[Haskell-cafe] A Very Simple Type Class Question

[Taylor Hedberg]

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 at 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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