When you declare a type signature such as f :: C x => a -> b there is an implicit "forall": f :: forall x. C x => a -> x That is, the type signature must hold *for all* x such that C x holds. However, in your case, it only holds for String – while A String does hold, that is irrelevant, because the type signature states that it must hold for *any* x such that A x that the caller wants. To put it another way, the caller decides the "b" in that type signature, not the function implementation. What you want is something like f :: exists x. C x => a -> x so that f will return some unknown value such that you know it is an instance of C x. Hope that helps, Andrew On Tue, Nov 11, 2014 at 12:59 PM, Larry Lee <llee454@gmail.com> wrote:
Hi
I have a very simple problem. I have a class and want to define a function in that class that returns a different instance of the same class.
I tried accomplishing this as follows:
class A a where f :: A b => a -> b
This fails however when I try to instantiate it. For example:
instance A String where f x = x
I get an error message that makes absolutely no sense to me:
src/CSVTree.hs:12:9: Could not deduce (b ~ [Char]) from the context (A b) bound by the type signature for f :: A b => String -> b at src/CSVTree.hs:12:3-9 `b' is a rigid type variable bound by the type signature for f :: A b => String -> b at src/CSVTree.hs:12:3 In the expression: x In an equation for `f': f x = x In the instance declaration for `A String' make: *** [compile] Error 1
Can someone please explain: how I can achieve my goal; and why my code is failing; simply and in plain English.
Thanks, Larry _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe