Form new groups of two for this assignment. We will not accept this assignment from people working individually or with a partner from the previous assignments. Email the names of the members of your group by 5pm on Friday, 11/09/07 (only one member needs to send this email). If you cannot find a partner, send us an email by the same deadline and you will be assigned a partner. In order to avoid being assigned a random partner, we suggest staying after class on Friday to find a partner in person. Missing the email deadline will lower your grade for this assignment by 10% per day late.

Type Inference

For this assignment, we will use PLAI Pretty Big Scheme.

Part I: Parsing the Language

Write a parser for this language:

 <expr> ::= <num>
          | true
          | false
          | {+ <expr> <expr>}
          | {- <expr> <expr>}
          | {* <expr> <expr>}
          | {iszero <expr>}
          | {bif <expr> <expr> <expr>}

          | <id>
          | {with {<id> <expr>} <expr>}
          | {rec {<id> <expr>} <expr>}
          | {fun {<id>} <expr>}
          | {<expr> <expr>}

          | tempty
          | {tcons <expr> <expr>}
          | {tempty? <expr>}
          | {tfirst <expr>}
          | {trest <expr>}

The support code specifies the abstract syntax that your parser should return. Your parse function must have the name and contract

parse :: S-exp -> Expr
You may assume that the s-expressions supplied to parse are valid.

Part II: Constraint Generation

Write a function called generate-constraints that accepts an expression and returns a list of constraints. You have the freedom to determine the precise contract of this function, including the representation of your constraints. However, we strongly recommend that you use the Type data structures as defined in the support code as part of your representation of constraints. A few things to note:

Part III: Unification

Implement the unification algorithm from the lecture notes. Call the function unify. The unify function should accept a list of constraints and return a substitution. However, unify should signal an error if two types cannot be unified or if the occurs check fails. Again, the precise contract of unify is up to you to define.

Part IV: Inferring Types

To infer the type of a program, parse it, generate constraints, and unify the constraints to get a substitution. From the substitution, you can determine the type of the program.

In particular, define the function

infer-type :: Expr -> Type

Part V: Testing and War-Grading

infer-type returns a Type, so you will need to test types for equality. This can be tricky due to type variables, particularly if generate-constraints calls (gensym) to generate unique type variables. The support code defines a function ((type=? t1) t2) that returns true if t1 and t2 are equal, modulo renaming of variables. We've included a few examples that show you how to use type=? with test/pred and test/exn. You are free to modify this function as you see fit or to not use it at all.

You should write unit tests for all functions that you write. However, for the purpose of war-grading, ensure that your test suites just use parse :: S-exp -> Expr and infer-type :: Expr -> Type. If you chose to use the type=? function we have provided, include it with your test suite.

Extra Credit

For a very small amount of extra credit, write a program in this language for which your algorithm infers the type

(t-fun (t-var 'a) (t-var 'b))
You shouldn’t attempt this problem until you’ve fully completed the assignment.

What Not To Do

You do not need to implement an interpreter for this language.

You do not need to implement let-based polymorphism.


A single member of your team should handin the assignment. From the directory containing the files for the assignment you wish to hand in, execute

/course/cs173/bin/cs173handin typei