TypeFunction example: ImplictConvTargets

[Stefan Koch]

On Wednesday, 7 October 2020 at 00:11:20 UTC, Paul Backus wrote:
> 
> I agree that allowing completely unrestricted tuple mutation 
> would be problematic, but I think it's still worth asking how 
> far we could potentially go in that direction.

I would be happy to discuss this over a coffee or beer sometime.
As a thought experiment it's exciting.

> For example, maybe you're only allowed to mutate tuples inside 
> the scope that declares them. That would let you implement 
> templates like Filter and staticMap iteratively, while still 
> presenting an immutable "interface" to the rest of the program.

If you can prove:
  - It never escapes a 'mutable' version into a monophonic scope.
  - Inside a polymorphic scope (template) all transforms are 
derived from the shape defining parameters (template parameters) 
or are invariant.

That is necessary, but I am not sure it's sufficient.