Folks
The folk at OGI have continued the current trend of discovering
unspecified corners of Haskell 98. That means taking some decisions,
and, as ever, I don't want to do that without giving you all a chance to
demur. Nevertheless, I doubt this'll be controversial, because it's
all dark-corner stuff.
| 1) Sec 5.3.1: In an "import A(x, ...)", the report tells us
| that x must be one of the names exported from A. There is no
| indication whether a similar rule applies to declarations
| like "import A hiding (x, ...)" if x is not exported from A.
I propose that hiding something that isn't exported should be
considered an error. It's not actually harmful but it is misleading.
| 2) The semantics of "module M" style entries in export lists seems to
| be underspecified. Here are some programs to illustrate:
|
| Program 1: module A(module B, ...) where
| ~~~~~~~~~~ ... code that doesn't import B ...
|
| Is this valid? We thought that it should probably be invalid,
| but could also see an interpretation in which it was valid, but
| with nothing exported for the "module B" part.
By the same token, I think this should be an error.
| Program 2: module A(module B, ...) where
| ~~~~~~~~~~ import qualified B
| ... code that doesn't import B ...
|
| A similar example: now B appears as an import, but there are no
| unqualified imports of B, so either "module B" adds nothing to the
| export list (except instances?), or else this program is an error.
|
| Item 5 on p65 (describing entities exported from modules) says that
| "module m" exports entities brought into scope from m by one or
more
| *unqualified* imports. But this example suggests more; that it
will
| export instances brought into scope from *qualified* imports too.
| Perhaps the intention is that "module m" exports all instances,
| and all entities that are imported from m with an unqualified name?
I don't think it should matter whether B is imported qualified or not; I
propose
to remove the *unqualified* adjective in the above quote.
The export list specifies what is exported, and can include qualified
names:
module A( B.f ) where
import qualified B
So 'module B' in an export list means "everything imported from B"
regardless of
whether the things imported from B are known by a qualified or
unqualified name.
| Program 3: module A(module B, ...) where
| ~~~~~~~~~~ import B as C
| ... code that doesn't import again from (or as) B ...
|
| Program 4: module A(module B, ...) where
| ~~~~~~~~~~ import C as B
| ... code that doesn't import again from (or as) B ...
|
| Which of these is valid? Does the "B" in "module B" refer to the
| literal name of the module from which entities are imported, the
| local alias by which they are referenced, ... or both?
Good question! Given
import X as Y
module X is known as Y throughout the importing module, so [proposal] I
think it should be module Y that appears in the export list, not X.
So a good summary of what is exported by "module Y" would be "every
entity
e for which Y.e is in scope". [Recall that everything is always
imported qualified,
and perhaps unqualified too.]
Simon
_______________________________________________
Haskell mailing list
[EMAIL PROTECTED]
http://www.haskell.org/mailman/listinfo/haskell
