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
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