[Haskell-cafe] Class Quantification

[Sean Leather]

> Now I would like to generalise 'foo' and 'bar' to 'bla' so that I can
> write:
>
>  testBla1 = bla fromInteger 1  :: (Int, Float)
>  testBla2 = bla read       "1" :: (Int, Float)
>
> My question is how to define 'bla'.
>
> I can write:
>
> > bla :: (forall b. a -> b) -> a -> (c, d)
> > bla f x = (f x, f x)
>
> But then 'testBla1' gives the following expected error:
>
>  Could not deduce (Num b) from the context ()
>    arising from a use of `fromInteger'
>
> [...]
>
> So, somehow I need to quantify over the type class.
>
>  bla :: forall cls. (cls c, cls d) => (forall b. cls b => a -> b) -> a ->
> (c, d)
>
> But this isn't legal.
>
> Is there another way of defining 'bla'?
>

The simple, from-the-gut answer involves the enumeration of all the classes
that you need in the context.

> bla :: (Read c, Read d, Num c, Num d) => (forall b. (Read b, Num b) => a
-> b) -> a -> (c, d)
> bla f x = (f x, f x)

But perhaps you're looking for potentially unknown classes?

Sean
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