The problem is that the type of f is
f :: (A a, A b) => a -> b
This means that given an 'a', you need to create a function which works for
*any* b in A.
However, the function you implement is of type `f :: String -> String`, not
of type `f :: A b => String -> b`, as needed. If you were to implement the
function in that way, you could use:
class A a where
f :: a -> a
2014-11-11 21:59 GMT+01:00 Larry Lee
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