First class modules

Tom Pledger
Thu, 9 Nov 2000 09:07:08 +1300 (NZDT)

Fergus Henderson writes:
 > On 07-Nov-2000, Tom Pledger <> wrote:
 > > Supposing that (some version of) Haskell had first class modules, and
 > > type variables could be universally quantified at the module level,
 > > would rule 2 of the monomorphism restriction go away?
 > No.
 >  |    Rule 2. Any monomorphic type variables that remain when type
 >  |    inference for an entire module is complete, are considered
 >  |    ambiguous, and are resolved to particular types using the
 >  |    defaulting rules (Section 4.3.4).
 > Although this rule refers to the "entire module", its typical for
 > the ambiguity to arise within a single function:
 > 	foo = show (read "whatever")
 > This expression is fundamentally ambiguous unless you somehow
 > disambiguate what type it is that you are trying to read.
 > I don't see how first class modules could solve that.

Thanks for the analysis.  I see how there's no way for another module
(or any other declarations in the same module) to disambiguate that.

How about the cases where the module boundary plays a part in
triggering the MR, like this?

    module MRDemo where
    x = return ()

I've got the impression that these cases are the ones which provoke
most of the MR criticism.  Would they be relieved by quantifying at
the module level, so that the ambiguity gets passed to modules which
import MRDemo and use x?