[Haskell-cafe] Re: Proposal: register a package asprovidingseveralAPI versions

[Simon Marlow]

Claus Reinke wrote:

> a few examples, of the top of my head:
> - consider the base split in reverse: if functionality is only repackaged,
>    the merged base would also provide for the previously separate
>    sub-package apis (that suggests a separate 'provides:' field, 
> though,       as merely listing version numbers wouldn't be sufficient)
> - consider the base split itself: if there was a way for the base split
>    authors to tell cabal that the collection of smaller packages can
>    provide for clients of the the old big base, those clients would
>    not run into trouble when the old big base is removed

These two cases could be solved by re-exports, no extra mechanism is required.

> - consider adding a new monad transformer to a monad transformer
>    library, or a new regex variant to a regex library - surely the new 
>    package version can still provide for clients of the old version

This case doesn't work - if you add *anything* to a library, I can write a 
module that can tell the difference.  So whether your new version is 
compatible in practice depends on the client.

> - consider various packages providing different implementations
>    of an api, say edison's - surely any of the implementations will
>    do for clients who depend only on the api, not on specifics

Yes, and in this case we should have another package that just re-exports 
one of the underlying packages.

You seem to want to add another layer of granularity in addition to 
packages, and I think that would be unnecessary complexity.

Cheers,
	Simon