// Copyright (c) 1996-2002 Brian D. Carlstrom

package bdc.scheme.expression;

import bdc.scheme.Environment;
import bdc.scheme.GlobalEnvironment;
import bdc.scheme.Scheme;
import bdc.scheme.SchemeException;
import bdc.scheme.Stack;
import bdc.scheme.Writer;
import bdc.scheme.compiler.CompileTimeEnvironment;
import bdc.scheme.exception.UndefinedVarException;
import bdc.util.Fmt;

public class GlobalAssignment extends Expression
{
    GlobalVariable globalVariable;
    Expression     expression;

    public GlobalAssignment (GlobalVariable globalVariable,
                             Expression     expression)
    {
        this.globalVariable = globalVariable;
        this.expression     = expression;
    }

    public Object eval (Environment environment, Stack stack)
      throws SchemeException
    {
        try {
            if (globalVariable.object == Scheme.Undefined) {
                throw new UndefinedVarException(globalVariable.name);
            }
            globalVariable.object = expression.eval(environment, stack);
            globalVariable.type = GlobalEnvironment.Location;
            return Scheme.Unspecified;
        }
        catch (SchemeException se) {
            backTrace(se);
            return Scheme.NotReached;
        }
    }

    /**
        Fixup sub-expressions and return

        Don't need to fixup GlobalVariable because it can't change
    */
    public Expression fixupVariables (CompileTimeEnvironment environment)
    {
        expression = expression.fixupVariables(environment);
        return this;
    }

    public String toString ()
    {
        return
            Fmt.S("(set! %s %s)",
                  globalVariable.name,
                  Writer.write(expression));
    }
}