consider the following types (from the paper:
"Stream Fusion From Lists to Streams to Nothing at All")
data Stream a = forall s. Stream ( s ->(Step a s)) s
data Step a s =
Done
| Yield a s
| Skip s
an instance of this data type is:
stream0 :: Stream ()
stream0 = Stream (\ s -> Yield () s) ()
now consider:
data StreamM m a = forall s. StreamM ( s ->m (Step a s)) s
and the following instance:
stream0IO :: StreamM IO ()
stream0IO = StreamM (\ s -> return (Yield () s)) ()
loads ok, but as soon as we introduce the following constraint:
data Monad m => StreamM' m a = forall s. StreamM' ( s ->m (Step a s)) s
stream0IO' :: StreamM' IO ()
stream0IO' = StreamM' (\ s -> return (Yield () s)) ()
we get the following message when we load into ghci (6.8.2)
Var/Type length mismatch:
[]
[base:GHC.Base.(){(w) tc 40}]
is this a bug? if not, what is this message telling us ?
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