Re: [Caml-list] Late adding of type variable constraints

[Markus Mottl <markus.mottl@gmail.com>]

On Mon, Sep 7, 2015 at 2:33 PM, Mikhail Mandrykin <mandrykin@ispras.ru> wrote:
> Not sure if it's exactly what's desired, but I'd suggest Format-like
> implementation in spirit of the following:

Though it only solves part of my particular problem, this is a pretty
clever encoding to create mapping functions for structures with
arbitrary numbers of type parameters so thanks for sharing.

In my case I also need to define a type ('a t) such that I can pass it
on in a functor.  The functor body needs to know that invariants are
maintained after each mapping: the skeleton, as captured by t, and
that the same constraints hold for type parameter 'a across mappings.
Only then can the functor body easily map back and forth between
various encodings of the type parameters while maintaining the
skeleton.

Right now my solution to stay extensible is to simply syntactically
include the file containing the functors "Option", "Pair", etc., using
a macro preprocessor.  The environment into which it is included
defines the constraints on the type variables for this particular
instance.  This approach works for arbitrary constraints, which is why
I'm disappointed that this apparently cannot be done without macros.
Somehow it feels like a problem that should be easily solvable within
the core language.

-- 
Markus Mottl        http://www.ocaml.info        markus.mottl@gmail.com