func (v *evalVariableAccess) Eval(scope ast.Scope, _ *ast.Stack) (interface{}, ast.Type, error) {
	// Look up the variable in the map
	variable, ok := scope.LookupVar(v.Name)
	if !ok {
		return nil, ast.TypeInvalid, fmt.Errorf(
			"unknown variable accessed: %s", v.Name)
	}

	return variable.Value, variable.Type, nil
}
func (v *evalCall) Eval(s ast.Scope, stack *ast.Stack) (interface{}, ast.Type, error) {
	// Look up the function in the map
	function, ok := s.LookupFunc(v.Func)
	if !ok {
		return nil, ast.TypeInvalid, fmt.Errorf(
			"unknown function called: %s", v.Func)
	}

	// The arguments are on the stack in reverse order, so pop them off.
	args := make([]interface{}, len(v.Args))
	for i, _ := range v.Args {
		node := stack.Pop().(*ast.LiteralNode)
		args[len(v.Args)-1-i] = node.Value
	}

	// Call the function
	result, err := function.Callback(args)
	if err != nil {
		return nil, ast.TypeInvalid, fmt.Errorf("%s: %s", v.Func, err)
	}

	return result, function.ReturnType, nil
}