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Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC

Commits:

33b546bd

by meooow25 at 2025-07-07T20:46:09-04:00

Keep scanl' strict in the head on rewrite

`scanl'` forces elements to WHNF when the corresponding `(:)`s are
forced. The rewrite rule for `scanl'` missed forcing the first element,
which is fixed here with a `seq`.

2 changed files:

libraries/base/changelog.md
libraries/ghc-internal/src/GHC/Internal/List.hs

Changes:

libraries/base/changelog.md

@@ -26,6 +26,7 @@
    * Expose constructor and field of `Backtraces` from `Control.Exception.Backtrace`, as per [CLC #199](https://github.com/haskell/core-libraries-committee/issues/199#issuecomment-1954662391)
    * Fix incorrect results of `integerPowMod` when the base is 0 and the exponent is negative, and `integerRecipMod` when the modulus is zero ([#26017](https://gitlab.haskell.org/ghc/ghc/-/issues/26017)).
 +  * Fix the rewrite rule for `scanl'` not being strict in the first element of the output list ([#26143](https://gitlab.haskell.org/ghc/ghc/-/issues/26143)).
  ## 4.21.0.0 *December 2024*

libraries/ghc-internal/src/GHC/Internal/List.hs

@@ -601,7 +601,7 @@ scanl' = scanlGo'
  -- See Note [scanl rewrite rules]
  {-# RULES
  "scanl'"  [~1] forall f a bs . scanl' f a bs =
 -  build (\c n -> a `c` foldr (scanlFB' f c) (flipSeq n) bs a)
 +  build (\c n -> a `seq` (a `c` foldr (scanlFB' f c) (flipSeq n) bs a))
  "scanlList'" [1] forall f a bs .
      foldr (scanlFB' f (:)) (flipSeq []) bs a = tail (scanl' f a bs)
   #-}