2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[Util]{Highly random utility functions}
7 -- IF_NOT_GHC is meant to make this module useful outside the context of GHC
13 Eager, thenEager, returnEager, mapEager, appEager, runEager,
16 -- general list processing
17 zipEqual, zipWithEqual, zipWith3Equal, zipWith4Equal,
18 zipLazy, stretchZipWith,
19 mapAndUnzip, mapAndUnzip3,
21 lengthExceeds, lengthIs, lengthAtLeast, listLengthCmp, atLength,
33 IF_NOT_GHC(quicksort COMMA stableSortLt COMMA mergesort COMMA)
35 IF_NOT_GHC(mergeSort COMMA) naturalMergeSortLe, -- from Carsten
36 IF_NOT_GHC(naturalMergeSort COMMA mergeSortLe COMMA)
38 -- transitive closures
42 mapAccumL, mapAccumR, mapAccumB,
45 takeList, dropList, splitAtList,
48 eqListBy, equalLength, compareLength,
49 thenCmp, cmpList, prefixMatch, suffixMatch,
55 IF_NOT_GHC(cfst COMMA applyToPair COMMA applyToFst COMMA)
56 IF_NOT_GHC(applyToSnd COMMA foldPair COMMA)
61 #if __GLASGOW_HASKELL__ <= 408
69 #include "../includes/config.h"
70 #include "HsVersions.h"
72 import qualified List ( elem, notElem )
73 import List ( zipWith4 )
74 import Maybe ( Maybe(..) )
75 import Panic ( panic, trace )
76 import IOExts ( IORef, newIORef, unsafePerformIO )
78 #if __GLASGOW_HASKELL__ <= 408
79 import Exception ( catchIO, justIoErrors, raiseInThread )
85 %************************************************************************
87 \subsection{The Eager monad}
89 %************************************************************************
91 The @Eager@ monad is just an encoding of continuation-passing style,
92 used to allow you to express "do this and then that", mainly to avoid
93 space leaks. It's done with a type synonym to save bureaucracy.
98 type Eager ans a = (a -> ans) -> ans
100 runEager :: Eager a a -> a
101 runEager m = m (\x -> x)
103 appEager :: Eager ans a -> (a -> ans) -> ans
104 appEager m cont = m cont
106 thenEager :: Eager ans a -> (a -> Eager ans b) -> Eager ans b
107 thenEager m k cont = m (\r -> k r cont)
109 returnEager :: a -> Eager ans a
110 returnEager v cont = cont v
112 mapEager :: (a -> Eager ans b) -> [a] -> Eager ans [b]
113 mapEager f [] = returnEager []
114 mapEager f (x:xs) = f x `thenEager` \ y ->
115 mapEager f xs `thenEager` \ ys ->
120 %************************************************************************
122 \subsection{A for loop}
124 %************************************************************************
127 -- Compose a function with itself n times. (nth rather than twice)
128 nTimes :: Int -> (a -> a) -> (a -> a)
131 nTimes n f = f . nTimes (n-1) f
134 %************************************************************************
136 \subsection{Maybe-ery}
138 %************************************************************************
141 unJust :: String -> Maybe a -> a
142 unJust who (Just x) = x
143 unJust who Nothing = panic ("unJust of Nothing, called by " ++ who)
146 %************************************************************************
148 \subsection[Utils-lists]{General list processing}
150 %************************************************************************
152 A paranoid @zip@ (and some @zipWith@ friends) that checks the lists
153 are of equal length. Alastair Reid thinks this should only happen if
154 DEBUGging on; hey, why not?
157 zipEqual :: String -> [a] -> [b] -> [(a,b)]
158 zipWithEqual :: String -> (a->b->c) -> [a]->[b]->[c]
159 zipWith3Equal :: String -> (a->b->c->d) -> [a]->[b]->[c]->[d]
160 zipWith4Equal :: String -> (a->b->c->d->e) -> [a]->[b]->[c]->[d]->[e]
164 zipWithEqual _ = zipWith
165 zipWith3Equal _ = zipWith3
166 zipWith4Equal _ = zipWith4
168 zipEqual msg [] [] = []
169 zipEqual msg (a:as) (b:bs) = (a,b) : zipEqual msg as bs
170 zipEqual msg as bs = panic ("zipEqual: unequal lists:"++msg)
172 zipWithEqual msg z (a:as) (b:bs)= z a b : zipWithEqual msg z as bs
173 zipWithEqual msg _ [] [] = []
174 zipWithEqual msg _ _ _ = panic ("zipWithEqual: unequal lists:"++msg)
176 zipWith3Equal msg z (a:as) (b:bs) (c:cs)
177 = z a b c : zipWith3Equal msg z as bs cs
178 zipWith3Equal msg _ [] [] [] = []
179 zipWith3Equal msg _ _ _ _ = panic ("zipWith3Equal: unequal lists:"++msg)
181 zipWith4Equal msg z (a:as) (b:bs) (c:cs) (d:ds)
182 = z a b c d : zipWith4Equal msg z as bs cs ds
183 zipWith4Equal msg _ [] [] [] [] = []
184 zipWith4Equal msg _ _ _ _ _ = panic ("zipWith4Equal: unequal lists:"++msg)
189 -- zipLazy is lazy in the second list (observe the ~)
191 zipLazy :: [a] -> [b] -> [(a,b)]
193 zipLazy (x:xs) ~(y:ys) = (x,y) : zipLazy xs ys
198 stretchZipWith :: (a -> Bool) -> b -> (a->b->c) -> [a] -> [b] -> [c]
199 -- (stretchZipWith p z f xs ys) stretches ys by inserting z in
200 -- the places where p returns *True*
202 stretchZipWith p z f [] ys = []
203 stretchZipWith p z f (x:xs) ys
204 | p x = f x z : stretchZipWith p z f xs ys
205 | otherwise = case ys of
207 (y:ys) -> f x y : stretchZipWith p z f xs ys
212 mapAndUnzip :: (a -> (b, c)) -> [a] -> ([b], [c])
214 mapAndUnzip f [] = ([],[])
218 (rs1, rs2) = mapAndUnzip f xs
222 mapAndUnzip3 :: (a -> (b, c, d)) -> [a] -> ([b], [c], [d])
224 mapAndUnzip3 f [] = ([],[],[])
225 mapAndUnzip3 f (x:xs)
228 (rs1, rs2, rs3) = mapAndUnzip3 f xs
230 (r1:rs1, r2:rs2, r3:rs3)
234 nOfThem :: Int -> a -> [a]
235 nOfThem n thing = replicate n thing
237 -- 'atLength atLen atEnd ls n' unravels list 'ls' to position 'n';
240 -- atLength atLenPred atEndPred ls n
241 -- | n < 0 = atLenPred n
242 -- | length ls < n = atEndPred (n - length ls)
243 -- | otherwise = atLenPred (drop n ls)
245 atLength :: ([a] -> b)
250 atLength atLenPred atEndPred ls n
251 | n < 0 = atEndPred n
252 | otherwise = go n ls
254 go n [] = atEndPred n
255 go 0 ls = atLenPred ls
256 go n (_:xs) = go (n-1) xs
259 lengthExceeds :: [a] -> Int -> Bool
260 -- (lengthExceeds xs n) = (length xs > n)
261 lengthExceeds = atLength (not.null) (const False)
263 lengthAtLeast :: [a] -> Int -> Bool
264 lengthAtLeast = atLength (not.null) (== 0)
266 lengthIs :: [a] -> Int -> Bool
267 lengthIs = atLength null (==0)
269 listLengthCmp :: [a] -> Int -> Ordering
270 listLengthCmp = atLength atLen atEnd
274 | x > 0 = LT -- not yet seen 'n' elts, so list length is < n.
280 isSingleton :: [a] -> Bool
281 isSingleton [x] = True
282 isSingleton _ = False
293 snocView :: [a] -> ([a], a) -- Split off the last element
294 snocView xs = go xs []
296 go [x] acc = (reverse acc, x)
297 go (x:xs) acc = go xs (x:acc)
300 Debugging/specialising versions of \tr{elem} and \tr{notElem}
303 isIn, isn'tIn :: (Eq a) => String -> a -> [a] -> Bool
306 isIn msg x ys = elem__ x ys
307 isn'tIn msg x ys = notElem__ x ys
309 --these are here to be SPECIALIZEd (automagically)
311 elem__ x (y:ys) = x==y || elem__ x ys
313 notElem__ x [] = True
314 notElem__ x (y:ys) = x /= y && notElem__ x ys
318 = elem (_ILIT 0) x ys
322 | i ># _ILIT 100 = trace ("Over-long elem in " ++ msg) $
324 | otherwise = x == y || elem (i +# _ILIT(1)) x ys
327 = notElem (_ILIT 0) x ys
329 notElem i x [] = True
331 | i ># _ILIT 100 = trace ("Over-long notElem in " ++ msg) $
332 x `List.notElem` (y:ys)
333 | otherwise = x /= y && notElem (i +# _ILIT(1)) x ys
337 %************************************************************************
339 \subsection[Utils-sorting]{Sorting}
341 %************************************************************************
343 %************************************************************************
345 \subsubsection[Utils-quicksorting]{Quicksorts}
347 %************************************************************************
352 -- tail-recursive, etc., "quicker sort" [as per Meira thesis]
353 quicksort :: (a -> a -> Bool) -- Less-than predicate
355 -> [a] -- Result list in increasing order
358 quicksort lt [x] = [x]
359 quicksort lt (x:xs) = split x [] [] xs
361 split x lo hi [] = quicksort lt lo ++ (x : quicksort lt hi)
362 split x lo hi (y:ys) | y `lt` x = split x (y:lo) hi ys
363 | True = split x lo (y:hi) ys
367 Quicksort variant from Lennart's Haskell-library contribution. This
368 is a {\em stable} sort.
371 stableSortLt = sortLt -- synonym; when we want to highlight stable-ness
373 sortLt :: (a -> a -> Bool) -- Less-than predicate
375 -> [a] -- Result list
377 sortLt lt l = qsort lt l []
379 -- qsort is stable and does not concatenate.
380 qsort :: (a -> a -> Bool) -- Less-than predicate
381 -> [a] -- xs, Input list
382 -> [a] -- r, Concatenate this list to the sorted input list
383 -> [a] -- Result = sort xs ++ r
387 qsort lt (x:xs) r = qpart lt x xs [] [] r
389 -- qpart partitions and sorts the sublists
390 -- rlt contains things less than x,
391 -- rge contains the ones greater than or equal to x.
392 -- Both have equal elements reversed with respect to the original list.
394 qpart lt x [] rlt rge r =
395 -- rlt and rge are in reverse order and must be sorted with an
396 -- anti-stable sorting
397 rqsort lt rlt (x : rqsort lt rge r)
399 qpart lt x (y:ys) rlt rge r =
402 qpart lt x ys (y:rlt) rge r
405 qpart lt x ys rlt (y:rge) r
407 -- rqsort is as qsort but anti-stable, i.e. reverses equal elements
409 rqsort lt [x] r = x:r
410 rqsort lt (x:xs) r = rqpart lt x xs [] [] r
412 rqpart lt x [] rle rgt r =
413 qsort lt rle (x : qsort lt rgt r)
415 rqpart lt x (y:ys) rle rgt r =
418 rqpart lt x ys rle (y:rgt) r
421 rqpart lt x ys (y:rle) rgt r
424 %************************************************************************
426 \subsubsection[Utils-dull-mergesort]{A rather dull mergesort}
428 %************************************************************************
432 mergesort :: (a -> a -> Ordering) -> [a] -> [a]
434 mergesort cmp xs = merge_lists (split_into_runs [] xs)
436 a `le` b = case cmp a b of { LT -> True; EQ -> True; GT -> False }
437 a `ge` b = case cmp a b of { LT -> False; EQ -> True; GT -> True }
439 split_into_runs [] [] = []
440 split_into_runs run [] = [run]
441 split_into_runs [] (x:xs) = split_into_runs [x] xs
442 split_into_runs [r] (x:xs) | x `ge` r = split_into_runs [r,x] xs
443 split_into_runs rl@(r:rs) (x:xs) | x `le` r = split_into_runs (x:rl) xs
444 | True = rl : (split_into_runs [x] xs)
447 merge_lists (x:xs) = merge x (merge_lists xs)
451 merge xl@(x:xs) yl@(y:ys)
453 EQ -> x : y : (merge xs ys)
454 LT -> x : (merge xs yl)
455 GT -> y : (merge xl ys)
459 %************************************************************************
461 \subsubsection[Utils-Carsten-mergesort]{A mergesort from Carsten}
463 %************************************************************************
466 Date: Mon, 3 May 93 20:45:23 +0200
467 From: Carsten Kehler Holst <kehler@cs.chalmers.se>
468 To: partain@dcs.gla.ac.uk
469 Subject: natural merge sort beats quick sort [ and it is prettier ]
471 Here is a piece of Haskell code that I'm rather fond of. See it as an
472 attempt to get rid of the ridiculous quick-sort routine. group is
473 quite useful by itself I think it was John's idea originally though I
474 believe the lazy version is due to me [surprisingly complicated].
475 gamma [used to be called] is called gamma because I got inspired by
476 the Gamma calculus. It is not very close to the calculus but does
477 behave less sequentially than both foldr and foldl. One could imagine
478 a version of gamma that took a unit element as well thereby avoiding
479 the problem with empty lists.
481 I've tried this code against
483 1) insertion sort - as provided by haskell
484 2) the normal implementation of quick sort
485 3) a deforested version of quick sort due to Jan Sparud
486 4) a super-optimized-quick-sort of Lennart's
488 If the list is partially sorted both merge sort and in particular
489 natural merge sort wins. If the list is random [ average length of
490 rising subsequences = approx 2 ] mergesort still wins and natural
491 merge sort is marginally beaten by Lennart's soqs. The space
492 consumption of merge sort is a bit worse than Lennart's quick sort
493 approx a factor of 2. And a lot worse if Sparud's bug-fix [see his
494 fpca article ] isn't used because of group.
501 group :: (a -> a -> Bool) -> [a] -> [[a]]
504 Date: Mon, 12 Feb 1996 15:09:41 +0000
505 From: Andy Gill <andy@dcs.gla.ac.uk>
507 Here is a `better' definition of group.
510 group p (x:xs) = group' xs x x (x :)
512 group' [] _ _ s = [s []]
513 group' (x:xs) x_min x_max s
514 | not (x `p` x_max) = group' xs x_min x (s . (x :))
515 | x `p` x_min = group' xs x x_max ((x :) . s)
516 | otherwise = s [] : group' xs x x (x :)
518 -- This one works forwards *and* backwards, as well as also being
519 -- faster that the one in Util.lhs.
524 let ((h1:t1):tt1) = group p xs
525 (t,tt) = if null xs then ([],[]) else
526 if x `p` h1 then (h1:t1,tt1) else
531 generalMerge :: (a -> a -> Bool) -> [a] -> [a] -> [a]
532 generalMerge p xs [] = xs
533 generalMerge p [] ys = ys
534 generalMerge p (x:xs) (y:ys) | x `p` y = x : generalMerge p xs (y:ys)
535 | otherwise = y : generalMerge p (x:xs) ys
537 -- gamma is now called balancedFold
539 balancedFold :: (a -> a -> a) -> [a] -> a
540 balancedFold f [] = error "can't reduce an empty list using balancedFold"
541 balancedFold f [x] = x
542 balancedFold f l = balancedFold f (balancedFold' f l)
544 balancedFold' :: (a -> a -> a) -> [a] -> [a]
545 balancedFold' f (x:y:xs) = f x y : balancedFold' f xs
546 balancedFold' f xs = xs
548 generalMergeSort p [] = []
549 generalMergeSort p xs = (balancedFold (generalMerge p) . map (: [])) xs
551 generalNaturalMergeSort p [] = []
552 generalNaturalMergeSort p xs = (balancedFold (generalMerge p) . group p) xs
554 mergeSort, naturalMergeSort :: Ord a => [a] -> [a]
556 mergeSort = generalMergeSort (<=)
557 naturalMergeSort = generalNaturalMergeSort (<=)
559 mergeSortLe le = generalMergeSort le
560 naturalMergeSortLe le = generalNaturalMergeSort le
563 %************************************************************************
565 \subsection[Utils-transitive-closure]{Transitive closure}
567 %************************************************************************
569 This algorithm for transitive closure is straightforward, albeit quadratic.
572 transitiveClosure :: (a -> [a]) -- Successor function
573 -> (a -> a -> Bool) -- Equality predicate
575 -> [a] -- The transitive closure
577 transitiveClosure succ eq xs
581 go done (x:xs) | x `is_in` done = go done xs
582 | otherwise = go (x:done) (succ x ++ xs)
585 x `is_in` (y:ys) | eq x y = True
586 | otherwise = x `is_in` ys
589 %************************************************************************
591 \subsection[Utils-accum]{Accumulating}
593 %************************************************************************
595 @mapAccumL@ behaves like a combination
596 of @map@ and @foldl@;
597 it applies a function to each element of a list, passing an accumulating
598 parameter from left to right, and returning a final value of this
599 accumulator together with the new list.
602 mapAccumL :: (acc -> x -> (acc, y)) -- Function of elt of input list
603 -- and accumulator, returning new
604 -- accumulator and elt of result list
605 -> acc -- Initial accumulator
607 -> (acc, [y]) -- Final accumulator and result list
609 mapAccumL f b [] = (b, [])
610 mapAccumL f b (x:xs) = (b'', x':xs') where
612 (b'', xs') = mapAccumL f b' xs
615 @mapAccumR@ does the same, but working from right to left instead. Its type is
616 the same as @mapAccumL@, though.
619 mapAccumR :: (acc -> x -> (acc, y)) -- Function of elt of input list
620 -- and accumulator, returning new
621 -- accumulator and elt of result list
622 -> acc -- Initial accumulator
624 -> (acc, [y]) -- Final accumulator and result list
626 mapAccumR f b [] = (b, [])
627 mapAccumR f b (x:xs) = (b'', x':xs') where
629 (b', xs') = mapAccumR f b xs
632 Here is the bi-directional version, that works from both left and right.
635 mapAccumB :: (accl -> accr -> x -> (accl, accr,y))
636 -- Function of elt of input list
637 -- and accumulator, returning new
638 -- accumulator and elt of result list
639 -> accl -- Initial accumulator from left
640 -> accr -- Initial accumulator from right
642 -> (accl, accr, [y]) -- Final accumulators and result list
644 mapAccumB f a b [] = (a,b,[])
645 mapAccumB f a b (x:xs) = (a'',b'',y:ys)
647 (a',b'',y) = f a b' x
648 (a'',b',ys) = mapAccumB f a' b xs
651 A strict version of foldl.
654 foldl' :: (a -> b -> a) -> a -> [b] -> a
655 foldl' f z xs = lgo z xs
658 lgo z (x:xs) = (lgo $! (f z x)) xs
661 A combination of foldl with zip. It works with equal length lists.
664 foldl2 :: (acc -> a -> b -> acc) -> acc -> [a] -> [b] -> acc
666 foldl2 k z (a:as) (b:bs) = foldl2 k (k z a b) as bs
669 Count the number of times a predicate is true
672 count :: (a -> Bool) -> [a] -> Int
674 count p (x:xs) | p x = 1 + count p xs
675 | otherwise = count p xs
678 @splitAt@, @take@, and @drop@ but with length of another
679 list giving the break-off point:
682 takeList :: [b] -> [a] -> [a]
687 (y:ys) -> y : takeList xs ys
689 dropList :: [b] -> [a] -> [a]
691 dropList _ xs@[] = xs
692 dropList (_:xs) (_:ys) = dropList xs ys
695 splitAtList :: [b] -> [a] -> ([a], [a])
696 splitAtList [] xs = ([], xs)
697 splitAtList _ xs@[] = (xs, xs)
698 splitAtList (_:xs) (y:ys) = (y:ys', ys'')
700 (ys', ys'') = splitAtList xs ys
705 %************************************************************************
707 \subsection[Utils-comparison]{Comparisons}
709 %************************************************************************
712 eqListBy :: (a->a->Bool) -> [a] -> [a] -> Bool
713 eqListBy eq [] [] = True
714 eqListBy eq (x:xs) (y:ys) = eq x y && eqListBy eq xs ys
715 eqListBy eq xs ys = False
717 equalLength :: [a] -> [b] -> Bool
718 equalLength [] [] = True
719 equalLength (_:xs) (_:ys) = equalLength xs ys
720 equalLength xs ys = False
722 compareLength :: [a] -> [b] -> Ordering
723 compareLength [] [] = EQ
724 compareLength (_:xs) (_:ys) = compareLength xs ys
725 compareLength [] _ys = LT
726 compareLength _xs [] = GT
728 thenCmp :: Ordering -> Ordering -> Ordering
729 {-# INLINE thenCmp #-}
731 thenCmp other any = other
733 cmpList :: (a -> a -> Ordering) -> [a] -> [a] -> Ordering
734 -- `cmpList' uses a user-specified comparer
736 cmpList cmp [] [] = EQ
737 cmpList cmp [] _ = LT
738 cmpList cmp _ [] = GT
739 cmpList cmp (a:as) (b:bs)
740 = case cmp a b of { EQ -> cmpList cmp as bs; xxx -> xxx }
744 prefixMatch :: Eq a => [a] -> [a] -> Bool
745 prefixMatch [] _str = True
746 prefixMatch _pat [] = False
747 prefixMatch (p:ps) (s:ss) | p == s = prefixMatch ps ss
750 suffixMatch :: Eq a => [a] -> [a] -> Bool
751 suffixMatch pat str = prefixMatch (reverse pat) (reverse str)
754 %************************************************************************
756 \subsection[Utils-pairs]{Pairs}
758 %************************************************************************
760 The following are curried versions of @fst@ and @snd@.
763 cfst :: a -> b -> a -- stranal-sem only (Note)
767 The following provide us higher order functions that, when applied
768 to a function, operate on pairs.
771 applyToPair :: ((a -> c),(b -> d)) -> (a,b) -> (c,d)
772 applyToPair (f,g) (x,y) = (f x, g y)
774 applyToFst :: (a -> c) -> (a,b)-> (c,b)
775 applyToFst f (x,y) = (f x,y)
777 applyToSnd :: (b -> d) -> (a,b) -> (a,d)
778 applyToSnd f (x,y) = (x,f y)
780 foldPair :: (a->a->a,b->b->b) -> (a,b) -> [(a,b)] -> (a,b)
781 foldPair fg ab [] = ab
782 foldPair fg@(f,g) ab ((a,b):abs) = (f a u,g b v)
783 where (u,v) = foldPair fg ab abs
787 unzipWith :: (a -> b -> c) -> [(a, b)] -> [c]
788 unzipWith f pairs = map ( \ (a, b) -> f a b ) pairs
792 seqList :: [a] -> b -> b
794 seqList (x:xs) b = x `seq` seqList xs b
800 global :: a -> IORef a
801 global a = unsafePerformIO (newIORef a)
807 #if __GLASGOW_HASKELL__ <= 408
809 ioErrors = justIoErrors
810 throwTo = raiseInThread