[Caml-list] Restrictions on signature substitutions

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

What's actually the deeper reason why the following is rejected:

  module type S = sig type t end
  module type T = S with type t := int list

Error message:

  Error: Only type constructors with identical parameters can be substituted.

I found this restriction quite annoying on multiple occasions.  At
first sight it seems easy to just introduce a workaround "type foo =
int list" and then substitute "foo".  But if the above definitions are
themselves part of a signature, this would require polluting the
containing signature with a new type or come up with more convoluted
solutions.

I suspect it's merely hard to support the above in the current type
system implementation, probably for a similar reason why substitutions
in sub-modules are disallowed.  Or is there some more fundamental
reason why this cannot be supported?

Regards,
Markus

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