Counter-intuitive behavior of min_member/2

Question

min_member(-Min, +List)

True when Min is the smallest member in the standard order of terms. Fails if List is empty.

?- min_member(3, [1,2,X]).
X = 3         


        

Answer 1

Clearly min_member/2 is not a true relation:

?- min_member(X, [X,0]), X = 1.
X = 1.

yet, after simply exchanging the two goals by (highly desirable) commutativity of conjunction, we get:

?- X = 1, min_member(X, [X,0]).
false.

This is clearly quite bad, as you correctly observe.

Constraints are a declarative solution for such problems. In the case of integers, finite domain constraints are a completely declarative solution for such problems.

Without constraints, it is best to throw an instantiation error when we know too little to give a sound answer.