I have some doubts about the operator: -op in Prolog, because I can not handle it. My exercise is focused on creating a predicate that from a list attribute-value, whose format is [a1 => v1, a2 => v2, ..., aN => vN], get the value of a attribute that is requested.
Example:? - value (shape, [color => blue, shape = spherical, weight => light, material = > plastic], X). X = spherical.
To do this, we suggest defining the operator = > in the logical base.
The op/3 directive allows you to declare atoms that will be treated syntactically as operators with a certain class (infix, suffix or prefix), associativity and priority.
In your case, you need an operator =>/2 infix that is not associative:
:- op(300, xfx, '=>').
Now you can handle the atom '=>'(x,y) as x => y (although internally these two structures are equivalent).
Finally, you can write your predicate valor/3 as:
valor(X, [X=>Y|_], Y).
valor(X, [_|T], Y) :- valor(X, T, Y).
Here you have more information about the operators in Prolog.
Wao, surprising. It worked for me without problems. Thanks for the collaboration.
?- valor(forma, [color=>azul, forma=>esferica, peso=>ligero, material=>plastico], X).
X = esferica ;
false.
?- valor(peso, [color=>azul, forma=>esferica, peso=>ligero, material=>plastico], X).
X = ligero ;
false.
?- valor(casa, [color=>azul, forma=>esferica, peso=>ligero, material=>plastico], X).
false.