[mercury-users] clause/1, functor/3, arg/3 ...

[Ralph Becket]

Fergus Henderson wrote on 8 Jul:
> ...
> For any given type, you can use a switch and pattern matching,
> but if you want to write a single predicate that works for every type,
> then you need something like arg/3 & functor/3.
> 
> An immediate application of this is the io__print, io__write, io__read,
> io__write_binary, and io__read_binary predicates that will be included
> in the next release.  The io__print predicate subsumes the functionality
> of io__write_int, io__write_string, io__write_float, etc.

io__write_many seems to be sufficient for all that stuff.  Although
it's nice to have `print whatever' procedures in an interactive/
debugging environment, I'm not sure I see what it buys you otherwise.
Why not have a programmer extensible overloaded function with
universal scope, for example:

:- func fmt(my_thing::in)=string::out.

fmt(X) = S :-
  X = case1(...), string__format("...", [...], S);
  ...

Programmers would similarly be expected to provide unmarshalling
procedures for reading things in.

> Another application of this sort of thing is for writing a generic
> hash function.  

Maybe... it just doesn't feel in the Mercury spirit.  By all means
provide these things as built-ins, such as standard ordering, where
the programmer can pretend they're overloaded functions.

I can see the point in having these things around, I'd just like to
see them as well hidden as possible.

'Nuff whingeing from me,

Ralph