[Haskell-cafe] casting

[Thomas Conway]

Hi All

I'm having some difficulty with typeclasses.

What I'm trying to do should be obvious, but it's still giving me
trouble. I want to take a packaged item, and strengthen the
constraints on its type. Rather than being just any type that is an
instance of A, I want to do a runtime check and do something different
if it is a type that is also an instance of B (which implies that it
is an instance of A also).

I've attached some code that attempts this, but ghci (er, 6.4.2 with
-fglasgow-exts) complains:

Compiling Main             ( Cast.hs, interpreted )

Cast.hs:15:22:
    Ambiguous type variable `b' in the constraint:
      `B b' arising from use of `qux' at Cast.hs:15:22-24
    Probable fix: add a type signature that fixes these type variable(s)
Failed, modules loaded: none.

So, can anyone suggest how I can achieve my goal?

thanks,
Tom
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