Switch statements in Prolog

Question

In Prolog predicates, I often write repetitive conditional statements like this one, but I wish they could be written more concisely:

output(Lang, Type, Outp         


        

Answer 1

In Prolog it is quite easy to define your own control structures, using meta-predicates (predicates that take goals or predicates as arguments).

For example, you could implement a switch construct like

switch(X, [
    a : writeln(case1),
    b : writeln(case2),
    c : writeln(case3)
])

by defining

switch(X, [Val:Goal|Cases]) :-
    ( X=Val ->
        call(Goal)
    ;
        switch(X, Cases)
    ).

If necessary, this can then be made more efficient by compile-time transformation as supported by many Prolog systems (inline/2 in ECLiPSe, or goal expansion in several other systems).

And via operator declarations you can tweak the syntax to pretty much anything you like.