Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC Commits: 003b715b by meooow25 at 2025-09-02T23:48:51+02:00 Adjust the strictness of Data.List.iterate' * Don't force the next element in advance when generating a (:). * Force the first element to WHNF like every other element. Now every element in the output list is forced to WHNF when the (:) containing it is forced. CLC proposal: https://github.com/haskell/core-libraries-committee/issues/335 - - - - - 2 changed files: - libraries/base/changelog.md - libraries/ghc-internal/src/GHC/Internal/List.hs Changes: ===================================== libraries/base/changelog.md ===================================== @@ -8,6 +8,7 @@ * `GHC.Exts.IOPort#` and its related operations have been removed ([CLC #213](https://github.com/haskell/core-libraries-committee/issues/213)) * Fix bug where `naturalAndNot` was incorrectly truncating results ([CLC proposal #350](github.com/haskell/core-libraries-committee/issues/350)) * Remove extra laziness from `Data.Bifunctor.Bifunctor` instances for all tuples to have the same laziness as their `Data.Functor.Functor` counterparts (i.e. they became more strict than before) ([CLC proposal #339](https://github.com/haskell/core-libraries-committee/issues/339)) + * Adjust the strictness of `Data.List.iterate'` to be more reasonable: every element of the output list is forced to WHNF when the `(:)` containing it is forced. ([CLC proposal #335)](https://github.com/haskell/core-libraries-committee/issues/335) ## 4.22.0.0 *TBA* * Shipped with GHC 9.14.1 ===================================== libraries/ghc-internal/src/GHC/Internal/List.hs ===================================== @@ -854,11 +854,17 @@ minimum xs = foldl1' min xs -- ==== __Laziness__ -- -- Note that 'iterate' is lazy, potentially leading to thunk build-up if --- the consumer doesn't force each iterate. See 'iterate'' for a strict +-- the consumer doesn't force each element. See 'iterate'' for a strict -- variant of this function. -- --- >>> take 1 $ iterate undefined 42 --- [42] +-- >>> let xs = iterate (\x -> if x == 0 then undefined else x - 1) 2 +-- >>> xs +-- [2,1,0,*** Exception: Prelude.undefined +-- >>> length (take 10 xs) +-- 10 +-- +-- In @xs@ every element following @0@ is bottom, but the list itself is +-- infinitely long because it is generated without forcing its elements. -- -- ==== __Examples__ -- @@ -889,24 +895,26 @@ iterateFB c f x0 = go x0 -- | 'iterate'' is the strict version of 'iterate'. -- --- It forces the result of each application of the function to weak head normal --- form (WHNF) --- before proceeding. +-- It forces each element to weak head normal form (WHNF) before proceeding. -- --- >>> take 1 $ iterate' undefined 42 +-- ==== __Laziness__ +-- +-- >>> let xs = iterate' (\x -> if x == 0 then undefined else x - 1) 2 +-- >>> xs +-- [2,1,0*** Exception: Prelude.undefined +-- >>> length (take 10 xs) -- *** Exception: Prelude.undefined +-- +-- The list @xs@ has 3 elements followed by a tail that is bottom. +-- {-# NOINLINE [1] iterate' #-} iterate' :: (a -> a) -> a -> [a] -iterate' f x = - let x' = f x - in x' `seq` (x : iterate' f x') +iterate' f !x = x : iterate' f (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') + where go !x = x `c` go (f x) {-# RULES "iterate'" [~1] forall f x. iterate' f x = build (\c _n -> iterate'FB c f x) View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/003b715b98101a19e20443529b62376d... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/003b715b98101a19e20443529b62376d... You're receiving this email because of your account on gitlab.haskell.org.