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prologunification

behaviour of unification in prolog in presence of arithmetic operators

12 ?- 3+4*5 = X+Y.
X = 3,
Y = 4*5.

13 ?- 3+4*5 = X*Y.
false.

16 ?- 3*4+5 = X*Y.
false.

I was expecting

13 ?- 3+4*5 = X*Y.
X = 3+4, Y = 5.

16 ?- 3*4+5 = X*Y.
X = 3, Y = 4+5.

Is there some "precedence" problem? I'm using the last swi-prolog release.

Solution

  • Yes, there is a precedence issue that you need to take into account.

    Prolog attaches a numeric precedence value to each operator defined, so that its parse can automatically treat, e.g., 3+4*5 the same as if parentheses had been used to state 3+(4*5).

    So your first example worked as expected, but not the second or third. There was simply no way to unify the terms, so Prolog returned false.