feat: allow mutual recursion in letrec

[StoneyJackson]

@jashelio
@fosler

This pull-request attempts to allow mutual recursion in TYPE1's letrec.

I had to use an instanceof in an if statement. I'm not happy with it. Should I add a new method to Exp that either returns the defined type for ProcExp or evaluates the expression for its type? Maybe call it evalTypeInLetrecDecl? Other suggestions?

[StoneyJackson]

For easy reference, here is the offending code fragment

     public TypeEnv addLetrecTypeBindings(TypeEnv tenv) {
        List<String> stringVarList = new ArrayList<String>(varList.size());
        for(Token t : varList) {
            stringVarList.add(t.toString());
        }
        tenv = tenv.extendTypeEnv(new TypeBindings());
        Iterator<Token> varIter = varList.iterator();
        Iterator<Exp> expIter = expList.iterator();
        while (varIter.hasNext()) {
            String str = varIter.next().toString();
            Exp e = expIter.next();
            Type typ = null;
            if (e instanceof ProcExp) {
                typ = ((ProcExp) e).proc.definedType();
            } else {
                typ = e.evalType(tenv);
            }
            tenv.add(new TypeBinding(str, typ));
        }
        expIter = expList.iterator();
        while (varIter.hasNext()) {
            Type typ = expIter.next().evalType(tenv);
        }
        return tenv;
    }

[jashelio]

Small comment first: I don’t see the point of stringVarList. It’s never used.
I don’t understand what the code after the first while loop is doing. The second while loop will be skipped, and even if that weren’t true, typ is never used. Side effect?
I am surprised that you have to make an exception for a ProcExp. I’m going to have to look at that code.
This is still not checking for repeated variable names, right?
We can talk more tomorrow if there’s time.

[jashelio]

PS I understand a need to go through twice. You’re trying to deal with mutual recursion.

[jashelio]

Reviewing the code in the file named "code” in TYPE1,, specifically the Proc class, I understand the dilemma. No quick answer revealed itself, of course. Will continue to think…