LANG-28: flesh out a concept for reflected type parameters

[ggreif]

Researching a type reflection mechanism. JIRA: https://dfinity.atlassian.net/browse/LANG-28.

I expect that deconstruction will be rather easy, but construction will be very tricky (needs CPS?).

[github-actions]

Comparing from 6ab555d to 14051b1:
The produced WebAssembly code seems to be completely unchanged.

[ggreif]

As of 14051b1 the following abilities are present

• analysis side (deconstruct A)
  • primitive types
  • pairs (tuples?)
  • up-structuring (integrate A into something bigger)
  • records
  • variants
• synthesis side (build an A knowing the type's structure)
  • ...

Currently all the administrative glue is manually written, but I expect that the compiler can fill this in by straightforward metaprogramming based on the call-site's known type structure resp. the passed-in Desc<B>.

[ggreif]

see also #2348 and #2096

[ggreif]

In today's ICP.Lab Motoko office hours the following simplified context came up:

func ws_send<to_candid A>(data : A)

would allow using to_candid(data) to obtain a Blob, where the caller would pass in a conversion (serialisation) function implicitly. Similarly

func ws_receive<from_candid A>(blob : Blob) : A { let ?(a : A) = from_candid(blob) else trap(); a }

would work for deserialising. Stacking would make sense too <switch from_candid to_candid A> etc. (not pretty though...)

[luc-blaeser]

Just doing some repo cleanup for better focussing on active work:
Closing PR as it seems inactive for a longer time. Please re-open if it is still relevant and after it has been updated with latest master changes.