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prologgnu-prolog

Why does gprolog not chain together uses of modus ponens?

I am reading Learn Prolog Now, 1.1.2 Knowledge Base 2 where they write about chaining together uses of modus ponens.

The KB2.pl file:

listensToMusic(mia).
happy(yolanda).
playsAirGuitar(mia) :- listensToMusic(mia).
playsAirGuitar(yolanda) :- listensToMusic(yolanda).
listensToMusic(yolanda) :- happy(yolanda).

When the query:

playsAirGuitar(yolanda).

is submitted to gprolog, it is supposed to respond yes, because it should be able to infer it from the fact that yolanda is happy.

But gprolog responds with no. Why is that?

Solution

  • i think that the problem is that the clauses of the predicate listensToMusic/1 are separated.

    the following code returns yes for me:

    listensToMusic(mia).
listensToMusic(yolanda) :- happy(yolanda).
happy(yolanda).
playsAirGuitar(mia) :- listensToMusic(mia).
playsAirGuitar(yolanda) :- listensToMusic(yolanda).

    you should get a warning like 

    warning: discontiguous predicate listensToMusic/1 - clause ignored