The problem with closures

[JohanAR]

I'm really getting frustrated with closures now. We finished lab 4
some time with substitutions, but I thought it would be fun to do it
with closures too. However, basically all I've got about them is 2
slides showing their respective judgements so I think it's difficult
to find out how to implement them.

Using Xavier Leroy's slides I think I managed to implement call-by-
value using closures, by doing like this:

Value = Closure | Integer

When I evaluate (fun arg) I first evaluate arg to argV. Then I
evaluate fun to (Closure env' (Abstraction x body)). After that I
modify env' so that x := argV and finally evaluate (body arg) in the
latest environment.

Because I need to put (argV : Value) in the environment/closure, I
felt that I had to make the environment a map from Ident to Value
(instead of Expression) which might or might not be correct.

Either way it passes all tests except good2.fun which loops until
there's a stack overflow.

So the real problem is that I now want to convert it to call-by-name
instead.

If I don't evaluate arg to argV, it will be an expression. Should I
change the environment back to map Ident Expression? If I do that, I
don't really need to know how to handle Idents..
If I look up an Ident x which maps to Expression body and I evaluate
body it breaks the closures and I can't use the same variable name in
several functions. If I don't evaluate the body I have to do something
else with it so it becomes a Value, but trying to enclose it breaks
just about everything.

I'm also a little uncertain about the first method being a real call-
by-value, as many times arg will just evaluate to a Closure which is
then inserted into the body.

[JohanAR]

Finally found the problem (with a little hint from some guy on the
Internet)..

The Environment should remain map Ident Value. When I look up an Ident
that maps to a Closure I should evaluate the body in the closure's
environment (I think I tried to do it in the current env). When I
apply (fun arg) I just stick arg Expression in a Closure with the
current environment.
And with that solved, it was pretty trivial to turn it into call-by-
need.

Suppose the next challenge is to modify the code so it uses StateT
instead of passing Environment. Managed to do it with our solution
that used substitutions, but I think this one will be more difficult
because some changes to environments should be discarded while other
ones saved.

Ah well, good luck on the exam tomorrow to everybody!