Prolog operator priority

Question

The user input is given in the format a:b>c>d>e... and so on. I would like to parse the input into :(a,>(b,>(c,>(d,e))) would that be possible and is there any suggestion to do that? I tested with this

prepare:-
    op(750,xfx,user:(:)), % change the default priority of : and >
    op(700,xfx,user:(>)),
    display(a: b > c),%this one worked fine and 
                      %the display value is :(a,>(b,c))
    display(a: b > c > d ). % I cannot have this works, 
                            %the error ERROR: 
                            %Syntax error: 
                            %Operator priority clash is thrown.

The input cannot be modified. Any suggestion is appreciate. Thanks!

Answer 1

Maybe first and foremostly: Both (:)/2 and (>)/2 are already infix operators defined in the standard like so:

:- op(600, xfy, :). % ISO/IEC 13211-2 5.2.1, table 1
:- op(700, xfx, >). % ISO/IEC 13211-1 6.3.4.4, table 7

Changing their priority means that you change their common meaning. This is very often not a good idea. Think of it: It's like you would (be able to) change the priority of operators in Java, C#, C++, Perl or PHP. They all did not dare to change the priority they inherited from C.

But, strictly speaking, you are able to do this.

To minimize the detrimental effect of such a change, try to keep the declaration in a module of your own. And, in a system without module-local operators, make sure you will recover to the original declarations safely.

You wrote user:(>) which affects the special user-module. Instead, write your own module.

... or maybe reconsider the operator to be changed.

Terms are read up to the next period in one fell swoop. So if you have a rule like you have shown, the operator declaration will become effective only when prepare will be executed. Therefore, it has no effect on the argument of the goals display/1. You probably had prepare loaded, executed and ; and reloaded it.

To make an operator declaration effective, either execute it directly on the top level (that is a quick hack); or write it as a directive into a file or module.

Then the >-like operator, I will use :> it its place, needs to be right-associative like the : and a priority below :. You declared xfx which means no associativity.

?- op(500,xfy,:>).
true.

?- write_canonical(a:b:>c:>d).
:(a,:>(b,:>(c,d)))
true.