#12173: foldl' semantics changed from 4.7 to 4.8 -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Core | Version: 7.10.2 Libraries | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Runtime crash Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- `foldl'` is now defined as {{{#!hs foldl' :: forall a b . (b -> a -> b) -> b -> [a] -> b {-# INLINE foldl' #-} foldl' k z0 xs = foldr (\(v::a) (fn::b->b) -> oneShot (\(z::b) -> z `seq` fn (k z v))) (id :: b -> b) xs z0 }}} As far as I can tell, the Haskell 2010 report does not specify anything about the behavior of `foldl'`. In base 4.7, it was defined {{{#!hs foldl' :: (b -> a -> b) -> b -> [a] -> b foldl' f z0 xs0 = lgo z0 xs0 where lgo z [] = z lgo z (x:xs) = let z' = f z x in z' `seq` lgo z' xs }}} These are ''not'' equivalent. In particular, with the old `foldl'`, {{{#!hs foldl' (\_ _ -> 3) undefined "hello" }}} evaluates to `3`, but with the new one, it throws an exception. If the old semantics are preferred, we can get them with {{{#!hs foldl' :: forall a b . (b -> a -> b) -> b -> [a] -> b {-# INLINE foldl' #-} foldl' k z0 xs = foldr (\(v::a) (fn::b->b) -> oneShot (\(z::b) -> fn $! k z v)) (id :: b -> b) xs z0 }}} The ''old'' semantics match the default `Foldable` instance. The advantage of the ''new'' semantics is that they're more consistent about strictness (unconditionally strict in the accumulator), but that blocks out idioms like {{{#!hs foldl' f (error "Empty list") ... }}} I don't remember this being discussed. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12173 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler