[Haskell-cafe] mapTuple

[Arthur van Leeuwen]

On 11-jan-2007, at 16:30, Marco Túlio Gontijo e Silva wrote:

> Em Qui, 2007-01-11 às 16:14 +0100, minh thu escreveu:
>> you might want invistigate "heterogeneous lists" : in your case, it's
>> "heterogeneous typle".
>
> But aren't tuples always heterogeneous?

Yes, and precisely therein lies the problem. There is no way
for the compiler to infer that in

tupleMap f (a, b) = (f a, f b)

the type of f should be polymorphic. If you want it to be,
explicitly require it to be so:

tupleMap :: (forall a b . a -> b) -> (b,c) -> (d,e)
tupleMap f (a,b) = (f a, f b)

However, this will still not allow you to write

tupleMap show ("string",3)

as you require f to be fully polymorphic, and not constrained by any
context.

With regards, Arthur.

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