[plt-dev] compound units and contracted signatures

(We do want to promote units but to be honest, I considered  
contracted units a 'warm up' exercise for Stevie :-)

Philippe, do you realize that

  F/F units used structural sig matching (they are Matthew's response  
to my finding faults with struct/functors)
  O/F units use nomimal sig matching?

While I was surprised at first, it started to make sense as Stevie  
and I worked through the exercise of adding contracts. Matthew and I  
lived in a "free" world where sigs just mentioned names and there was  
no meaning attached to them at all. I never figured out whether our  
type system was pragmatic. In this day and age, I'd say my  
grandmother could have designed it (if she were alive).

The very moment you introduce some amount of meaning (even an epsilon  
such as a type) or a lot (such as a contract) -- you seem to get some  
of the ML-style problems. BUT nominal matching ensures that you can  
say "the same t as over there" with importing appropriately. The name  
of the sig determines sameness, no sharing constraints needed. Yet.

The problem with cyclic export/import comes out most when you think  
about cyclic datatypes. ML style type systems can't allow that.  
Neither can we. So we will have to think hard about this one. For now  
I recommend using macros to overcome the problem.

-- Matthias