From: Matthias Luedtke <matthias-luedtke-usenet@...>
Date: 2005-07-31T18:56:02+09:00
Subject: Re: Idea for Ruby 2.0

Christian Neukirchen wrote:
[SWI-Prolog]
 > I'm not sure they are "old variables"... compare foo(A, A, B) and
 > foo(_, _, B).  I don't think they share the same semantics.

You're right, the semantics of both constructs differs:

foo(bar,baz,42).
foo(bar,bar,42).
foo(baz,baz,42).

% E:/Dokumente/Studium/Passau/5. 
Semester/DeduktiveDB/PrologWorkspace/RubyNEwsgroup.pl compiled 0.00 sec, 
944 bytes

Yes
3 ?- foo(A,A,C).

A = bar
C = 42 ;

A = baz
C = 42 ;

No

As you see in the above query Prolog unifies the query variable A with 
the same atom whereas the query

4 ?- foo(_,_,C).

yields

C = 42 ;

C = 42 ;

C = 42 ;

No

and therefore in this respect is equivalent to this query:

5 ?- foo(A,B,C).

A = bar
B = baz
C = 42 ;

A = bar
B = bar
C = 42 ;

A = baz
B = baz
C = 42 ;

No

However in the underscore version you don't get to see which atoms the 
variables got unified with.

In this example there is in effect no real query /condition/ when you 
write A,B,C or _,_,C. On the other hand the query :- foo(A,A,C) states 
that the first two parameters must hold the same value.

Although :- foo(A,B,C) and :- foo(_,_,C) have same query semantics the 
underscore version is a lot easier on the eye in my opinion. Writing 
4711.times{ |_| puts "Ruby rocks" } seems odd to me, though.

Regards,
Matthias