I've written a function similar to Data.Enumerator.List.map
that makes an Iteratee
compatible with an Enumerator
that feeds a different Stream
type.
import Data.Enumerator
test :: Monad m => (ao -> ai) -> Iteratee ai m b -> Iteratee ao m b
test f iter = go $$ iter
where go (Continue k) = continue $
\stream -> go $$ k (fmap f stream)
go (Yield res _) = yield res EOF
If I omit the type signature for go
, this will work just fine.
However, I'd like to include it but I'm unable to determine what the correct signature should be.
Here's what I think it should be:
go :: Monad m => Step ai m b -> Iteratee ao m b
but that doesn't work.
I need some advice on finding the correct type signature for go
.
You probably can't give go
a type signature as-is.
The reason for this is that it makes use of polymorphic arguments bound by test
. This means that, inside go
, the identifier f
has type (ao -> ai)
for some specific, but unknown types ao
and ai
.
Type variables are generally only in scope for the single type signature where they're introduced, so when you give go
its own type signature, the ao
and ai
there are new, polymorphic types, which of course causes a type error when trying to combine them with the similarly named, but fixed (and unknowable) types from test
's signature.
The end result is that you can't write the type of go
explicitly, which is not very satisfying. To solve this, GHC offers the ScopedTypeVariables extension, which allows bringing variables introduced in a type signature in scope inside the where
clause of the function, among other things.
Note that if you only use the where
clause to create an internal scope for definitions, and don't make use of identifiers bound by arguments to the outer function, you can write type signatures in the where
clause just like you can for top level bindings. If you don't want to use GHC extensions, you can simply pass the parameters in redundantly. Something like this should work in that case:
test :: Monad m => (ao -> ai) -> Iteratee ai m b -> Iteratee ao m b
test f iter = go f $$ iter
where go :: Monad m => (ao -> ai) -> Step ai m b -> Iteratee ao m b
go f (Continue k) = continue $
\stream -> go f $$ k (fmap f stream)
go _ (Yield res _) = yield res EOF
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