[Haskell-beginners] Beginners Digest, Vol 30, Issue 17

[Brent Yorgey]

On Sun, Dec 12, 2010 at 03:09:44PM -0800, Russ Abbott wrote:
> Is there no way to make the class approach (or something like it) work?
>  That's how I would do it in Java--define the component as an interface as
> far as the structure is concerned and then define the component itself to
> implement the interface.

Not really. Although in a sense, my version with a separate Types
module feels somehow morally equivalent; the Types module provides the
"interface" and then the other modules are defined in terms of that.
You could use a type class if you really wanted more of a "behavioral"
interface, but you would still have to put the type class in a
separate module to avoid circularity.

-Brent

> *
> -- Russ*
> 
> 
> On Sun, Dec 12, 2010 at 3:00 AM, <beginners-request at haskell.org> wrote:
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> > Today's Topics:
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> >   1. Re:  Linking modules (Brent Yorgey)
> >
> >
> > ----------------------------------------------------------------------
> >
> > Message: 1
> > Date: Sat, 11 Dec 2010 15:37:23 -0500
> > From: Brent Yorgey <byorgey at seas.upenn.edu>
> > Subject: Re: [Haskell-beginners] Linking modules
> > To: beginners at haskell.org
> > Message-ID: <20101211203723.GB9265 at seas.upenn.edu>
> > Content-Type: text/plain; charset=iso-8859-1
> >
> > On Sat, Dec 11, 2010 at 08:56:23PM +0100, Chadda? Fouch? 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
> >
> >
> >
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> > End of Beginners Digest, Vol 30, Issue 17
> > *****************************************
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