What is Expressiveness in a Computer Language

[Greg Buchholz]

Joe Marshall wrote:
> It isn't clear to me which programs we would have to give up, either.
> I don't have much experience in sophisticated typed languages.  It is
> rather easy to find programs that baffle an unsophisticated typed
> language (C, C++, Java, etc.).
>
> Looking back in comp.lang.lisp, I see these examples:
>
> (defun noisy-apply (f arglist)
>   (format t "I am now about to apply ~s to ~s" f arglist)
>   (apply f arglist))

    Just for fun, here is how you might do "apply" in Haskell...

{-# OPTIONS -fglasgow-exts #-}

class Apply x y z | x y -> z where
    apply :: x -> y -> z

instance Apply (a->b) a b where
    apply f x = f x

instance Apply b as c => Apply (a->b) (a,as) c where
    apply f (x,xs) = apply (f x) xs

f :: Int -> Double -> String -> Bool -> Int
f x y z True = x + floor y * length z
f x y z False= x * floor y + length z

args =  (1::Int,(3.1415::Double,("flub",True)))

main = print $ apply f args


...which depends on the fact that functions are automatically curried
in Haskell.  You could do something similar for fully curried function
objects in C++.   Also of possible interest...

    Functions with the variable number of (variously typed) arguments
    http://okmij.org/ftp/Haskell/types.html#polyvar-fn

...But now I'm curious about how to create the "apply" function in a
language like Scheme.  I suppose you could do something like...

  (define (apply fun args)
    (eval (cons fun args)))

...but "eval" seems a little like overkill.  Is there a better way?


Greg Buchholz