CODEWITHSUNDEEP
prolog
Boomer
Boomer

Reputation: 392

What are the conditions of the query in order to be True?

I'm new to Prolog and trying to figure out how it works. So could anyone help me with the following code:

whatisthis([]).
whatisthis([_, b| L]):- whatisthis(L).

So when would Prolog return 'True' if the query would have the following format:

?- whatisthis(X).

I experimented with some different inputs and the only X that returned 'True' was X = [].

Upvotes: 1

Views: 55

Answers (1)

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 477220

This query is even that declarative that you can query what X will be true:

?- whatisthis(X).
X = [] ;
X = [_G1242, b] ;
X = [_G1242, b, _G1248, b] ;
X = [_G1242, b, _G1248, b, _G1254, b] ;
X = [_G1242, b, _G1248, b, _G1254, b, _G1260, b] ;
X = [_G1242, b, _G1248, b, _G1254, b, _G1260, b, _G1266|...]

If you hit the semicolon ; it will emit the next result. The _Gxxxx parts are unbound variables. You can write anything there.

So every X that is a list of even length where every second element is a b. So the following examples will succeed:

[]
[a, b]
[2, b]
[1, b, 4, b]
[a, b, 1, b]
[f(a,b), b, b(f,a), b]
[1, b, 4, b, 2, b, 5, b]

Upvotes: 2

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