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
The only possible implementation for function f will be const undefined. Your function f promises that given a value of type a, it can return a value of *any* type b, as long as b is an instance of typeclass A. So, once we are given a value of type a, we have to produce a value of type b, however, we don't know what type b is! The only thing that we know about things of type b is that they are an instance of A. This is not enough information to be able to produce something of type 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) <snip>
See the above: you are producing of type String. However, according to the type signature, you should be able to produce something of an arbitrary type b. Clearly that is not possible. I am guessing that once you know the type a, you also know what the output type b will be. If that is the case then you can use type families: class A a where type B a f :: a -> B a instance A String where type B String = String f = id Hope this helps. Regards, -- - Frank