From: Michael Neumann <mneumann@...>
Date: 2003-11-20T16:59:39+09:00
Subject: Re: Ocaml's typing (was Re: "stereotyping")

On Thu, Nov 20, 2003 at 01:12:15PM +0900, Steven@ruby-lang.org wrote:
> Austin Ziegler <austin@halostatue.ca> writes:
> >Consider generics in Java (templates in C++). These effectively
> >introduce the sort of dynamicity that is found in Perl, Python, and
> >Ruby. Instead of having to do:
> >  int max(int x, int y) { return x >= y ? x : y }
> >  float max(float x, float y) { return x >= y ? x : y }
> >  double max(double x, double y) { return x >= y ? x : y }
> >You can do:
> >  def max(x, y); x >= y ? x : y; end
> >I know which I would prefer, and C++'s move toward generics is a
> >halting step toward that step.
> 
> BTW, to see a different point of view, you might investigate
> Ocaml (or other ML languages, or Haskell).  Ocaml is strongly,
> statically type-checked.  Here is max:
> 
>     let max x y = if x > y then x else y
> 
> (Note that there are no type declarations!)
> 
>     # max 3 4;;
>     - : int = 4
>     # max 3.0 4.0;;
>     - : float = 4

But in Ocaml 

  3.4 + 1.2

is not allowed, as + is only defined on integers (there's no type
overloading). So you have to use +. for floating point addition, *.  for
multiplication, print_int, print_string, print_float etc. Sometimes,
that's pretty ugly , IMHO.

> Why does this work?  How can max return an int the first time, and
> a float the second?  Because the type declaration for max (as
> inferred by the compiler) is:
> 
>      max : 'a -> 'a -> 'a = <fun>

Indeed, this does only work, as the comparison operators are defined
internally for every type. You cannot do this with your own funtioncs,
say: 

  if (greater x y) then x else y

This is where Haskell & Co is clearly better (or GCaml, generic OCaml).

Regards,

  Michael