transcendental-lisp/src/function/builtin/SYMBOL_FUNCTION.java

package function.builtin;

import function.*;
import function.builtin.cons.LENGTH;
import sexpression.*;

/**
 * <code>SYMBOL_FUNCTION</code> represents the SYMBOL-FUNCTION function in 
 * Lisp.
 */
public class SYMBOL_FUNCTION extends LispFunction {

    // The number of arguments that SYMBOL-FUNCTION takes.
    private static final int NUM_ARGS = 1;

    public SExpression call(Cons argList) {
        // retrieve the number of arguments passed to SYMBOL-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("SYMBOL-FUNCTION"),
                                          argList);
            String errMsg = "too " +
                            ((argListLength > NUM_ARGS) ? "many" : "few") +
                            " arguments given to SYMBOL-FUNCTION: " +
                            originalSExpr;

            throw new RuntimeException(errMsg);
        }

        SExpression arg = argList.getCar();

        // make sure the argument is a symbol
        if (arg.symbolp()) {
            LispFunction function = EVAL.lookupFunction(arg.toString());

            // make sure the function actually exists
            if (function != null) {
                if (function instanceof UserDefinedFunction) {
                    // this is a user-defined function

                    UserDefinedFunction udFunction = (UserDefinedFunction) function;

                    return udFunction.getLambdaExpression();
                }

                // this is a built-in function

                return new Symbol("SUBR-" + arg.toString());
            }

            throw new RuntimeException("SYMBOL-FUNCTION: undefined function " +
                                       arg);
        }

        throw new RuntimeException("SYMBOL-FUNCTION: " + arg +
                                   " is not a symbol");
    }

}