDoc p) 0 ps
761 fill1 :: Bool -> RDoc -> Int -> [Doc] -> Doc
762 fill1 g _ k ys | k `seq` False = undefined
763 fill1 g NoDoc k ys = NoDoc
764 fill1 g (p `Union` q) k ys = fill1 g p k ys
766 (aboveNest q False k (fill g ys))
768 fill1 g Empty k ys = mkNest k (fill g ys)
769 fill1 g (Nest n p) k ys = nest_ n (fill1 g p (k - n) ys)
771 fill1 g (NilAbove p) k ys = nilAbove_ (aboveNest p False k (fill g ys))
772 fill1 g (TextBeside s sl p) k ys = textBeside_ s sl (fillNB g p (k - sl) ys)
774 fillNB g _ k ys | k `seq` False = undefined
775 fillNB g (Nest _ p) k ys = fillNB g p k ys
776 fillNB g Empty k [] = Empty
777 fillNB g Empty k (y:ys) = nilBeside g (fill1 g (oneLiner (reduceDoc y)) k1 ys)
779 nilAboveNest False k (fill g (y:ys))
784 fillNB g p k ys = fill1 g p k ys
787 -- ---------------------------------------------------------------------------
788 -- Selecting the best layout
791 -> Int -- Line length
792 -> Int -- Ribbon length
794 -> RDoc -- No unions in here!
796 best OneLineMode w r p
801 get (NilAbove p) = nilAbove_ (get p)
802 get (TextBeside s sl p) = textBeside_ s sl (get p)
803 get (Nest k p) = get p -- Elide nest
804 get (p `Union` q) = first (get p) (get q)
809 get :: Int -- (Remaining) width of line
811 get w _ | w==0 && False = undefined
814 get w (NilAbove p) = nilAbove_ (get w p)
815 get w (TextBeside s sl p) = textBeside_ s sl (get1 w sl p)
816 get w (Nest k p) = nest_ k (get (w - k) p)
817 get w (p `Union` q) = nicest w r (get w p) (get w q)
819 get1 :: Int -- (Remaining) width of line
820 -> Int -- Amount of first line already eaten up
821 -> Doc -- This is an argument to TextBeside => eat Nests
822 -> Doc -- No unions in here!
824 get1 w _ _ | w==0 && False = undefined
825 get1 w sl Empty = Empty
826 get1 w sl NoDoc = NoDoc
827 get1 w sl (NilAbove p) = nilAbove_ (get (w - sl) p)
828 get1 w sl (TextBeside t tl p) = textBeside_ t tl (get1 w (sl + tl) p)
829 get1 w sl (Nest k p) = get1 w sl p
830 get1 w sl (p `Union` q) = nicest1 w r sl (get1 w sl p)
833 nicest w r p q = nicest1 w r 0 p q
834 nicest1 w r sl p q | fits ((w `minn` r) - sl) p = p
837 fits :: Int -- Space available
839 -> Bool -- True if *first line* of Doc fits in space available
841 fits n p | n < 0 = False
844 fits n (NilAbove _) = True
845 fits n (TextBeside _ sl p) = fits (n - sl) p
850 -- @first@ and @nonEmptySet@ are similar to @nicest@ and @fits@, only simpler.
851 -- @first@ returns its first argument if it is non-empty, otherwise its second.
853 first p q | nonEmptySet p = p
856 nonEmptySet NoDoc = False
857 nonEmptySet (p `Union` q) = True
858 nonEmptySet Empty = True
859 nonEmptySet (NilAbove p) = True -- NoDoc always in first line
860 nonEmptySet (TextBeside _ _ p) = nonEmptySet p
861 nonEmptySet (Nest _ p) = nonEmptySet p
863 -- @oneLiner@ returns the one-line members of the given set of @Doc@s.
865 oneLiner :: Doc -> Doc
866 oneLiner NoDoc = NoDoc
867 oneLiner Empty = Empty
868 oneLiner (NilAbove p) = NoDoc
869 oneLiner (TextBeside s sl p) = textBeside_ s sl (oneLiner p)
870 oneLiner (Nest k p) = nest_ k (oneLiner p)
871 oneLiner (p `Union` q) = oneLiner p
874 -- ---------------------------------------------------------------------------
875 -- Displaying the best layout
877 renderStyle style doc
878 = fullRender (mode style)
880 (ribbonsPerLine style)
885 render doc = showDoc doc ""
886 showDoc doc rest = fullRender PageMode 100 1.5 string_txt rest doc
888 string_txt (Chr c) s = c:s
889 string_txt (Str s1) s2 = s1 ++ s2
890 string_txt (PStr s1) s2 = s1 ++ s2
893 fullRender OneLineMode _ _ txt end doc = easy_display space_text txt end (reduceDoc doc)
894 fullRender LeftMode _ _ txt end doc = easy_display nl_text txt end (reduceDoc doc)
896 fullRender mode line_length ribbons_per_line txt end doc
897 = display mode line_length ribbon_length txt end best_doc
899 best_doc = best mode hacked_line_length ribbon_length (reduceDoc doc)
901 hacked_line_length, ribbon_length :: Int
902 ribbon_length = round (fromIntegral line_length / ribbons_per_line)
903 hacked_line_length = case mode of { ZigZagMode -> maxBound; other -> line_length }
905 display mode page_width ribbon_width txt end doc
906 = case page_width - ribbon_width of { gap_width ->
907 case gap_width `quot` 2 of { shift ->
909 lay k _ | k `seq` False = undefined
910 lay k (Nest k1 p) = lay (k + k1) p
913 lay k (NilAbove p) = nl_text `txt` lay k p
915 lay k (TextBeside s sl p)
917 ZigZagMode | k >= gap_width
919 Str (multi_ch shift '/') `txt` (
921 lay1 (k - shift) s sl p)))
925 Str (multi_ch shift '\\') `txt` (
927 lay1 (k + shift) s sl p )))
929 other -> lay1 k s sl p
931 lay1 k _ sl _ | k+sl `seq` False = undefined
932 lay1 k s sl p = Str (indent k) `txt` (s `txt` lay2 (k + sl) p)
934 lay2 k _ | k `seq` False = undefined
935 lay2 k (NilAbove p) = nl_text `txt` lay k p
936 lay2 k (TextBeside s sl p) = s `txt` (lay2 (k + sl) p)
937 lay2 k (Nest _ p) = lay2 k p
943 cant_fail = error "easy_display: NoDoc"
944 easy_display nl_text txt end doc
947 lay NoDoc no_doc = no_doc
948 lay (Union p q) no_doc = {- lay p -} (lay q cant_fail) -- Second arg can't be NoDoc
949 lay (Nest k p) no_doc = lay p no_doc
950 lay Empty no_doc = end
951 lay (NilAbove p) no_doc = nl_text `txt` lay p cant_fail -- NoDoc always on first line
952 lay (TextBeside s sl p) no_doc = s `txt` lay p no_doc
954 -- OLD version: we shouldn't rely on tabs being 8 columns apart in the output.
955 -- indent n | n >= 8 = '\t' : indent (n - 8)
956 -- | otherwise = spaces n
960 multi_ch n ch = ch : multi_ch (n - 1) ch
962 -- (spaces n) generates a list of n spaces
964 -- It should never be called with 'n' < 0, but that can happen for reasons I don't understand
965 -- Here's a test case:
966 -- ncat x y = nest 4 $ cat [ x, y ]
967 -- d1 = foldl1 ncat $ take 50 $ repeat $ char 'a'
968 -- d2 = parens $ sep [ d1, text "+" , d1 ]
970 -- I don't feel motivated enough to find the Real Bug, so meanwhile we just test for n<=0
971 spaces n | n <= 0 = ""
972 | otherwise = ' ' : spaces (n - 1)
974 {- Comments from Johannes Waldmann about what the problem might be:
976 In the example above, d2 and d1 are deeply nested, but `text "+"' is not,
977 so the layout function tries to "out-dent" it.
979 when I look at the Doc values that are generated, there are lots of
980 Nest constructors with negative arguments. see this sample output of
981 d1 (obtained with hugs, :s -u)
983 tBeside (TextDetails_Chr 'a') 1 Doc_Empty) (Doc_NilAbove (Doc_Nest
984 (-241) (Doc_TextBeside (TextDetails_Chr 'a') 1 Doc_Empty)))))
985 (Doc_NilAbove (Doc_Nest (-236) (Doc_TextBeside (TextDetails_Chr 'a') 1
986 (Doc_NilAbove (Doc_Nest (-5) (Doc_TextBeside (TextDetails_Chr 'a') 1
987 Doc_Empty)))))))) (Doc_NilAbove (Doc_Nest (-231) (Doc_TextBeside
988 (TextDetails_Chr 'a') 1 (Doc_NilAbove (Doc_Nest (-5) (Doc_TextBeside
989 (TextDetails_Chr 'a') 1 (Doc_NilAbove (Doc_Nest (-5) (Doc_TextBeside
990 (TextDetails_Chr 'a') 1 Doc_Empty))))))))))) (Doc_NilAbove (Doc_Nest