[mercury-users] boolean expressions from semidet predicates

[Nicholas Nethercote]

On Mon, 15 May 2006, Mark Brown wrote:

>> It has operators like '+', '++' and 'and',
>
> Yes, that means you can write `X and Y' instead of `and(X, Y)', etc.  As
> I mentioned earlier, this has no impact on the semantics: both mean the
> same thing.  Once Mercury has parsed the program, it no longer knows or
> cares whether infix notation was used.
>
>> but then there are functions int.+/2 and string.++/2 and bool.and/2.
>
> Yes.  Where is the problem?

So in Mercury some of the names, such as 'and', are treated specially in 
that you can write them infix.  So this program works:

  :- module a.

  :- interface.
  :- import_module io.
  :- pred main(io :: di, io :: uo) is det.

  :- implementation.
  :- import_module string.

  main(!IO) :-
     io.write_string("hello\n" and "goodbye\n", !IO).

  :- func and(string, string) = string.
  and(A, B) = A ++ B.

but if you change the name of and/2 to foo/2 it doesn't work.

So in Mercury some names are syntactically special (ie. they can be used 
infix) but you have to supply the meaning.  It's not a problem, but I find 
it really weird.  If you designed Mercury's syntax from scratch would you 
still treat these names specially?

Oh... I think I see how it's useful at least for the non-alphabetical 
operators.  Since you can't create new operators in Mercury, the language 
provides a bunch of built-in operators with no inherent meaning (literally 
just pieces of concrete syntax), and then you can "populate" them with 
whatever meaning you want.  That's how things like int.</2 work.

Hmm, it still feels weird to me, and not at all clear from the Reference 
Manual.  Perhaps if I knew Prolog better this wouldn't seem weird (and the 
"Mercury's syntax is almost identical to ISO-Prolog syntax" statements would 
be more helpful).

Nick
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