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.
56 badHead = errorEmptyList "head"
58 -- This rule is useful in cases like
59 -- head [y | (x,y) <- ps, x==t]
61 "head/build" forall g::forall b.(Bool->b->b)->b->b .
62 head (build g) = g (\x _ -> x) badHead
63 "head/augment" forall xs, g::forall b. (a->b->b) -> b -> b .
64 head (augment g xs) = g (\x _ -> x) (head xs)
69 tail [] = errorEmptyList "tail"
72 #ifdef USE_REPORT_PRELUDE
75 last [] = errorEmptyList "last"
77 -- eliminate repeated cases
78 last [] = errorEmptyList "last"
79 last (x:xs) = last' x xs
81 last' _ (y:ys) = last' y ys
85 #ifdef USE_REPORT_PRELUDE
87 init (x:xs) = x : init xs
88 init [] = errorEmptyList "init"
90 -- eliminate repeated cases
91 init [] = errorEmptyList "init"
92 init (x:xs) = init' x xs
94 init' y (z:zs) = y : init' z zs
101 -- length returns the length of a finite list as an Int; it is an instance
102 -- of the more general genericLength, the result type of which may be
103 -- any kind of number.
107 len :: [a] -> Int# -> Int
109 len (_:xs) a# = len xs (a# +# 1#)
111 -- filter, applied to a predicate and a list, returns the list of those
112 -- elements that satisfy the predicate; i.e.,
113 -- filter p xs = [ x | x <- xs, p x]
114 filter :: (a -> Bool) -> [a] -> [a]
115 {-# INLINE filter #-}
116 filter p xs = build (\c n -> foldr (filterFB c p) n xs)
118 filterFB c p x r | p x = x `c` r
122 "filterFB" forall c,p,q. filterFB (filterFB c p) q = filterFB c (\x -> p x && q x)
123 "filterList" forall p. foldr (filterFB (:) p) [] = filterList p
126 filterList :: (a -> Bool) -> [a] -> [a]
127 filterList _pred [] = []
128 filterList pred (x:xs)
129 | pred x = x : filterList pred xs
130 | otherwise = filterList pred xs
132 -- foldl, applied to a binary operator, a starting value (typically the
133 -- left-identity of the operator), and a list, reduces the list using
134 -- the binary operator, from left to right:
135 -- foldl f z [x1, x2, ..., xn] == (...((z `f` x1) `f` x2) `f`...) `f` xn
136 -- foldl1 is a variant that has no starting value argument, and thus must
137 -- be applied to non-empty lists. scanl is similar to foldl, but returns
138 -- a list of successive reduced values from the left:
139 -- scanl f z [x1, x2, ...] == [z, z `f` x1, (z `f` x1) `f` x2, ...]
140 -- Note that last (scanl f z xs) == foldl f z xs.
141 -- scanl1 is similar, again without the starting element:
142 -- scanl1 f [x1, x2, ...] == [x1, x1 `f` x2, ...]
144 foldl :: (a -> b -> a) -> a -> [b] -> a
146 foldl f z (x:xs) = foldl f (f z x) xs
148 foldl1 :: (a -> a -> a) -> [a] -> a
149 foldl1 f (x:xs) = foldl f x xs
150 foldl1 _ [] = errorEmptyList "foldl1"
152 scanl :: (a -> b -> a) -> a -> [b] -> [a]
153 scanl f q ls = q : (case ls of
155 x:xs -> scanl f (f q x) xs)
157 scanl1 :: (a -> a -> a) -> [a] -> [a]
158 scanl1 f (x:xs) = scanl f x xs
159 scanl1 _ [] = errorEmptyList "scanl1"
161 -- foldr, foldr1, scanr, and scanr1 are the right-to-left duals of the
164 foldr1 :: (a -> a -> a) -> [a] -> a
166 foldr1 f (x:xs) = f x (foldr1 f xs)
167 foldr1 _ [] = errorEmptyList "foldr1"
169 scanr :: (a -> b -> b) -> b -> [a] -> [b]
171 scanr f q0 (x:xs) = f x q : qs
172 where qs@(q:_) = scanr f q0 xs
174 scanr1 :: (a -> a -> a) -> [a] -> [a]
176 scanr1 f (x:xs) = f x q : qs
177 where qs@(q:_) = scanr1 f xs
178 scanr1 _ [] = errorEmptyList "scanr1"
180 -- iterate f x returns an infinite list of repeated applications of f to x:
181 -- iterate f x == [x, f x, f (f x), ...]
182 iterate :: (a -> a) -> a -> [a]
183 {-# INLINE iterate #-}
184 iterate f x = build (\c n -> iterateFB c f x)
186 iterateFB c f x = x `c` iterateFB c f (f x)
188 iterateList f x = x : iterateList f (f x)
191 "iterate" iterateFB (:) = iterateList
195 -- repeat x is an infinite list, with x the value of every element.
197 {-# INLINE repeat #-}
198 repeat x = build (\c n -> repeatFB c x)
200 repeatFB c x = xs where xs = x `c` xs
201 repeatList x = xs where xs = x : xs
204 "repeat" repeatFB (:) = repeatList
207 -- replicate n x is a list of length n with x the value of every element
208 replicate :: Int -> a -> [a]
209 replicate n x = take n (repeat x)
211 -- cycle ties a finite list into a circular one, or equivalently,
212 -- the infinite repetition of the original list. It is the identity
213 -- on infinite lists.
216 cycle [] = error "Prelude.cycle: empty list"
217 cycle xs = xs' where xs' = xs ++ xs'
219 -- takeWhile, applied to a predicate p and a list xs, returns the longest
220 -- prefix (possibly empty) of xs of elements that satisfy p. dropWhile p xs
221 -- returns the remaining suffix. Span p xs is equivalent to
222 -- (takeWhile p xs, dropWhile p xs), while break p uses the negation of p.
224 takeWhile :: (a -> Bool) -> [a] -> [a]
227 | p x = x : takeWhile p xs
230 dropWhile :: (a -> Bool) -> [a] -> [a]
232 dropWhile p xs@(x:xs')
233 | p x = dropWhile p xs'
236 -- take n, applied to a list xs, returns the prefix of xs of length n,
237 -- or xs itself if n > length xs. drop n xs returns the suffix of xs
238 -- after the first n elements, or [] if n > length xs. splitAt n xs
239 -- is equivalent to (take n xs, drop n xs).
240 #ifdef USE_REPORT_PRELUDE
241 take :: Int -> [a] -> [a]
244 take n (x:xs) | n > 0 = x : take (n-1) xs
245 take _ _ = errorNegativeIdx "take"
247 drop :: Int -> [a] -> [a]
250 drop n (_:xs) | n > 0 = drop (n-1) xs
251 drop _ _ = errorNegativeIdx "drop"
254 splitAt :: Int -> [a] -> ([a],[a])
255 splitAt 0 xs = ([],xs)
256 splitAt _ [] = ([],[])
257 splitAt n (x:xs) | n > 0 = (x:xs',xs'') where (xs',xs'') = splitAt (n-1) xs
258 splitAt _ _ = errorNegativeIdx "splitAt"
260 #else /* hack away */
261 take :: Int -> [b] -> [b]
262 take (I# n#) xs = takeUInt n# xs
264 -- The general code for take, below, checks n <= maxInt
265 -- No need to check for maxInt overflow when specialised
266 -- at type Int or Int# since the Int must be <= maxInt
268 takeUInt :: Int# -> [b] -> [b]
270 | n >=# 0# = take_unsafe_UInt n xs
271 | otherwise = errorNegativeIdx "take"
273 take_unsafe_UInt :: Int# -> [b] -> [b]
274 take_unsafe_UInt 0# _ = []
275 take_unsafe_UInt m ls =
278 (x:xs) -> x : take_unsafe_UInt (m -# 1#) xs
280 takeUInt_append :: Int# -> [b] -> [b] -> [b]
281 takeUInt_append n xs rs
282 | n >=# 0# = take_unsafe_UInt_append n xs rs
283 | otherwise = errorNegativeIdx "take"
285 take_unsafe_UInt_append :: Int# -> [b] -> [b] -> [b]
286 take_unsafe_UInt_append 0# _ rs = rs
287 take_unsafe_UInt_append m ls rs =
290 (x:xs) -> x : take_unsafe_UInt_append (m -# 1#) xs rs
292 drop :: Int -> [b] -> [b]
294 | n# <# 0# = errorNegativeIdx "drop"
295 | otherwise = drop# n# ls
297 drop# :: Int# -> [a] -> [a]
300 drop# m# (_:xs) = drop# (m# -# 1#) xs
302 splitAt :: Int -> [b] -> ([b], [b])
304 | n# <# 0# = errorNegativeIdx "splitAt"
305 | otherwise = splitAt# n# ls
307 splitAt# :: Int# -> [a] -> ([a], [a])
308 splitAt# 0# xs = ([], xs)
309 splitAt# _ xs@[] = (xs, xs)
310 splitAt# m# (x:xs) = (x:xs', xs'')
312 (xs', xs'') = splitAt# (m# -# 1#) xs
314 #endif /* USE_REPORT_PRELUDE */
316 span, break :: (a -> Bool) -> [a] -> ([a],[a])
317 span _ xs@[] = (xs, xs)
319 | p x = let (ys,zs) = span p xs' in (x:ys,zs)
320 | otherwise = ([],xs)
322 #ifdef USE_REPORT_PRELUDE
323 break p = span (not . p)
325 -- HBC version (stolen)
326 break _ xs@[] = (xs, xs)
329 | otherwise = let (ys,zs) = break p xs' in (x:ys,zs)
332 -- reverse xs returns the elements of xs in reverse order. xs must be finite.
333 reverse :: [a] -> [a]
334 #ifdef USE_REPORT_PRELUDE
335 reverse = foldl (flip (:)) []
340 rev (x:xs) a = rev xs (x:a)
343 -- and returns the conjunction of a Boolean list. For the result to be
344 -- True, the list must be finite; False, however, results from a False
345 -- value at a finite index of a finite or infinite list. or is the
346 -- disjunctive dual of and.
347 and, or :: [Bool] -> Bool
348 #ifdef USE_REPORT_PRELUDE
349 and = foldr (&&) True
350 or = foldr (||) False
353 and (x:xs) = x && and xs
355 or (x:xs) = x || or xs
358 "and/build" forall g::forall b.(Bool->b->b)->b->b .
359 and (build g) = g (&&) True
360 "or/build" forall g::forall b.(Bool->b->b)->b->b .
361 or (build g) = g (||) False
365 -- Applied to a predicate and a list, any determines if any element
366 -- of the list satisfies the predicate. Similarly, for all.
367 any, all :: (a -> Bool) -> [a] -> Bool
368 #ifdef USE_REPORT_PRELUDE
373 any p (x:xs) = p x || any p xs
376 all p (x:xs) = p x && all p xs
378 "any/build" forall p, g::forall b.(a->b->b)->b->b .
379 any p (build g) = g ((||) . p) False
380 "all/build" forall p, g::forall b.(a->b->b)->b->b .
381 all p (build g) = g ((&&) . p) True
385 -- elem is the list membership predicate, usually written in infix form,
386 -- e.g., x `elem` xs. notElem is the negation.
387 elem, notElem :: (Eq a) => a -> [a] -> Bool
388 #ifdef USE_REPORT_PRELUDE
390 notElem x = all (/= x)
393 elem x (y:ys) = x==y || elem x ys
396 notElem x (y:ys)= x /= y && notElem x ys
399 -- lookup key assocs looks up a key in an association list.
400 lookup :: (Eq a) => a -> [(a,b)] -> Maybe b
401 lookup _key [] = Nothing
402 lookup key ((x,y):xys)
404 | otherwise = lookup key xys
407 -- maximum and minimum return the maximum or minimum value from a list,
408 -- which must be non-empty, finite, and of an ordered type.
409 {-# SPECIALISE maximum :: [Int] -> Int #-}
410 {-# SPECIALISE minimum :: [Int] -> Int #-}
411 maximum, minimum :: (Ord a) => [a] -> a
412 maximum [] = errorEmptyList "maximum"
413 maximum xs = foldl1 max xs
415 minimum [] = errorEmptyList "minimum"
416 minimum xs = foldl1 min xs
418 concatMap :: (a -> [b]) -> [a] -> [b]
419 concatMap f = foldr ((++) . f) []
421 concat :: [[a]] -> [a]
422 {-# INLINE concat #-}
423 concat = foldr (++) []
428 -- List index (subscript) operator, 0-origin
429 (!!) :: [a] -> Int -> a
430 #ifdef USE_REPORT_PRELUDE
432 (_:xs) !! n | n > 0 = xs !! (n-1)
433 (_:_) !! _ = error "Prelude.(!!): negative index"
434 [] !! _ = error "Prelude.(!!): index too large"
436 -- HBC version (stolen), then unboxified
437 -- The semantics is not quite the same for error conditions
438 -- in the more efficient version.
440 xs !! (I# n) | n <# 0# = error "Prelude.(!!): negative index\n"
441 | otherwise = sub xs n
443 sub :: [a] -> Int# -> a
444 sub [] _ = error "Prelude.(!!): index too large\n"
445 sub (y:ys) n = if n ==# 0#
447 else sub ys (n -# 1#)
452 %*********************************************************
454 \subsection{The zip family}
456 %*********************************************************
461 foldr2 k z (x:xs) (y:ys) = k x y (foldr2 k z xs ys)
463 foldr2_left k z x r [] = z
464 foldr2_left k z x r (y:ys) = k x y (r ys)
466 foldr2_right k z y r [] = z
467 foldr2_right k z y r (x:xs) = k x y (r xs)
469 -- foldr2 k z xs ys = foldr (foldr2_left k z) (\_ -> z) xs ys
470 -- foldr2 k z xs ys = foldr (foldr2_right k z) (\_ -> z) ys xs
472 "foldr2/left" forall k,z,ys,g::forall b.(a->b->b)->b->b .
473 foldr2 k z (build g) ys = g (foldr2_left k z) (\_ -> z) ys
475 "foldr2/right" forall k,z,xs,g::forall b.(a->b->b)->b->b .
476 foldr2 k z xs (build g) = g (foldr2_right k z) (\_ -> z) xs
480 zip takes two lists and returns a list of corresponding pairs. If one
481 input list is short, excess elements of the longer list are discarded.
482 zip3 takes three lists and returns a list of triples. Zips for larger
483 tuples are in the List library
486 ----------------------------------------------
487 zip :: [a] -> [b] -> [(a,b)]
489 zip xs ys = build (\c n -> foldr2 (zipFB c) n xs ys)
491 zipFB c x y r = (x,y) `c` r
494 zipList :: [a] -> [b] -> [(a,b)]
495 zipList (a:as) (b:bs) = (a,b) : zipList as bs
499 "zipList" foldr2 (zipFB (:)) [] = zipList
504 ----------------------------------------------
505 zip3 :: [a] -> [b] -> [c] -> [(a,b,c)]
507 -- zip3 = zipWith3 (,,)
508 zip3 (a:as) (b:bs) (c:cs) = (a,b,c) : zip3 as bs cs
513 -- The zipWith family generalises the zip family by zipping with the
514 -- function given as the first argument, instead of a tupling function.
515 -- For example, zipWith (+) is applied to two lists to produce the list
516 -- of corresponding sums.
520 ----------------------------------------------
521 zipWith :: (a->b->c) -> [a]->[b]->[c]
522 {-# INLINE zipWith #-}
523 zipWith f xs ys = build (\c n -> foldr2 (zipWithFB c f) n xs ys)
525 zipWithFB c f x y r = (x `f` y) `c` r
527 zipWithList :: (a->b->c) -> [a] -> [b] -> [c]
528 zipWithList f (a:as) (b:bs) = f a b : zipWithList f as bs
529 zipWithList f _ _ = []
532 "zipWithList" forall f. foldr2 (zipWithFB (:) f) [] = zipWithList f
537 zipWith3 :: (a->b->c->d) -> [a]->[b]->[c]->[d]
538 zipWith3 z (a:as) (b:bs) (c:cs)
539 = z a b c : zipWith3 z as bs cs
540 zipWith3 _ _ _ _ = []
542 -- unzip transforms a list of pairs into a pair of lists.
543 unzip :: [(a,b)] -> ([a],[b])
544 unzip = foldr (\(a,b) ~(as,bs) -> (a:as,b:bs)) ([],[])
546 unzip3 :: [(a,b,c)] -> ([a],[b],[c])
547 unzip3 = foldr (\(a,b,c) ~(as,bs,cs) -> (a:as,b:bs,c:cs))
552 %*********************************************************
554 \subsection{Error code}
556 %*********************************************************
558 Common up near identical calls to `error' to reduce the number
559 constant strings created when compiled:
562 errorEmptyList :: String -> a
564 error (prel_list_str ++ fun ++ ": empty list")
566 errorNegativeIdx :: String -> a
567 errorNegativeIdx fun =
568 error (prel_list_str ++ fun ++ ": negative index")
570 prel_list_str :: String
571 prel_list_str = "Prelude."