[jhc] new extension in jhc: explicit namespaces in import/export lists

[John Meacham]

Hi, I implemented an extension in jhc that I thought the community might
have feedback on and feel that a variant of it would be a candidate for
future standardization. Here is the section from the jhc manual describing
it.

The 'kind' information is specific to jhc, and with this extension I'd
really like to get rid of the wart in haskell 98 that names in 'hiding'
lists are treated differently than names otherwise. I will likely make that
change in jhc in the future once I work out the details of the best way to
be backwards compatible.

I have seen this exact proposal more-or-less come up on the lists
periodically, so I guess I am just pointing to a concrete implementation
now.

# Explicit namespaces in import/export lists

jhc allows explicit namespaces in import export lists. In haskell 98, a name
such as 'Foo' in an export list will indicate that all of a class named Foo,
a type named 'Foo' and a data constructor named 'Foo' shoud be exported.
Jhc allows finer grained control by prefixing exports with a
namespace indicator. The valid namespace indicators are

    * 'type' - The name is a type, as in something defined by type ,
      newtype , or data , or the constructors of a kind declaration.
    * 'class' - Specifies that the name is that of a class.
    * 'data'  - Specifies that the name is a data constructor.
    * 'kind'  - specifies that the name is a user defined kind.

In addition, classes and types are in independent namespaces, so a type and
a     class of the same name may be in scope and not conflict.

When an unqualified name is given in a import/export list then all names
that match it that are in scope are matched. The only exception is a
holdover from haskell 98 where in hiding lists data constructors are not
matched. This behavior may change in the future when explicit
namespaces are used, so it should not be relied upon when using
explicit namespaces.

     John