[GHC] #16366: Strict version of `foldlM`.

[GHC]

#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:
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 `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>
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