Correction by the author:
It seems that ghc doesn't like the fact that I am saying 'foo' must return a class 'b' of typeclass 'Bar', while providing a function that returns a concrete data instance of 'Bar' (viz., FU or FI) later on when I implement 'foo' in each type classes.
Should read:
It seems that ghc doesn't like the fact that I am saying 'foo' must
return something of TYPE 'b' implementing typeclass 'Bar', while
providing
a function that returns a concrete data instance of 'Bar' (viz., FU or
FI)
later on when I implement 'foo' in each type classes.
On Sep 29, 10:43 pm, DNM
N.B. I'm a newbie to Haskell, and this problem is a bit complex, so bear with me.
I'm using typeclasses to implement a sort of common interface for all things -- call them things of type 'Cls' -- that can be expected to implement a set of functions -- an 'interface' in OOP-speak. (Yes, yes, I'm aware that typeclasses are subtly different and far superior, but my Haskell-ese is still a bit rudimentary.)
Essentially, I want to have a typeclass that expects its instances to have an accessor function that results in something that is an instance of another typeclass whose instances can perform some operation. The ghc type-checker doesn't seem to like my code, though, and I can't seem to figure out why.
To make it concrete, I've typed up some dummy typeclasses and a dummy function that uses their instances to illustrate what I mean, as well as the form of the ghc(i) error.
------------- BEGIN CODE ------------------ class Cls c where foo :: (Bar b) => c -> b
class Bar b where toNum :: b -> Int
-- | One implementation of Cls data D = D {fu :: FU} data FU = FU {num :: Int}
instance Cls D where foo = fu instance Bar FU where toNum f = (num f) + 47
-- | Another implementation of Cls data E = E {fi :: FI} data FI = FI {nuum :: Int}
instance Cls E where foo = fi instance Bar FI where toNum f = (nuum f) + 100
-- | Yet another (this one re-uses FI) data F = F {fii :: FI}
instance Cls F where foo = fii
-- | And one last one, just to stress that -- I really need to implement multiple -- instances of Cls. data G = G {fuu :: FU}
instance Cls G where foo = fuu
-- | Good. Now, the function 'useThisStuff' need -- not know anything about it's payload -- other than that it its args are Cls's -- (hence they are foo'able things that -- can be used to construct an Int answer). useThisStuff :: (Cls x, Cls y) => x -> y -> Int useThisStuff x y = (toNum $ foo x) + (toNum $ foo y) ------------- END CODE --------------------
When I type this up in a file and try to load it in ghci, I get the following error message(s):
------------- BEGIN ERROR MSG ---------- Prelude> :load Typeclasses.hs [1 of 1] Compiling Typeclasses ( Typeclasses.hs, interpreted )
Typeclasses.hs:14:10: Couldn't match expected type `b' against inferred type `FU' `b' is a rigid type variable bound by the type signature for `foo' at Typeclasses.hs:4:16 Expected type: D -> b Inferred type: D -> FU In the expression: fu In the definition of `foo': foo = fu
Typeclasses.hs:23:10: Couldn't match expected type `b' against inferred type `FI' `b' is a rigid type variable bound by the type signature for `foo' at Typeclasses.hs:4:16 Expected type: E -> b Inferred type: E -> FI In the expression: fi In the definition of `foo': foo = fi
Typeclasses.hs:31:10: Couldn't match expected type `b' against inferred type `FI' `b' is a rigid type variable bound by the type signature for `foo' at Typeclasses.hs:4:16 Expected type: F -> b Inferred type: F -> FI In the expression: fii In the definition of `foo': foo = fii
Typeclasses.hs:39:10: Couldn't match expected type `b' against inferred type `FU' `b' is a rigid type variable bound by the type signature for `foo' at Typeclasses.hs:4:16 Expected type: G -> b Inferred type: G -> FU In the expression: fuu In the definition of `foo': foo = fuu Failed, modules loaded: none. ------------- END ERROR MSG ------------
It seems that ghc doesn't like the fact that I am saying 'foo' must return a class 'b' of typeclass 'Bar', while providing a function that returns a concrete data instance of 'Bar' (viz., FU or FI) later on when I implement 'foo' in each type classes. Repeated for convenience:
class Cls c where foo :: (Bar b) => c -> b ... -- (e.g.) data G = G {fuu :: FU} instance Cls G where foo = fuu
Does anyone have any clue as to what I'm doing wrong (language extensions that I may need, etc.)?
Is is because I'm using context restrictions on the *result* type of a typeclass method? I've written other typeclasses with methods that say, essentially:
class A a where blah :: (MonadPlus m) => a -> a -> m a
with no issues. The restriction there is not on the return type a, but rather on some monadic 'wrapper' around it. This may be why that code works.
Please advise. Any help is greatly appreciated.
--D.N. (Dennis) _______________________________________________ Haskell-Cafe mailing list Haskell-C...@haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe