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Prolog Variable

I have a small problem when we are talking about anonymous variables. For example when we make this:

?- [_,2]=[X|Y].
Y=[2].

but my question is about the variable X. Does it have the '_'?

Solution

  • No, X does not "have the _". It is bound to an anonymous variable, which is never bound to anything else. This binding of X to an anonymous variable does not create any additional limitations on X - for all practical purposes, it remains unbound.

    The _ variable has been introduced to let Prolog coders express in code that they do not care about a value in a particular position. One could emulate this behavior by using variables that look like UNUSED1, UNUSED2, UNUSED3 and so on instead of the _, and ignoring Prolog warnings about singleton variables:

    [UNUSED123,2]=[X|Y].

    Using the underscore _ is like telling Prolog that you know that the unused variable is singleton, and that it is indeed your intention.