🚀 go-pugleaf

In article <1991Feb13.123619.23855@rodan.acs.syr.edu>, mitanu@goedel.top.cis.syr.edu (Mitanu Paul) writes:
> 	:- op(550, xf, !), op(550, xfx, !).
[This is not accepted as valid syntax:]
> 	?- X = (a+b!).

> Does the parser get confused when confronted with such operators?
> Both C-prolog and Quintus prolog produce this error.

The version number *does* make a difference.  There were versions of
C Prolog that should have got this right.
There are Prolog systems around that can handle this:

% np
NU-Prolog 1.5#18
1?- op(550, xf, !), op(550, xfx, !).

2?- display( a+b! ).
!(+(a, b))

% sicstus
SICStus 0.7 #1: Tue Sep 4 21:39:16 EST 1990
| ?- op(550, xf, !), op(550, xfx, !).

| ?- display(a+b!).
!(+(a,b))

| ?- compile(tokens), compile(read).	% Public-domain parser.
| ?- portable_read(X), display(X).
|: a+b!.
** expression expected
a + b ! <<here>> 
no
| ?- portable_read(X), display(X).
|: (a+b!).
** ) cannot start an expression
 ( a + b ! ) <<here>> 
no

This is a mistake, as the term is quite unambiguous.  To users of
the public domain Prolog parser, I can only apologise for the mistake.

-- 
Professional programming is paranoid programming