[Haskell] Simple IO Regions

[Sean Seefried]

> Dominic Steinitz wrote:
>> Can someone give an explanation of how the marks get built up?
>
> Suppose we have a class TypeEq a b so that the constraint TypeEq holds
> whenever a and b are the same. Then the type Int and the type
> 	TypeEq a Int => a
> are kind of equivalent, right? The HList library plays many tricks
> like that. The idea is that we can replace a definite type with a type
> variable subject to some constraint (which we can fix later on).
> In some respects, constraints are more convenient: they float, their
> order does not matter, their duplicates are automatically
> eliminated. Just what we need to build a set...
> It ``follows'' then that instead of building the union of types, we
> can build the union of constraints. The latter operation is trivial:
> the typechecker does that all the time. If we restrict the scope of
> the type variable by quantification, the scope of the corresponding
> constraint is likewise restricted. The quantification also builds
> eigen-variables (which are distinct from anything else) -- so we get
> 'gensym' on the type level for free. That is all there is to it...

Olge, I found this explanation really useful. I was wondering if you  
could explain to me what you mean by eigen-variable though?

Sean