Homework 3: Semantic Checker

Due Tuesday, 19 February 2008, 6 PM

Homework 3 is to read Chapter 5 of the text and build a static-semantics checker for Appel's Tiger language.

Your code should perform the static semantic checks decribed in Chapter 5:

• Type-check the program.
• Check for break statements that occur out of scope.

Your Semant module should export a function with this signature:

      transProg : Absyn.exp -> unit

that checks an AST and outputs error messages if anything is wrong. (Why is our type-checker called transProg rather than something like typeCheck? Because we'll be extending it in Homework 4 to type-check and translate the AST to our intermediate representation.)

You will find a structure Types defining a datatype to represent Tiger types in $TIGER/chap5/types.sml.

You should submit:

• A semant.sml file containing your code
• Any other source files you wrote to support your checker
• Your sources.cm file
• A text file describing
  • The members of your team
  • Anything you think is of interest about your code.

Words to the wise:

• Appel's advice on pages 121/122 about breaking the the work into a part A and a part B is good advice.
• One potentially confusing thing is that there are two "ty" types:

  Types.ty

  These are semantic values—they represent the various types found in Tiger. This is carefully designed to assist the type-checking process, e.g. handling circularities.

  Absyn.ty

  These are syntactic values—they represent the notation in Tiger for describing types.

  Obviously, we can map any Absyn.ty chunk of Tiger type syntax to a Types.ty value. But don't confuse the two different datatypes. Anytime you must handle a description of a Tiger type, be clear in your mind: are you handling a semantic value or a chunk of syntax?
• This is a significant programming task. I found it harder than the parser as a matter of software engineering. Start yesterday.

---

Last updated 18 February 2008.