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Luiso Vega
Asked: 2020-09-01 08:25:26 +0800 CST 2020-09-01 08:25:26 +0800 CST

Create predicate using :-op operator?

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I have some doubts about the :-op operator in Prolog, because I don't know how to handle it. My exercise is focused on creating a predicate that, from an attribute-value list, whose format is [a1=>v1, a2=>v2,...,aN=>vN], obtains the value of an attribute that is Ask him.

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.

prolog

2 Answers

  1. Best Answer

    The directive op/3allows you to declare atoms that will be syntactically treated as operators with a certain class (infix, suffix, or prefix), associativity, and precedence.

    =>/2In your case, you need an infix operator 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/3as:

    valor(X, [X=>Y|_], Y).
valor(X, [_|T], Y) :- valor(X, T, Y).

    Here is more information about operators in Prolog.

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  2. Wow, amazing. 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.
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