Proposal: Scoping rule change

[Lennart Augustsson]

It's not often that one gets the chance to change something as
fundamental as the scoping rules of a language.  Nevertheless, I would
like to propose a change to Haskell's scoping rules.

The change is quite simple.  As it is, top level entities in a module
are in the same scope as all imported entities.  I suggest that this
is changed to that the entities from the module are in an inner scope
and do not clash with imported identifiers.

Why?  Consider the following snippet

    module M where
    import I
    foo = True

Assume this compiles.  Now change the module I so it exports something
called foo.  After this change the module M no longer compiles since
(under the current scoping rules) the imported foo clashes with the
foo in M.

Pros: Module compilation becomes more robust under library changes.
Fewer imports with hiding are necessary.

Cons: There's the chance that you happen to define a module identifier
with the same name as something imported.  This will typically lead to
a type error, but there is a remote chance it could have the same
type.

Implementation status: The Mu compiler has used the scoping rule for
several years now and it works very well in practice.

  -- Lennart
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