What is up with the cut (!)

Discussion
43

The only reason for the list in msup([a, 2]): it’s a html tag, so it will be translated to

<msup><mi>a</mi><mn>2</mn></msup>

I agree it doesn’t make much sense in another context.

44

I think you can create a wrapper predicate to enumerate args like a list – and I guess one can also envision a non-deterministic member predicate applied to args.

Something like this, perhaps:

arg_member(Value, Term) :-
    ground(Term),
    functor(Term, _, Arity),
    between(1, Arity, Arg),  
    arg(Arg, Term, Value).

Dan

45

We are drifting away a bit, but I am not sure if this definition is necessary. What is it that you are getting that you don’t get with arg/3 already?

Like this:

?- arg(_, foo(a, b, c), X).
X = a ;
X = b ;
X = c.

?- arg(_, foo(a, B, c), X).
X = a ;
B = X ;
X = c.

?- arg(_, Foo, X).
ERROR: Arguments are not sufficiently instantiated

?- arg(_, foo, X).
ERROR: Type error: `compound' expected, found `foo' (an atom)

The only difference is that this (correctly) has an error if your term is not a compound.

Alternatively, if you need a list, Term =.. [Name|Args].

2 Likes
46

You are absolutely right …

I didn’t know you can call arg/3 without instantiated index.

Dan