Revamping the module hierarchy

Edward Kmett ekmett at
Fri Jun 19 09:44:59 EDT 2009

On Fri, Jun 19, 2009 at 6:06 AM, wren ng thornton <
wren at> wrote:

>  I agree with Maurico that what we really need is to have the tools to be
> able to rearrange the tree at will. The Haskell language has no business
> dealing with the provenance of where modules come from--- and forcing
> modules to be named after their packages would make it do so. Currently,
> ghc-pkg (or whatever) handles the provenance of making sure that packages
> are visible to have their modules be loaded. As it stands, this provenance
> mechanism automatically roots all packages at the same place, but there's no
> reason it needs to. We just have to come up with the right DSL for scripting
> ghc-pkg (or equivalently, the right CLI) to be able to play around with the
> module namespace in a more intelligent way.
> For instance, let's assume we have:
>    > ghc-pkg describe libfoo-0.0.0
>    ...
>    exposed-modules: Data.Foo Control.Bar Control.Bar.Baz
>    ...
> Now, if we say:
>    ghc-pkg expose libfoo-0.0.0
> Then any Haskell programs can now load the modules mentioned above, by the
> names mentioned above. If instead we said something like:
>    ghc-pkg expose libfoo-0.0.0 at Zot
> Then Haskell programs would be able to load the modules by the names
> Zot.Data.Foo, Zot.Control.Bar, and Zot.Control.Bar.Baz instead. And if we
> wanted to rebase subtrees then we could say something like:
>    ghc-pkg expose libfoo-0.0.0:Control.Bar as Quux
> Which would make the modules Quux and Quux.Baz available for loading, and
> would effectively hide libfoo-0.0.0:Data.Foo from being loadable.
> To implement this we need to update not only ghc-pkg, but also the Cabal
> format. Rather than just specifying which dependent packages must be
> exposed, we also need to specify *where* the package expects them to be
> exposed in the module namespace. Assuming this is implemented sanely, then
> all of the renaming for changing the root and for rebasing subtrees can be
> boiled out and undone during the linking phase (that is, when GHC is
> "linking" things to follow imports etc; not when ld is actually linking
> things). An import declaration is a reference to an actual compiled module,
> the name is just a proxy to know where to find it, the name doesn't have any
> meaning in itself.
> Since every package gets their own local view of the module namespace,
> every package can choose their own names for things. Moreover, since every
> package must specify their local view, if one wants to have some crazy
> jumbled view then the burden is on them to specify how to do it. Since every
> package exposes a view of its exposed module namespace, this serves as the
> default view. Since it takes work for people to rearrange things, there will
> still be a force to give things good names in the first place. Only we would
> no longer be stuck with bad decisions.


I really like this proposal.

I agree that I much prefer the current orthogonality of modules provided to
package names. It lets you refactor packages into several smaller chunks,
and this would not even be possible under the other namespacing schemes I've
seen bandied about without breaking other code.

The biggest problem that I have with the current scheme is the inability to
work with packages with conflicting namespaces (i.e. to support both the mtl
and one of its competitors that overlap it). This quite elegantly works
around that restriction.

It is not perfect because there is still the corner case that you still
can't work with conflicting instance declarations for types from the
Prelude, but its a damn sight better than anything else I've seen put

-Edward Kmett
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