2 {-# OPTIONS_GHC -XNoImplicitPrelude #-}
3 {-# OPTIONS_HADDOCK hide #-}
4 -----------------------------------------------------------------------------
7 -- Copyright : (c) The University of Glasgow 1994-2002
8 -- License : see libraries/base/LICENSE
10 -- Maintainer : cvs-ghc@haskell.org
11 -- Stability : internal
12 -- Portability : non-portable (GHC Extensions)
14 -- The List data type and its operations
16 -----------------------------------------------------------------------------
20 -- [] (..), -- Not Haskell 98; built in syntax
22 map, (++), filter, concat,
23 head, last, tail, init, null, length, (!!),
24 foldl, scanl, scanl1, foldr, foldr1, scanr, scanr1,
25 iterate, repeat, replicate, cycle,
26 take, drop, splitAt, takeWhile, dropWhile, span, break,
28 any, all, elem, notElem, lookup,
30 zip, zip3, zipWith, zipWith3, unzip, unzip3,
33 #ifndef USE_REPORT_PRELUDE
34 -- non-standard, but hidden when creating the Prelude
45 infix 4 `elem`, `notElem`
48 %*********************************************************
50 \subsection{List-manipulation functions}
52 %*********************************************************
55 -- | Extract the first element of a list, which must be non-empty.
60 badHead = errorEmptyList "head"
62 -- This rule is useful in cases like
63 -- head [y | (x,y) <- ps, x==t]
65 "head/build" forall (g::forall b.(a->b->b)->b->b) .
66 head (build g) = g (\x _ -> x) badHead
67 "head/augment" forall xs (g::forall b. (a->b->b) -> b -> b) .
68 head (augment g xs) = g (\x _ -> x) (head xs)
71 -- | Extract the elements after the head of a list, which must be non-empty.
74 tail [] = errorEmptyList "tail"
76 -- | Extract the last element of a list, which must be finite and non-empty.
78 #ifdef USE_REPORT_PRELUDE
81 last [] = errorEmptyList "last"
83 -- eliminate repeated cases
84 last [] = errorEmptyList "last"
85 last (x:xs) = last' x xs
87 last' _ (y:ys) = last' y ys
90 -- | Return all the elements of a list except the last one.
91 -- The list must be finite and non-empty.
93 #ifdef USE_REPORT_PRELUDE
95 init (x:xs) = x : init xs
96 init [] = errorEmptyList "init"
98 -- eliminate repeated cases
99 init [] = errorEmptyList "init"
100 init (x:xs) = init' x xs
101 where init' _ [] = []
102 init' y (z:zs) = y : init' z zs
105 -- | Test whether a list is empty.
110 -- | 'length' returns the length of a finite list as an 'Int'.
111 -- It is an instance of the more general 'Data.List.genericLength',
112 -- the result type of which may be any kind of number.
116 len :: [a] -> Int# -> Int
118 len (_:xs) a# = len xs (a# +# 1#)
120 -- | 'filter', applied to a predicate and a list, returns the list of
121 -- those elements that satisfy the predicate; i.e.,
123 -- > filter p xs = [ x | x <- xs, p x]
125 filter :: (a -> Bool) -> [a] -> [a]
128 | pred x = x : filter pred xs
129 | otherwise = filter pred xs
131 {-# NOINLINE [0] filterFB #-}
132 filterFB c p x r | p x = x `c` r
136 "filter" [~1] forall p xs. filter p xs = build (\c n -> foldr (filterFB c p) n xs)
137 "filterList" [1] forall p. foldr (filterFB (:) p) [] = filter p
138 "filterFB" forall c p q. filterFB (filterFB c p) q = filterFB c (\x -> q x && p x)
141 -- Note the filterFB rule, which has p and q the "wrong way round" in the RHS.
142 -- filterFB (filterFB c p) q a b
143 -- = if q a then filterFB c p a b else b
144 -- = if q a then (if p a then c a b else b) else b
145 -- = if q a && p a then c a b else b
146 -- = filterFB c (\x -> q x && p x) a b
147 -- I originally wrote (\x -> p x && q x), which is wrong, and actually
148 -- gave rise to a live bug report. SLPJ.
151 -- | 'foldl', applied to a binary operator, a starting value (typically
152 -- the left-identity of the operator), and a list, reduces the list
153 -- using the binary operator, from left to right:
155 -- > foldl f z [x1, x2, ..., xn] == (...((z `f` x1) `f` x2) `f`...) `f` xn
157 -- The list must be finite.
159 -- We write foldl as a non-recursive thing, so that it
160 -- can be inlined, and then (often) strictness-analysed,
161 -- and hence the classic space leak on foldl (+) 0 xs
163 foldl :: (a -> b -> a) -> a -> [b] -> a
164 foldl f z xs = lgo z xs
167 lgo z (x:xs) = lgo (f z x) xs
169 -- | 'scanl' is similar to 'foldl', but returns a list of successive
170 -- reduced values from the left:
172 -- > scanl f z [x1, x2, ...] == [z, z `f` x1, (z `f` x1) `f` x2, ...]
176 -- > last (scanl f z xs) == foldl f z xs.
178 scanl :: (a -> b -> a) -> a -> [b] -> [a]
179 scanl f q ls = q : (case ls of
181 x:xs -> scanl f (f q x) xs)
183 -- | 'scanl1' is a variant of 'scanl' that has no starting value argument:
185 -- > scanl1 f [x1, x2, ...] == [x1, x1 `f` x2, ...]
187 scanl1 :: (a -> a -> a) -> [a] -> [a]
188 scanl1 f (x:xs) = scanl f x xs
191 -- foldr, foldr1, scanr, and scanr1 are the right-to-left duals of the
194 -- | 'foldr1' is a variant of 'foldr' that has no starting value argument,
195 -- and thus must be applied to non-empty lists.
197 foldr1 :: (a -> a -> a) -> [a] -> a
199 foldr1 f (x:xs) = f x (foldr1 f xs)
200 foldr1 _ [] = errorEmptyList "foldr1"
202 -- | 'scanr' is the right-to-left dual of 'scanl'.
205 -- > head (scanr f z xs) == foldr f z xs.
207 scanr :: (a -> b -> b) -> b -> [a] -> [b]
209 scanr f q0 (x:xs) = f x q : qs
210 where qs@(q:_) = scanr f q0 xs
212 -- | 'scanr1' is a variant of 'scanr' that has no starting value argument.
214 scanr1 :: (a -> a -> a) -> [a] -> [a]
217 scanr1 f (x:xs) = f x q : qs
218 where qs@(q:_) = scanr1 f xs
220 -- | 'iterate' @f x@ returns an infinite list of repeated applications
223 -- > iterate f x == [x, f x, f (f x), ...]
225 iterate :: (a -> a) -> a -> [a]
226 iterate f x = x : iterate f (f x)
228 iterateFB c f x = x `c` iterateFB c f (f x)
232 "iterate" [~1] forall f x. iterate f x = build (\c _n -> iterateFB c f x)
233 "iterateFB" [1] iterateFB (:) = iterate
237 -- | 'repeat' @x@ is an infinite list, with @x@ the value of every element.
239 {-# INLINE [0] repeat #-}
240 -- The pragma just gives the rules more chance to fire
241 repeat x = xs where xs = x : xs
243 {-# INLINE [0] repeatFB #-} -- ditto
244 repeatFB c x = xs where xs = x `c` xs
248 "repeat" [~1] forall x. repeat x = build (\c _n -> repeatFB c x)
249 "repeatFB" [1] repeatFB (:) = repeat
252 -- | 'replicate' @n x@ is a list of length @n@ with @x@ the value of
254 -- It is an instance of the more general 'Data.List.genericReplicate',
255 -- in which @n@ may be of any integral type.
256 {-# INLINE replicate #-}
257 replicate :: Int -> a -> [a]
258 replicate n x = take n (repeat x)
260 -- | 'cycle' ties a finite list into a circular one, or equivalently,
261 -- the infinite repetition of the original list. It is the identity
262 -- on infinite lists.
265 cycle [] = error "Prelude.cycle: empty list"
266 cycle xs = xs' where xs' = xs ++ xs'
268 -- | 'takeWhile', applied to a predicate @p@ and a list @xs@, returns the
269 -- longest prefix (possibly empty) of @xs@ of elements that satisfy @p@:
271 -- > takeWhile (< 3) [1,2,3,4,1,2,3,4] == [1,2]
272 -- > takeWhile (< 9) [1,2,3] == [1,2,3]
273 -- > takeWhile (< 0) [1,2,3] == []
276 takeWhile :: (a -> Bool) -> [a] -> [a]
279 | p x = x : takeWhile p xs
282 -- | 'dropWhile' @p xs@ returns the suffix remaining after 'takeWhile' @p xs@:
284 -- > dropWhile (< 3) [1,2,3,4,5,1,2,3] == [3,4,5,1,2,3]
285 -- > dropWhile (< 9) [1,2,3] == []
286 -- > dropWhile (< 0) [1,2,3] == [1,2,3]
289 dropWhile :: (a -> Bool) -> [a] -> [a]
291 dropWhile p xs@(x:xs')
292 | p x = dropWhile p xs'
295 -- | 'take' @n@, applied to a list @xs@, returns the prefix of @xs@
296 -- of length @n@, or @xs@ itself if @n > 'length' xs@:
298 -- > take 5 "Hello World!" == "Hello"
299 -- > take 3 [1,2,3,4,5] == [1,2,3]
300 -- > take 3 [1,2] == [1,2]
302 -- > take (-1) [1,2] == []
303 -- > take 0 [1,2] == []
305 -- It is an instance of the more general 'Data.List.genericTake',
306 -- in which @n@ may be of any integral type.
307 take :: Int -> [a] -> [a]
309 -- | 'drop' @n xs@ returns the suffix of @xs@
310 -- after the first @n@ elements, or @[]@ if @n > 'length' xs@:
312 -- > drop 6 "Hello World!" == "World!"
313 -- > drop 3 [1,2,3,4,5] == [4,5]
314 -- > drop 3 [1,2] == []
316 -- > drop (-1) [1,2] == [1,2]
317 -- > drop 0 [1,2] == [1,2]
319 -- It is an instance of the more general 'Data.List.genericDrop',
320 -- in which @n@ may be of any integral type.
321 drop :: Int -> [a] -> [a]
323 -- | 'splitAt' @n xs@ returns a tuple where first element is @xs@ prefix of
324 -- length @n@ and second element is the remainder of the list:
326 -- > splitAt 6 "Hello World!" == ("Hello ","World!")
327 -- > splitAt 3 [1,2,3,4,5] == ([1,2,3],[4,5])
328 -- > splitAt 1 [1,2,3] == ([1],[2,3])
329 -- > splitAt 3 [1,2,3] == ([1,2,3],[])
330 -- > splitAt 4 [1,2,3] == ([1,2,3],[])
331 -- > splitAt 0 [1,2,3] == ([],[1,2,3])
332 -- > splitAt (-1) [1,2,3] == ([],[1,2,3])
334 -- It is equivalent to @('take' n xs, 'drop' n xs)@.
335 -- 'splitAt' is an instance of the more general 'Data.List.genericSplitAt',
336 -- in which @n@ may be of any integral type.
337 splitAt :: Int -> [a] -> ([a],[a])
339 #ifdef USE_REPORT_PRELUDE
340 take n _ | n <= 0 = []
342 take n (x:xs) = x : take (n-1) xs
344 drop n xs | n <= 0 = xs
346 drop n (_:xs) = drop (n-1) xs
348 splitAt n xs = (take n xs, drop n xs)
350 #else /* hack away */
352 "take" [~1] forall n xs . take n xs = takeFoldr n xs
353 "takeList" [1] forall n xs . foldr (takeFB (:) []) (takeConst []) xs n = takeUInt n xs
356 {-# INLINE takeFoldr #-}
357 takeFoldr :: Int -> [a] -> [a]
359 = build (\c nil -> if n# <=# 0# then nil else
360 foldr (takeFB c nil) (takeConst nil) xs n#)
362 {-# NOINLINE [0] takeConst #-}
363 -- just a version of const that doesn't get inlined too early, so we
364 -- can spot it in rules. Also we need a type sig due to the unboxed Int#.
365 takeConst :: a -> Int# -> a
368 {-# NOINLINE [0] takeFB #-}
369 takeFB :: (a -> b -> b) -> b -> a -> (Int# -> b) -> Int# -> b
370 takeFB c n x xs m | m <=# 1# = x `c` n
371 | otherwise = x `c` xs (m -# 1#)
373 {-# INLINE [0] take #-}
374 take (I# n#) xs = takeUInt n# xs
376 -- The general code for take, below, checks n <= maxInt
377 -- No need to check for maxInt overflow when specialised
378 -- at type Int or Int# since the Int must be <= maxInt
380 takeUInt :: Int# -> [b] -> [b]
382 | n >=# 0# = take_unsafe_UInt n xs
385 take_unsafe_UInt :: Int# -> [b] -> [b]
386 take_unsafe_UInt 0# _ = []
387 take_unsafe_UInt m ls =
390 (x:xs) -> x : take_unsafe_UInt (m -# 1#) xs
392 takeUInt_append :: Int# -> [b] -> [b] -> [b]
393 takeUInt_append n xs rs
394 | n >=# 0# = take_unsafe_UInt_append n xs rs
397 take_unsafe_UInt_append :: Int# -> [b] -> [b] -> [b]
398 take_unsafe_UInt_append 0# _ rs = rs
399 take_unsafe_UInt_append m ls rs =
402 (x:xs) -> x : take_unsafe_UInt_append (m -# 1#) xs rs
406 | otherwise = drop# n# ls
408 drop# :: Int# -> [a] -> [a]
411 drop# m# (_:xs) = drop# (m# -# 1#) xs
414 | n# <# 0# = ([], ls)
415 | otherwise = splitAt# n# ls
417 splitAt# :: Int# -> [a] -> ([a], [a])
418 splitAt# 0# xs = ([], xs)
419 splitAt# _ xs@[] = (xs, xs)
420 splitAt# m# (x:xs) = (x:xs', xs'')
422 (xs', xs'') = splitAt# (m# -# 1#) xs
424 #endif /* USE_REPORT_PRELUDE */
426 -- | 'span', applied to a predicate @p@ and a list @xs@, returns a tuple where
427 -- first element is longest prefix (possibly empty) of @xs@ of elements that
428 -- satisfy @p@ and second element is the remainder of the list:
430 -- > span (< 3) [1,2,3,4,1,2,3,4] == ([1,2],[3,4,1,2,3,4])
431 -- > span (< 9) [1,2,3] == ([1,2,3],[])
432 -- > span (< 0) [1,2,3] == ([],[1,2,3])
434 -- 'span' @p xs@ is equivalent to @('takeWhile' p xs, 'dropWhile' p xs)@
436 span :: (a -> Bool) -> [a] -> ([a],[a])
437 span _ xs@[] = (xs, xs)
439 | p x = let (ys,zs) = span p xs' in (x:ys,zs)
440 | otherwise = ([],xs)
442 -- | 'break', applied to a predicate @p@ and a list @xs@, returns a tuple where
443 -- first element is longest prefix (possibly empty) of @xs@ of elements that
444 -- /do not satisfy/ @p@ and second element is the remainder of the list:
446 -- > break (> 3) [1,2,3,4,1,2,3,4] == ([1,2,3],[4,1,2,3,4])
447 -- > break (< 9) [1,2,3] == ([],[1,2,3])
448 -- > break (> 9) [1,2,3] == ([1,2,3],[])
450 -- 'break' @p@ is equivalent to @'span' ('not' . p)@.
452 break :: (a -> Bool) -> [a] -> ([a],[a])
453 #ifdef USE_REPORT_PRELUDE
454 break p = span (not . p)
456 -- HBC version (stolen)
457 break _ xs@[] = (xs, xs)
460 | otherwise = let (ys,zs) = break p xs' in (x:ys,zs)
463 -- | 'reverse' @xs@ returns the elements of @xs@ in reverse order.
464 -- @xs@ must be finite.
465 reverse :: [a] -> [a]
466 #ifdef USE_REPORT_PRELUDE
467 reverse = foldl (flip (:)) []
472 rev (x:xs) a = rev xs (x:a)
475 -- | 'and' returns the conjunction of a Boolean list. For the result to be
476 -- 'True', the list must be finite; 'False', however, results from a 'False'
477 -- value at a finite index of a finite or infinite list.
478 and :: [Bool] -> Bool
480 -- | 'or' returns the disjunction of a Boolean list. For the result to be
481 -- 'False', the list must be finite; 'True', however, results from a 'True'
482 -- value at a finite index of a finite or infinite list.
484 #ifdef USE_REPORT_PRELUDE
485 and = foldr (&&) True
486 or = foldr (||) False
489 and (x:xs) = x && and xs
491 or (x:xs) = x || or xs
494 "and/build" forall (g::forall b.(Bool->b->b)->b->b) .
495 and (build g) = g (&&) True
496 "or/build" forall (g::forall b.(Bool->b->b)->b->b) .
497 or (build g) = g (||) False
501 -- | Applied to a predicate and a list, 'any' determines if any element
502 -- of the list satisfies the predicate.
503 any :: (a -> Bool) -> [a] -> Bool
505 -- | Applied to a predicate and a list, 'all' determines if all elements
506 -- of the list satisfy the predicate.
507 all :: (a -> Bool) -> [a] -> Bool
508 #ifdef USE_REPORT_PRELUDE
513 any p (x:xs) = p x || any p xs
516 all p (x:xs) = p x && all p xs
518 "any/build" forall p (g::forall b.(a->b->b)->b->b) .
519 any p (build g) = g ((||) . p) False
520 "all/build" forall p (g::forall b.(a->b->b)->b->b) .
521 all p (build g) = g ((&&) . p) True
525 -- | 'elem' is the list membership predicate, usually written in infix form,
526 -- e.g., @x \`elem\` xs@.
527 elem :: (Eq a) => a -> [a] -> Bool
529 -- | 'notElem' is the negation of 'elem'.
530 notElem :: (Eq a) => a -> [a] -> Bool
531 #ifdef USE_REPORT_PRELUDE
533 notElem x = all (/= x)
536 elem x (y:ys) = x==y || elem x ys
539 notElem x (y:ys)= x /= y && notElem x ys
542 -- | 'lookup' @key assocs@ looks up a key in an association list.
543 lookup :: (Eq a) => a -> [(a,b)] -> Maybe b
544 lookup _key [] = Nothing
545 lookup key ((x,y):xys)
547 | otherwise = lookup key xys
549 -- | Map a function over a list and concatenate the results.
550 concatMap :: (a -> [b]) -> [a] -> [b]
551 concatMap f = foldr ((++) . f) []
553 -- | Concatenate a list of lists.
554 concat :: [[a]] -> [a]
555 concat = foldr (++) []
558 "concat" forall xs. concat xs = build (\c n -> foldr (\x y -> foldr c y x) n xs)
559 -- We don't bother to turn non-fusible applications of concat back into concat
566 -- | List index (subscript) operator, starting from 0.
567 -- It is an instance of the more general 'Data.List.genericIndex',
568 -- which takes an index of any integral type.
569 (!!) :: [a] -> Int -> a
570 #ifdef USE_REPORT_PRELUDE
571 xs !! n | n < 0 = error "Prelude.!!: negative index"
572 [] !! _ = error "Prelude.!!: index too large"
574 (_:xs) !! n = xs !! (n-1)
576 -- HBC version (stolen), then unboxified
577 -- The semantics is not quite the same for error conditions
578 -- in the more efficient version.
580 xs !! (I# n) | n <# 0# = error "Prelude.(!!): negative index\n"
581 | otherwise = sub xs n
583 sub :: [a] -> Int# -> a
584 sub [] _ = error "Prelude.(!!): index too large\n"
585 sub (y:ys) n = if n ==# 0#
587 else sub ys (n -# 1#)
592 %*********************************************************
594 \subsection{The zip family}
596 %*********************************************************
599 foldr2 _k z [] _ys = z
600 foldr2 _k z _xs [] = z
601 foldr2 k z (x:xs) (y:ys) = k x y (foldr2 k z xs ys)
603 foldr2_left _k z _x _r [] = z
604 foldr2_left k _z x r (y:ys) = k x y (r ys)
606 foldr2_right _k z _y _r [] = z
607 foldr2_right k _z y r (x:xs) = k x y (r xs)
609 -- foldr2 k z xs ys = foldr (foldr2_left k z) (\_ -> z) xs ys
610 -- foldr2 k z xs ys = foldr (foldr2_right k z) (\_ -> z) ys xs
612 "foldr2/left" forall k z ys (g::forall b.(a->b->b)->b->b) .
613 foldr2 k z (build g) ys = g (foldr2_left k z) (\_ -> z) ys
615 "foldr2/right" forall k z xs (g::forall b.(a->b->b)->b->b) .
616 foldr2 k z xs (build g) = g (foldr2_right k z) (\_ -> z) xs
620 The foldr2/right rule isn't exactly right, because it changes
621 the strictness of foldr2 (and thereby zip)
623 E.g. main = print (null (zip nonobviousNil (build undefined)))
624 where nonobviousNil = f 3
625 f n = if n == 0 then [] else f (n-1)
627 I'm going to leave it though.
630 Zips for larger tuples are in the List module.
633 ----------------------------------------------
634 -- | 'zip' takes two lists and returns a list of corresponding pairs.
635 -- If one input list is short, excess elements of the longer list are
637 zip :: [a] -> [b] -> [(a,b)]
638 zip (a:as) (b:bs) = (a,b) : zip as bs
641 {-# INLINE [0] zipFB #-}
642 zipFB c x y r = (x,y) `c` r
645 "zip" [~1] forall xs ys. zip xs ys = build (\c n -> foldr2 (zipFB c) n xs ys)
646 "zipList" [1] foldr2 (zipFB (:)) [] = zip
651 ----------------------------------------------
652 -- | 'zip3' takes three lists and returns a list of triples, analogous to
654 zip3 :: [a] -> [b] -> [c] -> [(a,b,c)]
656 -- zip3 = zipWith3 (,,)
657 zip3 (a:as) (b:bs) (c:cs) = (a,b,c) : zip3 as bs cs
662 -- The zipWith family generalises the zip family by zipping with the
663 -- function given as the first argument, instead of a tupling function.
666 ----------------------------------------------
667 -- | 'zipWith' generalises 'zip' by zipping with the function given
668 -- as the first argument, instead of a tupling function.
669 -- For example, @'zipWith' (+)@ is applied to two lists to produce the
670 -- list of corresponding sums.
671 zipWith :: (a->b->c) -> [a]->[b]->[c]
672 zipWith f (a:as) (b:bs) = f a b : zipWith f as bs
675 {-# INLINE [0] zipWithFB #-}
676 zipWithFB c f x y r = (x `f` y) `c` r
679 "zipWith" [~1] forall f xs ys. zipWith f xs ys = build (\c n -> foldr2 (zipWithFB c f) n xs ys)
680 "zipWithList" [1] forall f. foldr2 (zipWithFB (:) f) [] = zipWith f
685 -- | The 'zipWith3' function takes a function which combines three
686 -- elements, as well as three lists and returns a list of their point-wise
687 -- combination, analogous to 'zipWith'.
688 zipWith3 :: (a->b->c->d) -> [a]->[b]->[c]->[d]
689 zipWith3 z (a:as) (b:bs) (c:cs)
690 = z a b c : zipWith3 z as bs cs
691 zipWith3 _ _ _ _ = []
693 -- | 'unzip' transforms a list of pairs into a list of first components
694 -- and a list of second components.
695 unzip :: [(a,b)] -> ([a],[b])
697 unzip = foldr (\(a,b) ~(as,bs) -> (a:as,b:bs)) ([],[])
699 -- | The 'unzip3' function takes a list of triples and returns three
700 -- lists, analogous to 'unzip'.
701 unzip3 :: [(a,b,c)] -> ([a],[b],[c])
702 {-# INLINE unzip3 #-}
703 unzip3 = foldr (\(a,b,c) ~(as,bs,cs) -> (a:as,b:bs,c:cs))
708 %*********************************************************
710 \subsection{Error code}
712 %*********************************************************
714 Common up near identical calls to `error' to reduce the number
715 constant strings created when compiled:
718 errorEmptyList :: String -> a
720 error (prel_list_str ++ fun ++ ": empty list")
722 prel_list_str :: String
723 prel_list_str = "Prelude."