[Haskell-beginners] Linking modules

[Brent Yorgey]

On Sat, Dec 11, 2010 at 08:56:23PM +0100, Chaddaï Fouché wrote:
> On Sat, Dec 11, 2010 at 7:37 PM, Russ Abbott <russ.abbott at gmail.com> wrote:
> > I have two modules Structure and Component.  Module Structure defines
> >      data Structure = Component ...
> > Module Component defines
> >     data Component = ...
> > I would like Structure to import Component.  But Component
> > includes functions that take a Structure as an argument. So I have (or would
> > like) something like this organization.
> >
> > [snip]
> >
> > The functions in Component are really very Component related and should not
> > be moved to Structure. So how can I set up this circular relationship?
> 
> GHC allows you to compile mutually recursive modules, see
> http://www.haskell.org/ghc/docs/latest/html/users_guide/separate-compilation.html#mutual-recursion
> for how to do it.

Yes, this is possible, but it has always seemed sort of fragile and
ugly to me.  Another suggestion is to put the definitions of the
Structure and Component data types into a separate module called
Types, and then import Types into both the Structure module (which
defines functions over Structures) and the Component module (which
defines functions over Components).  Then no circularity is needed.

-Brent