Write the code given in example 6.11 to implement the get operator in the calculator. Be sure to test your program. When you test it, the generated code will display a question mark each time it is waiting for input.

example 6.11

To read a value from the keyboard while evaluating a calculator expression the lexical specification in calc.lex must be modified to add the get keyword to the list of keyword tokens. Keywords are added to the alpha followed by zero or more alphanumeric characters rule. That rule would be modified as shown here.

The AST definition in calcast.sml must be modified so get’ can appear as an AST node. Finally, the code generator in calc.grm must be modified to generate code for a get’ node.

In this short example, the language was extended to allow input to be read from the keyboard. Implementing this makes it possible for a programmer to write a short program to read values from the keyboard and use them in an expression. This was accomplished by changing the scanner, parser, AST definition, and the code generator. Other more interesting extensions to the language are also possible.