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Comment by nemetroid | Hacker News Reader

It will share with you its wisdom (the last line is printed by the interpreter):

  ?- FOO = 1,
  BAR = 2,
  FOO = BAR.
  false.

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And if you haven't bound them previously, it does exactly the right thing, and regards them as now the same variable.

    ?- FOO = BAR, FOO = 1, print(BAR).
    1

Erlang is interesting too here, in that it basically works the same way (unification essentially via substitution). If you can't make it work, a program exception is thrown instead of a false language. This all makes sense since prolog talks more about logic, and erlang is GP language.

That’s neat. Is there a separate syntax for var to var assignment or are variables immutable in Prolog?

Variables are immutable, but note that it's the contradiction that leads to the "false" response, there's no issue with the assignment itself:

  ?- FOO = 1,
  BAR = 1,
  FOO = BAR.
  FOO = BAR, BAR = 1.

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