diff options
Diffstat (limited to 'libraries/base/GHC/List.hs')
| -rw-r--r-- | libraries/base/GHC/List.hs | 55 |
1 files changed, 45 insertions, 10 deletions
diff --git a/libraries/base/GHC/List.hs b/libraries/base/GHC/List.hs index 70bfbe4de0..92b5952cbe 100644 --- a/libraries/base/GHC/List.hs +++ b/libraries/base/GHC/List.hs @@ -1,7 +1,6 @@ {-# LANGUAGE Trustworthy #-} {-# LANGUAGE CPP, NoImplicitPrelude, ScopedTypeVariables, MagicHash #-} {-# LANGUAGE BangPatterns #-} -{-# OPTIONS_HADDOCK hide #-} ----------------------------------------------------------------------------- -- | @@ -23,7 +22,7 @@ module GHC.List ( map, (++), filter, concat, head, last, tail, init, uncons, null, length, (!!), foldl, foldl', foldl1, foldl1', scanl, scanl1, scanl', foldr, foldr1, - scanr, scanr1, iterate, repeat, replicate, cycle, + scanr, scanr1, iterate, iterate', repeat, replicate, cycle, take, drop, sum, product, maximum, minimum, splitAt, takeWhile, dropWhile, span, break, reverse, and, or, any, all, elem, notElem, lookup, @@ -442,7 +441,10 @@ minimum xs = foldl1 min xs -- of @f@ to @x@: -- -- > iterate f x == [x, f x, f (f x), ...] - +-- +-- Note that 'iterate' is lazy, potentially leading to thunk build-up if +-- the consumer doesn't force each iterate. See 'iterate'' for a strict +-- variant of this function. {-# NOINLINE [1] iterate #-} iterate :: (a -> a) -> a -> [a] iterate f x = x : iterate f (f x) @@ -458,6 +460,29 @@ iterateFB c f x0 = go x0 #-} +-- | 'iterate'' is the strict version of 'iterate'. +-- +-- It ensures that the result of each application of force to weak head normal +-- form before proceeding. +{-# NOINLINE [1] iterate' #-} +iterate' :: (a -> a) -> a -> [a] +iterate' f x = + let x' = f x + in x' `seq` (x : iterate' f x') + +{-# INLINE [0] iterate'FB #-} -- See Note [Inline FB functions] +iterate'FB :: (a -> b -> b) -> (a -> a) -> a -> b +iterate'FB c f x0 = go x0 + where go x = + let x' = f x + in x' `seq` (x `c` go x') + +{-# RULES +"iterate'" [~1] forall f x. iterate' f x = build (\c _n -> iterate'FB c f x) +"iterate'FB" [1] iterate'FB (:) = iterate' + #-} + + -- | 'repeat' @x@ is an infinite list, with @x@ the value of every element. repeat :: a -> [a] {-# INLINE [0] repeat #-} @@ -921,12 +946,19 @@ foldr2_left k _z x r (y:ys) = k x y (r ys) ---------------------------------------------- -- | 'zip' takes two lists and returns a list of corresponding pairs. +-- +-- > zip [1, 2] ['a', 'b'] = [(1, 'a'), (2, 'b')] +-- -- If one input list is short, excess elements of the longer list are --- discarded. +-- discarded: +-- +-- > zip [1] ['a', 'b'] = [(1, 'a')] +-- > zip [1, 2] ['a'] = [(1, 'a')] -- -- 'zip' is right-lazy: -- -- > zip [] _|_ = [] +-- > zip _|_ [] = _|_ {-# NOINLINE [1] zip #-} zip :: [a] -> [b] -> [(a,b)] zip [] _bs = [] @@ -966,9 +998,11 @@ zip3 _ _ _ = [] -- > zipWith f [] _|_ = [] {-# NOINLINE [1] zipWith #-} zipWith :: (a->b->c) -> [a]->[b]->[c] -zipWith _f [] _bs = [] -zipWith _f _as [] = [] -zipWith f (a:as) (b:bs) = f a b : zipWith f as bs +zipWith f = go + where + go [] _ = [] + go _ [] = [] + go (x:xs) (y:ys) = f x y : go xs ys -- zipWithFB must have arity 2 since it gets two arguments in the "zipWith" -- rule; it might not get inlined otherwise @@ -985,9 +1019,10 @@ zipWithFB c f = \x y r -> (x `f` y) `c` r -- elements, as well as three lists and returns a list of their point-wise -- combination, analogous to 'zipWith'. zipWith3 :: (a->b->c->d) -> [a]->[b]->[c]->[d] -zipWith3 z (a:as) (b:bs) (c:cs) - = z a b c : zipWith3 z as bs cs -zipWith3 _ _ _ _ = [] +zipWith3 z = go + where + go (a:as) (b:bs) (c:cs) = z a b c : go as bs cs + go _ _ _ = [] -- | 'unzip' transforms a list of pairs into a list of first components -- and a list of second components. |