package prolog.components.expressions

import prolog.components.terms.Functor
import prolog.components.terms.Goal
import prolog.components.terms.Head
import prolog.components.terms.Term
import prolog.unify

// TODO Change this to the right type, supporting operators and normal Clauses
//  Probably needs a new interface or abstract class (?)
typealias Body = List<Term>

/**
 * ‘Sentence’ of a Prolog program.
 *
 * A clause consists of a [Head] and body separated by the [neck][prolog.terms.Neck] operator, or it is a [Fact].
 *
 * @see [prolog.components.terms.Variable]
 * @see [Predicate]
 */
abstract class Clause(val head: Head, val body: Body = emptyList()) : Expression {
    val functor: Functor = head.functor
    override fun evaluate(goal: Goal): Boolean {
        val result = unify(goal, head)
        // TODO Evaluate the body
        return result.isEmpty
    }

    override fun toString(): String {
        return when {
            body.isEmpty() -> head.toString()
            else -> "$head :- ${body.joinToString(", ")}"
        }
    }
}