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prologswi-prologjpl

Query working in the consult, but not in JPL and command line

I'm using JPL and launch some queries to Swi-Prolog. The program works fine with all queries, but one.

This query is not working:

assert(like(X, Y) :- element(I, [1, 2, 3], X), element(I, [2, 3, 4], Y) ).

Exception in thread "main" jpl.PrologException: PrologException: error(uninstantiation_error(element(_3, '.'(2, '.'(3, '.'(4, []))), _4)), context(:(system, /(assert, 2)), '2-nd argument'))

If I put the query in a .pl fle (without assert, obvisously) and run it with consult, works fine! But if I type the query in console mode, return the same error like JPL. So, where is the problem?

Solution

  • You are very near to get the illumination. It will not work neither on console ! Then you will try to change the syntax. Understanding the Prolog operator model, try

    ?- assert((like(X, Y) :- element(I, [1, 2, 3], X), element(I, [2, 3, 4], Y))).

    and it will work!

    Your problem was the precedence of operator (:-)/2 WRT operator (,)/2.