[Haskell-beginners] Module import problem

Chaddaï Fouché chaddai.fouche at gmail.com
Sun Sep 28 15:09:45 EDT 2008

2008/9/28 Casey Rodarmor <caseyrodarmor at gmail.com>:
> The comparison I was thinking of was with python, where a module can
> be placed in an arbitrary directory, and then accessed using a
> relative path. If I have 'stuff.py' that contains class 'Foo', I can
> move it to 'hello/module/stuff.py', and then import it is
> hello.module.stuff.

And if you have something in hello/ it must refer to the same module
by module.stuff instead. I'm not sure it's clearer...
Anyway the hierarchical modules in Haskell means that every code must
import the same module with the same name which I find saner (though I
could wish the import/export syntax would be more advanced to allow
for shorter imports when I have a lot of imports in the same hierarchy

> A python module doesn't need to know where it
> lives.

Well, it doesn't need to, but everyone else needs to, in Haskell, the
module need to know where it lives, but everyone else don't care.

> I often find myself working on computers where I don't have
> administrative privileges, which means I might not be able to install
> libraries in the 'right' places. This approach also makes it simple to
> create self-contained projects that can easily be moved from machine
> to machine, where only the local directory structure is important.
> Would there be any downside to this in haskell?

Note that if the modules are in the same project, they're all in the
same hierarchy (at least that's how I see a project) and so their
names are coherent with each other and with the relative directory
hierarchy, there is no problem there.
Now, I think your preoccupation has more to do with packaging several
libraries you're not sure will be available on the final machine with
your project, and reasonably, you don't want to put them all in the
same base directory (yeah, it would be a mess). I would argue that
Haskell being a compiled language, its distribution problematics
aren't exactly the same as Python... If you're just distributing an
application you presumably would just compile it on your development
setup and then distribute the executable.
But if you really want to distribute the code source in a self
sufficient package (assuming GHC on the final machine I guess ?), you
should use Cabal : it automates dependency checking, compilation and
installation with several source directory and configurable target
installation directory. It's pretty easy to use too (mainly I suggest
you do copy/paste of a good project file on Hackage).


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