Assignment 8: Metaprogramming

Overview

In this assignment, you will write a Haskell-to-Java compiler!

In particular, you will translate Haskell data type definitions to the corresponding Java definitions, with support for the visitor pattern.

More than any other assignment we have done before, most of the work for this assignment happens in your head. The challenge is to understand programming languages at many levels—concrete and abstract syntax—and to understand how all the parts of the visitor pattern work together to simulate pattern-matching.

There is quite a bit to read here. Start early. Skim over things so that you know what information is available to you, then dive in and read in more detail.

Materials

Use the assignment workflow and the link provided on Piazza to get access to this week’s files.

Part 1: Understand makevisitors

Your Haskell-to-Java compiler is called makevisitors. The makevisitors program translates Haskell data types to equivalent Java classes. Furthermore, the Java code supports the visitor pattern, so that people can add new operations, without modifying the generated Java code.

Your task: Learn about what makevisitors does and how it works, so that you will know what we as asking you to implement. Refer back to this description as you work.

Part 2: Familiarize yourself with the provided code

To implement your code-generator, you will build on the provided code. The most important files and directories to understand are the ones that translate from the Haskell AST to the Java AST and the ones that build the full program:

• HaskellAST.hs: the AST for Haskell declarations
• JavaAST.hs: the AST for Java declarations
• Translate.hs: transforms Haskell AST to Java AST
• haskell: a directory with some example Haskell declarations
• java: a directory where makevisitor will place generated Java code
• Makefile: builds the makevisitor program and runs it on examples

Your task: Read over this short description of the provided code before you start your implementation. Refer back to it as you work.

Part 3: Implement makevisitors

As you work, be sure to use good programming practices: choose descriptive variable names, document your code, define helper functions, and generally aim for readable code. Ten percent of your grade will be based on how well you follow these guidelines for code quality.

Although the Haskell code you write should be as professional as possible, the Java code that makevisitors generates is not required to be “industrial strength” Java. In particular, note that:

• All generated classes declare the fields to be public (rather than making them private and generating getters and setters).

• There are few comments.

• The generated code does not use the @Override annotation when overriding a method.

Your task: Implement makevisitors in the following order, referring to the linked pages for more details:

• Implement pass 1: Generate Java enums from simple Haskell data types.
• Implement pass 2: Generate the Java visitor interfaces.
• Implement pass 3: Generate the Java abstract classes.
• Implement pass 4: Generate the Java concrete classes.

Part 4: Bring it all together

Your task: Implement a visitor and use your generated code.