The caller of your function f, rather than the implementation of f, gets to
decide which instance of A f should return. In other words, f's
implementation must be polymorphic in its return type. Your example f
x = x does
not satisfy that property. That implementation has type f :: A a => a -> a,
not the required type f :: (A a, A b) => a -> b.
The error message is saying that, from the class definition, GHC has
deduced that f must return a value of type A b => b, but your
implementation is returning a String (a.k.a. [Char]) instead. The notation b
~ [Char] means "b is equivalent to [Char]".
On Tue, Nov 11, 2014 at 12:59 PM, 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