Re: [Haskell-cafe] Re: Problem with result-type context restrictions in typeclasses.

In your class, you have: class Cls c where foo :: (Bar b) => c -> b There's an implicit forall for b, meaning that the caller of the method gets to choose what it wants for b (as long as it's an instance of Bar). For you to be able to write such a method you'd need to write functions that can return any instance of Bar. One solution to this is to turn on the GHC extension -XTypeFamilies, and then modify your code as follows: class Cls c where type Ret c :: * -- or a better name foo :: c -> Ret c instance Cls G where type Ret G = FU foo = fuu That should work (although I haven't tested it). What type families do in this case is allow you to write not only methods associated with typeclasses, but type functions associated with them too. In this case you can think of Ret as a function that takes a type (G in the instance above) and returns another type (FU). Each instance can define new mappings for Ret. Hope this helps! Dan On Tue, Sep 29, 2009 at 10:48 PM, DNM wrote:

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 wrote:

...

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

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