Prolog error arguments

Question

I'm wondering how to go about adding error checking in Prolog. For instance I have a program that will find how long a list is:

listlen([],0). 
listlen([_|T],N) :-
   listlen(T,X),
   N is X+1.

How would I print out an error like "The 1st argument has to be a list" when it happens?

Territorialism

In this particular case, there are good reasons to not produce an error. See this comparison for the behavior of various systems in this situation. – Eldoree

Answer 1

SWI-Prolog has ISO-compliant exception handling, so you can actually throw errors as defined in the standard.

?- throw(error(type_error(list, foo), context(foo/0, 'Must be a list'))).
ERROR: foo/0: Type error: `list' expected, found `foo' (an atom) (Must be a list)

This is not only difficult to type/use: it is also implementation dependent. Instead, you can (and should) use library(error), which provides the must_be/2 predicate (sadly, it is very difficult to find this on the SWI-Prolog website if you don't know what you are looking for):

?- must_be(list, [foo]).
true.

?- must_be(list, foo).
ERROR: Type error: `list' expected, found `foo' (an atom)

I assume that other Prolog implementations that support exception handling provide very similar facilities.

Albaugh

There is another reason why writing throw(error(E,Imp_def)) is problematic: The second argument of error/2 is implementation defined. So code that gives an explicit term that is not specific to an ISO conforming implementation may not work. – Eldoree

@Eldoree Yes. Actually, it is quite difficult (because documentation) to figure out the exact "ISO error term definition". – Albaugh

You know this? – Eldoree

@Eldoree no I did not. This is a great reference, thank you. – Albaugh