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From: hopp@nbs-amrf.UUCP (Ted Hopp)
Newsgroups: net.lang.prolog
Subject: Re: "name" and type conversion
Message-ID: <77@nbs-amrf.UUCP>
Date: Sun, 24-Nov-85 15:37:41 EST
Article-I.D.: nbs-amrf.77
Posted: Sun Nov 24 15:37:41 1985
Date-Received: Mon, 25-Nov-85 08:07:58 EST
References: <382@bcsaic.UUCP>
Organization: National Bureau of Standards
Lines: 48

>  ... one of our programmers discovered that the following succeeds:
> 	...name ('12', X), name (Y, X), integer (Y)...
> In other words, running such an atom into and then back out of "name"
> converts it from an atom to an integer!
> 
> Now this is a very handy thing to have.  However, I suspect that it's
> not supposed to work that way, and probably doesn't in other Prologs.
> Any comments?  Does anyone have another (simple) way of doing such a
> conversion?
> -- 
> Mike Maxwell
> Boeing Artificial Intelligence Center
> 	...uw-beaver!uw-june!bcsaic!michaelm

This works in C-Prolog v 1.3 as well.  According to the C-Prolog manual,
name(X,L) succeeds when L is a list of ASCII codes of the characters for
X, not only when X is an atom, but when X is an integer.  Thus,

	name('12',L), name(12,L).

succeeds.  C-Prolog (and, I suspect, DEC-10 Prolog, on which C-Prolog is
based) differ in this from Clocksin and Mellish, where the first argument
to name/2 must be an atom.

One way of evaluating a string of ASCII digits as an integer is as follows:

	int(N,[],N) .
	int(N,[X|L],M) :- ascii_digit(X), M1 is 10*N + X - 48, int(M1,L,M) .

	ascii_digit(X) :- 48 =< X, X =< 57 .

A sample call is:

	name('123',L), int(0,L,M)

which binds M to 123.  If int(0,L,M) fails, L is not a list of digits, or
M cannot be unified to the evaluation of L (e.g., M was already unified to
a different integer value).  (If you can assume that the second argument
will always be a list of digits, the call to ascii_digit/1 can be omitted
from the second clause of int/3.)

Prologs that define name/2 in strict accordance with Clocksin and Mellish,
I suspect, will not be able to use the name-in/name-out trick.  Use of
a predicate such as int/3 above should always be safe.

-- 

Ted Hopp	{seismo,umcp-cs}!nbs-amrf!hopp