Nicolas Frisby wrote:
The inferred type for rite_t1 is rite_t1 :: (Iso (Either Char a) (Either f' g')) => () -> Either f' g'
Why isn't the inferred type of rite_t1 the same as the ascribed type of rite_t1'?
rite_t1' :: Iso b b' => () -> Either MyChar b' rite_t1' () = rite t1
I think GHC does not know whether the given instance declaration instance ... => Iso (Either a b) (Either a' b') even applies to the special case of (a = Char) because it mostly ignores the preconditions on the left side of (=>). Hugs is much different. Maybe throwing away undecidable instances will drastically change things.
Last post until a response I promise! Another demonstration:
bar () = runIdentity . flip runStateT 0 $ return 'c'
Inferred signature: bar :: (Monad (StateT s Identity), Num s) => () -> (Char, s)
Why not? bar :: Num s => () -> (Char, s)
I am not coming up with an s that could prevent (StateT s Identity) from being a monad. Is there one?
The same might go on for this case. By not looking at the preconditions in the instance declaration instance Monad m => Monad (StateT s m) GHC concludes that (Monad (StateT s Identity)) might or might not hold depending on s. Regards, apfelmus