Cannot create apply function with static language?

Question

I have read that with a statically typed language like Scala or Haskell there is no way to create or provide a Lisp apply function:

(apply #\'+ (lis         


        

Answer 1

In Haskell, there is no datatype for multi-types lists, although I believe, that you can hack something like this together whith the mysterious Typeable typeclass. As I see, you're looking for a function, which takes a function, a which contains exactly the same amount of values as needed by the function and returns the result.

For me, this looks very familiar to haskells uncurryfunction, just that it takes a tuple instead of a list. The difference is, that a tuple has always the same count of elements (so (1,2) and (1,2,3) are of different types (!)) and there contents can be arbitrary typed.

The uncurry function has this definition:

uncurry :: (a -> b -> c) -> (a,b) -> c
uncurry f (a,b) = f a b

What you need is some kind of uncurry which is overloaded in a way to provide an arbitrary number of params. I think of something like this:

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}

class MyApply f t r where
  myApply :: f -> t -> r

instance MyApply (a -> b -> c) (a,b) c where
  myApply f (a,b) = f a b

instance MyApply (a -> b -> c -> d) (a,b,c) d where
  myApply f (a,b,c) = f a b c

-- and so on

But this only works, if ALL types involved are known to the compiler. Sadly, adding a fundep causes the compiler to refuse compilation. As I'm not a haskell guru, maybe domeone else knows, howto fix this. Sadly, I don't know how to archieve this easier.

Résumee: apply is not very easy in Haskell, although possible. I guess, you'll never need it.

Edit I have a better idea now, give me ten minutes and I present you something whithout these problems.