src/lang/Predicate.java

package com.googlecode.prolog_cafe.lang;

import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.util.LinkedList;

/**
 * Superclass of any predicate's implementation class.
 * <p>
 * Concrete implementations must provide a definition of the Operation's
 * {@link Operation#exec(Prolog)} method. On a successful execution the
 * method must return {@link #cont}.
 * <p>
 * To be compatible with the {@code am2j} compiler implementations must
 * define a constructor matching the signature:
 * <pre>
 *   public ClassName(Term arg1, Term arg2, ..., Operation cont) {
 *     // save arg1, arg2
 *     this.cont = cont;
 *   }
 * </pre>
 */
public abstract class Predicate extends Operation {
  /**
   * Holds the continuation goal of this predicate.
   * <p>
   * In a Prolog program this is the next operation to perform if this operation
   * was executed successfully.
   */
  public Operation cont;

  @Override
  public String toString() {
    LinkedList<Class<?>> toScan = new LinkedList<>();
    Class<?> clazz = getClass();
    while (clazz != Predicate.class) {
      toScan.addFirst(clazz);
      clazz = clazz.getSuperclass();
    }

    StringBuffer sb = new StringBuffer();
    sb.append(PredicateEncoder.decodeFunctor(getClass().getName()));
    boolean first = true;
    for (Class<?> c : toScan) {
      for (Field f : c.getDeclaredFields()) {
        if ((f.getModifiers() & Modifier.STATIC) == 0
            && f.getType() == Term.class
            && f.getName().startsWith("arg")) {
          Term val;
          try {
            f.setAccessible(true);
            val = (Term) f.get(this);
          } catch (IllegalArgumentException e) {
            continue;
          } catch (IllegalAccessException e) {
            continue;
          }

          if (first) {
            sb.append('(');
            first = false;
          } else
            sb.append(", ");
          sb.append(val);
        }
      }
    }
    if (!first)
      sb.append(')');
    return sb.toString();
  }

  public static abstract class P1 extends Predicate {
    protected Term arg1;
  }

  public static abstract class P2 extends Predicate {
    protected Term arg1;
    protected Term arg2;
  }

  public static abstract class P3 extends Predicate {
    protected Term arg1;
    protected Term arg2;
    protected Term arg3;
  }

  public static abstract class P4 extends Predicate {
    protected Term arg1;
    protected Term arg2;
    protected Term arg3;
    protected Term arg4;
  }
}