GHC doesn't know that seq# produces something in WHNF

data Str a = Str !a
bar :: Maybe a -> IO (Str (Maybe a))
bar x = do
  x' <- evaluate x
  pure (Str x')

This compiles to

Test.bar1
  = \ (@ a_a3Ld)
      (x_a3Ah :: Maybe a_a3Ld)
      (s_i3Nz :: GHC.Prim.State# GHC.Prim.RealWorld) ->
      case GHC.Prim.seq#
             @ (Maybe a_a3Ld) @ GHC.Prim.RealWorld x_a3Ah s_i3Nz
      of
      { (# ipv_i3NC, ipv1_i3ND #) ->
      (# ipv_i3NC, Test.$WStr @ (Maybe a_a3Ld) ipv1_i3ND #)
      }

We suspend the application of $WStr to ipv1_i3ND, when all we actually need to do is apply Str directly. We could work around this in base by defining

evaluate x = IO $ \s ->
  case seq# x s of
    (# s', !x' #) -> (# s', x' #)

but that seems more than a little bit silly.

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonmar
Operating system
Architecture