On Fri, Jan 28, 2005 at 10:01:33AM -0500, Jacques Carette wrote:
> The previous post on record syntax reminded me of some 'problems' I had
> noticed where Haskell and mathematics have a (deep) usage mismatch.
>
> First, consider a syntax for other component-wise function application?
> For example, it would be convenient to have (f,g) @ (x,y)
> be (f x, g y). In some languages [with dynamic typing], one can even do
> (f,g) (x,y) :-)
> Yes, I am aware that this untypeable in Haskell, because polymorphism is
> straight-jacketed by structural rules.
It's not as bad as you think. You can do this:
{-# OPTIONS -fglasgow-exts #-}
module Apply where
class Apply f a b | f -> a, f -> b where
apply :: f -> a -> b
instance Apply (a -> b) a b where
apply f a = f a
instance Apply (a1 -> b1, a2 -> b2) (a1, a2) (b1, b2) where
apply (f1, f2) (a1, a2) = (f1 a1, f2 a2)
instance Apply (a1 -> b1, a2 -> b2, a3 -> b3) (a1, a2, a3) (b1, b2, b3)
where
apply (f1, f2, f3) (a1, a2, a3) = (f1 a1, f2 a2, f3 a3)
And then:
*Apply> (succ, pred, show) `apply` (1, 2, ())
(2,1,"()")
*Apply> (\x -> 2 * x + 1/2, length) `apply` (10, [1,2,3,4,5])
(20.5,5)
Best regards,
Tomasz
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