transcendental-lisp/src/eval/UDFunction.java

/*
 * Name: Mike Cifelli
 * Course: CIS 443 - Programming Languages
 * Assignment: Lisp Interpreter 2
 */

package eval;

import parser.*;
import sexpression.Cons;
import sexpression.SExpression;
import sexpression.Symbol;

import java.util.ArrayList;

/**
 * A <code>UDFunction</code> is an internal representation of a user-defined
 * function in the Lisp programming language.
 */
public class UDFunction extends LispFunction {

    // the number of arguments that this user-defined function takes.
    private final int NUM_ARGS;

    private String name;
    private Cons body;
    private Cons lexpr;
    private SymbolTable environment;
    private ArrayList<String> parameters;

    /**
     * Create a new user-defined function with the specified name, lambda list
     * and body.
     *
     * @param name
     *  the name of this user-defined function
     * @param lambdaList
     *  a list of the formal parameters of this user-defined function (MUST BE
     *  A PROPER LIST)
     * @param body
     *  the body of this user-defined function (MUST BE A PROPER LIST)
     */
    public UDFunction(String name, Cons lambdaList, Cons body) {
        this.name = name;
        this.body = body;
        this.lexpr = new Cons(new Symbol(name), new Cons(lambdaList, body));
        this.environment = SETF.getEnvironment();
        this.parameters = new ArrayList<String>();

        // retrieve the names of all the formal parameters of this function
        while (lambdaList.consp()) {
            this.parameters.add(lambdaList.getCar().toString());

            lambdaList = (Cons) lambdaList.getCdr();
        }

        this.NUM_ARGS = this.parameters.size();
    }

    public SExpression call(Cons argList) {
        // retrieve the number of arguments passed to this function
        int argListLength = LENGTH.getLength(argList);

        // make sure we have received the proper number of arguments
        if (argListLength != NUM_ARGS) {
            Cons originalSExpr = new Cons(new Symbol(name), argList);
            String errMsg = "too " +
                            ((argListLength > NUM_ARGS) ? "many" : "few") +
                            " arguments given to " + name + ": " +
                            originalSExpr;

            throw new RuntimeException(errMsg);
        }

        // push a new symbol table onto this function's environment (for its
        // parameters)
        environment = new SymbolTable(environment);

        // bind the values of the arguments to the formal parameter names
        for (String param : parameters) {
            SExpression currentArg = argList.getCar();

            environment.put(param, currentArg);

            argList = (Cons) argList.getCdr();
        }

        // store the environment of the S-expression that called this function
        // (the current environment)
        SymbolTable currentEnvironment = SETF.getEnvironment();

        // replace the current environment with the environment of this
        // function
        SETF.setEnvironment(environment);

        Cons currentSExpression = body;
        SExpression retval = null;

        // evaluate all the S-expressions making up this function's body
        while (currentSExpression.consp()) {
            retval = EVAL.eval(currentSExpression.getCar());
            currentSExpression = (Cons) currentSExpression.getCdr();
        }

        // replace the environment of the S-expression that called this
        // function
        SETF.setEnvironment(currentEnvironment);

        // remove the bindings of the arguments to the formal parameter names
        // in the environment of this function
        environment = new SymbolTable(environment.getParent());

        return retval;
    }

    /**
     * Return the lambda expression that represents this user-defined function.
     *
     * @return
     *  the lambda expression that represents this user-defined function
     */
    public Cons getLexpr() {
        return lexpr;
    }

}