Calculate a list of cubes in Prolog

Question

So I'm doing some Prolog in SWI-Prolog and I've come across a little snag. I must create a list of cubes, given an input list. The code I currently have is

cubes([Head|Tail],Cubes) :-
    cubes([Head|Tail],Cubes,[]).
cubes([Head|Tail],Cubes,ActualCubes) :-
    X is Head^3,
    append(ActualCubes,[X],NewCubes),
    cubes(Tail,Cubes,NewCubes).
cubes([Tail],Cubes,ActualCubes) :-
    X is Tail^3,
    append(ActualCubes,[X],NewCubes),
    Cubes is NewCubes.

When I run that it gives an error, specifically...

ERROR: '.'/2: Type error: `[]' expected, found `[8]' ("x" must hold one character)
   Exception: (7) cbList([1, 2], _G296, []) ? creep

I'm not entirely sure why this error is occurring but it seems to happen around the very last line, Cubes is NewCubes. Any help is appreciated :)

Answer 1

First, you're making different cubes predicates with differing numbers of arguments. This is bound to cause both conceptual and syntactical problems, so at that point, re-think what you're doing. In this case, try to expand the ways you can use pattern matching and recursion:

cubes([],[]).
cubes([H|T], [Y|Z]):-
        Y is H*H*H,
        cubes(T,Z).


betterCubes([],[]).
betterCubes([H|T], [Y|Z]):-
        ( 
          var(Y) , nonvar(H)    -> Y is H*H*H
        ; nonvar(Y) , var(H)    -> H is Y**(1.0/3.0) 
        ; nonvar(Y) , nonvar(H) -> H*H*H =:= Y
        ),
        betterCubes(T,Z).