2 % (c) The AQUA Project, Glasgow University, 1994-1996
5 \section[PrelList]{Module @PrelList@}
7 The List data type and its operations
10 {-# OPTIONS -fno-implicit-prelude #-}
15 map, (++), filter, concat,
16 head, last, tail, init, null, length, (!!),
17 foldl, foldl1, scanl, scanl1, foldr, foldr1, scanr, scanr1,
18 iterate, repeat, replicate, cycle,
19 take, drop, splitAt, takeWhile, dropWhile, span, break,
21 any, all, elem, notElem, lookup,
22 maximum, minimum, concatMap,
23 zip, zip3, zipWith, zipWith3, unzip, unzip3,
25 -- non-standard, but hidden when creating the Prelude
31 import {-# SOURCE #-} PrelErr ( error )
37 infix 4 `elem`, `notElem`
40 %*********************************************************
42 \subsection{List-manipulation functions}
44 %*********************************************************
47 -- head and tail extract the first element and remaining elements,
48 -- respectively, of a list, which must be non-empty. last and init
49 -- are the dual functions working from the end of a finite list,
50 -- rather than the beginning.
54 head [] = errorEmptyList "head"
58 tail [] = errorEmptyList "tail"
61 #ifdef USE_REPORT_PRELUDE
64 last [] = errorEmptyList "last"
66 -- eliminate repeated cases
67 last [] = errorEmptyList "last"
68 last (x:xs) = last' x xs
70 last' _ (y:ys) = last' y ys
74 #ifdef USE_REPORT_PRELUDE
76 init (x:xs) = x : init xs
77 init [] = errorEmptyList "init"
79 -- eliminate repeated cases
80 init [] = errorEmptyList "init"
81 init (x:xs) = init' x xs
83 init' y (z:zs) = y : init' z zs
90 -- length returns the length of a finite list as an Int; it is an instance
91 -- of the more general genericLength, the result type of which may be
92 -- any kind of number.
96 len :: [a] -> Int# -> Int
98 len (_:xs) a# = len xs (a# +# 1#)
100 -- filter, applied to a predicate and a list, returns the list of those
101 -- elements that satisfy the predicate; i.e.,
102 -- filter p xs = [ x | x <- xs, p x]
103 filter :: (a -> Bool) -> [a] -> [a]
104 {-# INLINE filter #-}
105 filter p xs = build (\c n -> foldr (filterFB c p) n xs)
107 filterFB c p x r | p x = x `c` r
111 "filterFB" forall c,p,q. filterFB (filterFB c p) q = filterFB c (\x -> p x && q x)
112 "filterList" forall p. foldr (filterFB (:) p) [] = filterList p
115 filterList :: (a -> Bool) -> [a] -> [a]
116 filterList _pred [] = []
117 filterList pred (x:xs)
118 | pred x = x : filterList pred xs
119 | otherwise = filterList pred xs
121 -- foldl, applied to a binary operator, a starting value (typically the
122 -- left-identity of the operator), and a list, reduces the list using
123 -- the binary operator, from left to right:
124 -- foldl f z [x1, x2, ..., xn] == (...((z `f` x1) `f` x2) `f`...) `f` xn
125 -- foldl1 is a variant that has no starting value argument, and thus must
126 -- be applied to non-empty lists. scanl is similar to foldl, but returns
127 -- a list of successive reduced values from the left:
128 -- scanl f z [x1, x2, ...] == [z, z `f` x1, (z `f` x1) `f` x2, ...]
129 -- Note that last (scanl f z xs) == foldl f z xs.
130 -- scanl1 is similar, again without the starting element:
131 -- scanl1 f [x1, x2, ...] == [x1, x1 `f` x2, ...]
133 foldl :: (a -> b -> a) -> a -> [b] -> a
135 foldl f z (x:xs) = foldl f (f z x) xs
137 foldl1 :: (a -> a -> a) -> [a] -> a
138 foldl1 f (x:xs) = foldl f x xs
139 foldl1 _ [] = errorEmptyList "foldl1"
141 scanl :: (a -> b -> a) -> a -> [b] -> [a]
142 scanl f q ls = q : (case ls of
144 x:xs -> scanl f (f q x) xs)
146 scanl1 :: (a -> a -> a) -> [a] -> [a]
147 scanl1 f (x:xs) = scanl f x xs
148 scanl1 _ [] = errorEmptyList "scanl1"
150 -- foldr, foldr1, scanr, and scanr1 are the right-to-left duals of the
153 foldr1 :: (a -> a -> a) -> [a] -> a
155 foldr1 f (x:xs) = f x (foldr1 f xs)
156 foldr1 _ [] = errorEmptyList "foldr1"
158 scanr :: (a -> b -> b) -> b -> [a] -> [b]
160 scanr f q0 (x:xs) = f x q : qs
161 where qs@(q:_) = scanr f q0 xs
163 scanr1 :: (a -> a -> a) -> [a] -> [a]
165 scanr1 f (x:xs) = f x q : qs
166 where qs@(q:_) = scanr1 f xs
167 scanr1 _ [] = errorEmptyList "scanr1"
169 -- iterate f x returns an infinite list of repeated applications of f to x:
170 -- iterate f x == [x, f x, f (f x), ...]
171 iterate :: (a -> a) -> a -> [a]
172 {-# INLINE iterate #-}
173 iterate f x = build (\c n -> iterateFB c f x)
175 iterateFB c f x = x `c` iterateFB c f (f x)
177 iterateList f x = x : iterateList f (f x)
180 "iterate" iterateFB (:) = iterateList
184 -- repeat x is an infinite list, with x the value of every element.
186 {-# INLINE repeat #-}
187 repeat x = build (\c n -> repeatFB c x)
189 repeatFB c x = xs where xs = x `c` xs
190 repeatList x = xs where xs = x : xs
193 "repeat" repeatFB (:) = repeatList
196 -- replicate n x is a list of length n with x the value of every element
197 replicate :: Int -> a -> [a]
198 replicate n x = take n (repeat x)
200 -- cycle ties a finite list into a circular one, or equivalently,
201 -- the infinite repetition of the original list. It is the identity
202 -- on infinite lists.
205 cycle [] = error "Prelude.cycle: empty list"
206 cycle xs = xs' where xs' = xs ++ xs'
208 -- takeWhile, applied to a predicate p and a list xs, returns the longest
209 -- prefix (possibly empty) of xs of elements that satisfy p. dropWhile p xs
210 -- returns the remaining suffix. Span p xs is equivalent to
211 -- (takeWhile p xs, dropWhile p xs), while break p uses the negation of p.
213 takeWhile :: (a -> Bool) -> [a] -> [a]
216 | p x = x : takeWhile p xs
219 dropWhile :: (a -> Bool) -> [a] -> [a]
221 dropWhile p xs@(x:xs')
222 | p x = dropWhile p xs'
225 -- take n, applied to a list xs, returns the prefix of xs of length n,
226 -- or xs itself if n > length xs. drop n xs returns the suffix of xs
227 -- after the first n elements, or [] if n > length xs. splitAt n xs
228 -- is equivalent to (take n xs, drop n xs).
229 #ifdef USE_REPORT_PRELUDE
230 take :: Int -> [a] -> [a]
233 take n (x:xs) | n > 0 = x : take (n-1) xs
234 take _ _ = errorNegativeIdx "take"
236 drop :: Int -> [a] -> [a]
239 drop n (_:xs) | n > 0 = drop (n-1) xs
240 drop _ _ = errorNegativeIdx "drop"
243 splitAt :: Int -> [a] -> ([a],[a])
244 splitAt 0 xs = ([],xs)
245 splitAt _ [] = ([],[])
246 splitAt n (x:xs) | n > 0 = (x:xs',xs'') where (xs',xs'') = splitAt (n-1) xs
247 splitAt _ _ = errorNegativeIdx "splitAt"
249 #else /* hack away */
250 take :: Int -> [b] -> [b]
251 take (I# n#) xs = takeUInt n# xs
253 -- The general code for take, below, checks n <= maxInt
254 -- No need to check for maxInt overflow when specialised
255 -- at type Int or Int# since the Int must be <= maxInt
257 takeUInt :: Int# -> [b] -> [b]
259 | n >=# 0# = take_unsafe_UInt n xs
260 | otherwise = errorNegativeIdx "take"
262 take_unsafe_UInt :: Int# -> [b] -> [b]
263 take_unsafe_UInt 0# _ = []
264 take_unsafe_UInt m ls =
267 (x:xs) -> x : take_unsafe_UInt (m -# 1#) xs
269 takeUInt_append :: Int# -> [b] -> [b] -> [b]
270 takeUInt_append n xs rs
271 | n >=# 0# = take_unsafe_UInt_append n xs rs
272 | otherwise = errorNegativeIdx "take"
274 take_unsafe_UInt_append :: Int# -> [b] -> [b] -> [b]
275 take_unsafe_UInt_append 0# _ rs = rs
276 take_unsafe_UInt_append m ls rs =
279 (x:xs) -> x : take_unsafe_UInt_append (m -# 1#) xs rs
281 drop :: Int -> [b] -> [b]
283 | n# <# 0# = errorNegativeIdx "drop"
284 | otherwise = drop# n# ls
286 drop# :: Int# -> [a] -> [a]
289 drop# m# (_:xs) = drop# (m# -# 1#) xs
291 splitAt :: Int -> [b] -> ([b], [b])
293 | n# <# 0# = errorNegativeIdx "splitAt"
294 | otherwise = splitAt# n# ls
296 splitAt# :: Int# -> [a] -> ([a], [a])
297 splitAt# 0# xs = ([], xs)
298 splitAt# _ xs@[] = (xs, xs)
299 splitAt# m# (x:xs) = (x:xs', xs'')
301 (xs', xs'') = splitAt# (m# -# 1#) xs
303 #endif /* USE_REPORT_PRELUDE */
305 span, break :: (a -> Bool) -> [a] -> ([a],[a])
306 span _ xs@[] = (xs, xs)
308 | p x = let (ys,zs) = span p xs' in (x:ys,zs)
309 | otherwise = ([],xs)
311 #ifdef USE_REPORT_PRELUDE
312 break p = span (not . p)
314 -- HBC version (stolen)
315 break _ xs@[] = (xs, xs)
318 | otherwise = let (ys,zs) = break p xs' in (x:ys,zs)
321 -- reverse xs returns the elements of xs in reverse order. xs must be finite.
322 reverse :: [a] -> [a]
323 #ifdef USE_REPORT_PRELUDE
324 reverse = foldl (flip (:)) []
329 rev (x:xs) a = rev xs (x:a)
332 -- and returns the conjunction of a Boolean list. For the result to be
333 -- True, the list must be finite; False, however, results from a False
334 -- value at a finite index of a finite or infinite list. or is the
335 -- disjunctive dual of and.
336 and, or :: [Bool] -> Bool
337 #ifdef USE_REPORT_PRELUDE
338 and = foldr (&&) True
339 or = foldr (||) False
342 and (x:xs) = x && and xs
344 or (x:xs) = x || or xs
347 "and/build" forall g::forall b.(Bool->b->b)->b->b .
348 and (build g) = g (&&) True
349 "or/build" forall g::forall b.(Bool->b->b)->b->b .
350 or (build g) = g (||) False
354 -- Applied to a predicate and a list, any determines if any element
355 -- of the list satisfies the predicate. Similarly, for all.
356 any, all :: (a -> Bool) -> [a] -> Bool
357 #ifdef USE_REPORT_PRELUDE
362 any p (x:xs) = p x || any p xs
365 all p (x:xs) = p x && all p xs
367 "any/build" forall p, g::forall b.(a->b->b)->b->b .
368 any p (build g) = g ((||) . p) False
369 "all/build" forall p, g::forall b.(a->b->b)->b->b .
370 all p (build g) = g ((&&) . p) True
374 -- elem is the list membership predicate, usually written in infix form,
375 -- e.g., x `elem` xs. notElem is the negation.
376 elem, notElem :: (Eq a) => a -> [a] -> Bool
377 #ifdef USE_REPORT_PRELUDE
379 notElem x = all (/= x)
382 elem x (y:ys) = x==y || elem x ys
385 notElem x (y:ys)= x /= y && notElem x ys
388 -- lookup key assocs looks up a key in an association list.
389 lookup :: (Eq a) => a -> [(a,b)] -> Maybe b
390 lookup _key [] = Nothing
391 lookup key ((x,y):xys)
393 | otherwise = lookup key xys
396 -- maximum and minimum return the maximum or minimum value from a list,
397 -- which must be non-empty, finite, and of an ordered type.
398 {-# SPECIALISE maximum :: [Int] -> Int #-}
399 {-# SPECIALISE minimum :: [Int] -> Int #-}
400 maximum, minimum :: (Ord a) => [a] -> a
401 maximum [] = errorEmptyList "maximum"
402 maximum xs = foldl1 max xs
404 minimum [] = errorEmptyList "minimum"
405 minimum xs = foldl1 min xs
407 concatMap :: (a -> [b]) -> [a] -> [b]
408 concatMap f = foldr ((++) . f) []
410 concat :: [[a]] -> [a]
411 {-# INLINE concat #-}
412 concat = foldr (++) []
417 -- List index (subscript) operator, 0-origin
418 (!!) :: [a] -> Int -> a
419 #ifdef USE_REPORT_PRELUDE
421 (_:xs) !! n | n > 0 = xs !! (n-1)
422 (_:_) !! _ = error "Prelude.(!!): negative index"
423 [] !! _ = error "Prelude.(!!): index too large"
425 -- HBC version (stolen), then unboxified
426 -- The semantics is not quite the same for error conditions
427 -- in the more efficient version.
429 xs !! (I# n) | n <# 0# = error "Prelude.(!!): negative index\n"
430 | otherwise = sub xs n
432 sub :: [a] -> Int# -> a
433 sub [] _ = error "Prelude.(!!): index too large\n"
434 sub (y:ys) n = if n ==# 0#
436 else sub ys (n -# 1#)
441 %*********************************************************
443 \subsection{The zip family}
445 %*********************************************************
450 foldr2 k z (x:xs) (y:ys) = k x y (foldr2 k z xs ys)
452 foldr2_left k z x r [] = z
453 foldr2_left k z x r (y:ys) = k x y (r ys)
455 foldr2_right k z y r [] = z
456 foldr2_right k z y r (x:xs) = k x y (r xs)
458 -- foldr2 k z xs ys = foldr (foldr2_left k z) (\_ -> z) xs ys
459 -- foldr2 k z xs ys = foldr (foldr2_right k z) (\_ -> z) ys xs
461 "foldr2/left" forall k,z,ys,g::forall b.(a->b->b)->b->b .
462 foldr2 k z (build g) ys = g (foldr2_left k z) (\_ -> z) ys
464 "foldr2/right" forall k,z,xs,g::forall b.(a->b->b)->b->b .
465 foldr2 k z xs (build g) = g (foldr2_right k z) (\_ -> z) xs
469 zip takes two lists and returns a list of corresponding pairs. If one
470 input list is short, excess elements of the longer list are discarded.
471 zip3 takes three lists and returns a list of triples. Zips for larger
472 tuples are in the List library
475 ----------------------------------------------
476 zip :: [a] -> [b] -> [(a,b)]
478 zip xs ys = build (\c n -> foldr2 (zipFB c) n xs ys)
480 zipFB c x y r = (x,y) `c` r
483 zipList :: [a] -> [b] -> [(a,b)]
484 zipList (a:as) (b:bs) = (a,b) : zipList as bs
488 "zipList" foldr2 (zipFB (:)) [] = zipList
493 ----------------------------------------------
494 zip3 :: [a] -> [b] -> [c] -> [(a,b,c)]
496 -- zip3 = zipWith3 (,,)
497 zip3 (a:as) (b:bs) (c:cs) = (a,b,c) : zip3 as bs cs
502 -- The zipWith family generalises the zip family by zipping with the
503 -- function given as the first argument, instead of a tupling function.
504 -- For example, zipWith (+) is applied to two lists to produce the list
505 -- of corresponding sums.
509 ----------------------------------------------
510 zipWith :: (a->b->c) -> [a]->[b]->[c]
511 {-# INLINE zipWith #-}
512 zipWith f xs ys = build (\c n -> foldr2 (zipWithFB c f) n xs ys)
514 zipWithFB c f x y r = (x `f` y) `c` r
516 zipWithList :: (a->b->c) -> [a] -> [b] -> [c]
517 zipWithList f (a:as) (b:bs) = f a b : zipWithList f as bs
518 zipWithList f _ _ = []
521 "zipWithList" forall f. foldr2 (zipWithFB (:) f) [] = zipWithList f
526 zipWith3 :: (a->b->c->d) -> [a]->[b]->[c]->[d]
527 zipWith3 z (a:as) (b:bs) (c:cs)
528 = z a b c : zipWith3 z as bs cs
529 zipWith3 _ _ _ _ = []
531 -- unzip transforms a list of pairs into a pair of lists.
532 unzip :: [(a,b)] -> ([a],[b])
533 unzip = foldr (\(a,b) ~(as,bs) -> (a:as,b:bs)) ([],[])
535 unzip3 :: [(a,b,c)] -> ([a],[b],[c])
536 unzip3 = foldr (\(a,b,c) ~(as,bs,cs) -> (a:as,b:bs,c:cs))
541 %*********************************************************
543 \subsection{Error code}
545 %*********************************************************
547 Common up near identical calls to `error' to reduce the number
548 constant strings created when compiled:
551 errorEmptyList :: String -> a
553 error (prel_list_str ++ fun ++ ": empty list")
555 errorNegativeIdx :: String -> a
556 errorNegativeIdx fun =
557 error (prel_list_str ++ fun ++ ": negative index")
559 prel_list_str :: String
560 prel_list_str = "Prelude."