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Cannot create apply function with static language?

Asked 2010-09-12T02:03:22.143
21

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 #'+ (list 1 2 3)) => 6

or maybe

(apply #'list '(list :foo 1 2 "bar")) => (:FOO 1 2 "bar")
(apply #'nth (list 1 '(1 2 3))) => 2

Is this a truth?

scala programming-languages haskell lisp static-typing
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1 Answer

-1

A list in Haskell can only store values of one type, so you couldn't do funny stuff like (apply substring ["Foo",2,3]). Neither does Haskell have variadic functions, so (+) can only ever take two arguments.

There is a $ function in Haskell:

($)                     :: (a -> b) -> a -> b
f $ x                   =  f x

But that's only really useful because it has very low precedence, or as passing around HOFs.

I imagine you might be able to do something like this using tuple types and fundeps though?

class Apply f tt vt | f -> tt, f -> vt where
  apply :: f -> tt -> vt

instance Apply (a -> r) a r where
  apply f t = f t

instance Apply (a1 -> a2 -> r) (a1,a2) r where
  apply f (t1,t2) = f t1 t2

instance Apply (a1 -> a2 -> a3 -> r) (a1,a2,a3) r where
  apply f (t1,t2,t3) = f t1 t2 t3

I guess that's a sort of 'uncurryN', isn't it?

Edit: this doesn't actually compile; superseded by @FUZxxl's answer.

answered 2010-09-12T02:33:04.043

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