Homework # 10
due November 15

Reading

Please read Chapters 20 and 21 in your textbook.

Problems

Please do the following problems

• Exercise 20.1.2
• Exercise 20.2.1 (convert your code for the previous question)
• Exercise 21.2.2
• Exercise 21.3.4

Please check your answers against the solutions in the book. Do not turn in your answers.

Implementation

Combine the checkers fullequirec and fullref in a single implementation that handles types such as:

Counter = Rec S . { get : S -> Nat, 
                    set : S -> Nat -> Unit, 
                    inc : S -> Unit };

counterClass = 
   lambda rep: {x: Ref Nat}.
          {get = lambda s:Counter . !(rep.x),
	   set = lambda s:Counter . lambda n:Nat . (rep.x) := n,
           inc = lambda s:Counter . (s.set) s (succ ((s.get) s))}
	as Counter;

makeCounter =
  lambda i:nat . counterClass {x = (ref i)};

Put this combined code in a subdirectory fullequirecref in your homework10 folder.

Discussion

Can you typecheck your answer to Question 4.3 of Homework #9 (object.f) now that you have recursive types? (Of course, you will need to fix/specify the types.) Why or why not? What happens?

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