Hi all,
I'm hoping someone can explain some weirdnesses to me. I want to have a
class that's like the standard Monad class, but is a bit more
general. That is, I want:
class Monad2 m a ma | m a -> ma, ma -> m a where
return2 :: a -> ma
bind2 :: Monad2 m b mb => ma -> (a -> mb) -> mb
_unused :: m a -> ()
_unused = \_ -> ()
Here, all the real work happens in the combined "mb" parameter. The
_unused definition is just to get Hugs to realize that 'm' should be of
kind * -> * (for GHC, a kind definition does the same thing).
I want to be able to define instances like:
instance Monad2 [] a [a] where
return2 x = [x]
bind2 l f = concatMap f l
However, in GHC, if I just give the instance declaration (without the
function definitions), we get:
Cannot unify the type-signature variable `mb' with the type `[b]'
Expected type: [b]
Inferred type: mb
When using functional dependencies to combine
Monad2 [] a [a],
arising from the instance declaration at
/nfs/isd/hdaume/projects/Trials/UsefulDTs/UsefulDTs.hs:114
Monad2 [] b mb,
arising from a type signature at
/nfs/isd/hdaume/projects/Trials/UsefulDTs/UsefulDTs.hs:114
When trying to generalise the type inferred for `bind2'
Signature type: forall a mb1 b1.
(Monad2 [] b1 mb1) =>
[a] -> (a -> mb1) -> mb1
Type to generalise: forall mb1 b1.
(Monad2 [] b1 mb1) =>
[a] -> (a -> mb1) -> mb1
This doesn't make sense to me. Hugs accepts this without
complaint. Here's why I don't understand it:
We know that m is []. We have an instance definition (the one we're
defining) that says that for any type x, we have 'Monad2 [] x [x]' where
[] and x determine [x]. This should tell us that mb=[b], but somehow GHC
doesn't get that (or there's something wrong with my reasoning).
When the funciton definitions are provided for the instance, GHC
complains:
/nfs/isd/hdaume/projects/Trials/UsefulDTs/UsefulDTs.hs:119:
Cannot unify the type-signature variable `mb' with the type `[b]'
Expected type: a -> [b]
Inferred type: a -> mb
In the first argument of `concatMap', namely `f'
In the definition of `bind2': concatMap f l
and Hugs complains similarly:
*** Expected type : (Monad2 [] a [a], Monad2 [] b c) => [a] -> (a -> c) ->
c
*** Inferred type : (Monad2 [] a [a], Monad2 [] b [c]) => [a] -> (a ->
[c]) -> [c]
Both are saying that they don't know that mb=[b], but I think the fundeps
guarentee this.
Note that if we change the type definition of bind2 to:
bind2 :: Monad2 m b mb => ma -> (a -> m b) -> m b
then Hugs accepts the instance declaration but GHC complains as it did the
first time.
Can someone explain which is right and why?
Thanks!
- Hal
--
Hal Daume III
"Computer science is no more about computers | [EMAIL PROTECTED]
than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume
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