#16366: Strict version of `foldlM`. -------------------------------------+------------------------------------- Reporter: autotaker | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: | Version: 8.6.3 libraries/base | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- `Data.Foldable.foldlM` is lazy on the accumulation parameter. A user can define {{{foldlM (\acc x -> acc `seq` step acc x) z xs}}}, but actually this function is NOT strict! It is simplified to the following core: {{{#!hs foldlM (\acc x -> acc `seq` step acc x) z xs == go xs z where go [] acc = pure acc go (x:xs) acc = (acc `seq` step acc x) >>= go xs }}} DemandAnalysis infers that `go` is lazy on the second argument because `pure` is lazy in general (e.g. `IO` Monad). Thus `foldlM'` is needed. Its definition would be: {{{#!hs foldlM' :: (Monad m, Foldable t) => (b -> a -> m b) -> b -> t a -> m b foldlM' f z0 xs = foldr c (\x -> x `seq` pure x) xs z0 where c x k = \z -> z `seq` (f z x >>= k) {-# INLINE c #-} }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16366 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler