Submission instructions:

Submit a Scheme file for part 1.
Write your answer to 2. as a comment in that file.

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1. Extend your typed lambda calculus implementation from part 2 of the
last lecture with subtyping, as described in the lecture on 24/4/09.
[ don't worry, this is a lot less work than the last programming exercise ]

You can omit the "variants" and "fixpoint" language constructs if you want to.

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2. Give a brief discussion of subtyping. Is it a useful language feature?
What kinds of problems are particularly well-suited for using subtyping?
How would you solve these problems without subtyping, say, in Haskell