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 <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
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