Question

I’m writing a function called subseq which checks if one list is a subsequence of another.

subseq([],[]).
subseq([],[Y|Ys]).
subseq([X|Xs],[Y|Ys]) :- X=:=Y, subseq(Xs,Ys).
subseq([X|Xs],[Y|Ys]) :- X=\=Y, subseq([X|Xs],Ys).

When I try subseq(X,[1,2]) I get:

X = [] ? ;
uncaught exception: error(instantiation_error,(=:=)/2)

Why is this happening? My guess is that [] is being operated on by =:=, but how do I check for/prevent this error?

Answer 1

You use =:= and =\= in the wrong context here. Those two operators should be used when you have two expressions at hand and want to evaluate and compare them. In your test, because X is not known beforehand, Prolog couldn’t evaluate X and compare with Y. More information about =:= and =\= could be found here: Prolog Operator =:=.

In your code you only need unification for atoms so one possible fix could be:

subseq([],[]).
subseq([],[_|_]).
subseq([X|Xs],[Y|Ys]) :- X=Y, subseq(Xs,Ys).
subseq([X|Xs],[Y|Ys]) :- X\=Y, subseq([X|Xs],Ys).