Circular Instance Declarations

[Hal Daume III]

I have also wanted this, in the context of John Hughes' restricted data
types.  In order to simulate these properly, you need circular instances
of this form.

 - Hal

On Sun, 2003-09-07 at 21:07, Brandon Michael Moore wrote:
> Hi Ashley
> 
>   See the thread "Type Class Problem". In his post on Aug 22 Simon
> Peyton-Jones said that it shouldn't be hard to implement, and mentioned
> that it would ruin the property that dictionaries can be evaluated by
> call-by-value. I couldn't puzzle out enough of the type class system to
> make the change on my first try, and since then I've been looking for a
> more general solution
> 
> Actually, I'm surprised someone else has a use for this. I wanted
> circular instances for playing with the paper "Recursion Schemes from
> Comonads". What are you trying to do?
> 
> Detecting circularity in a derivation is equivalent to accepting a regular
> infinite derivation for instances. Would you have a use for irregular
> derivations?
> 
> Brandon
> 
> On Sat, 6 Sep 2003, Ashley Yakeley wrote:
> 
> > When -fallow-undecidable-instances is switched on, is there any reason
> > why circular instances are forbidden? For instance:
> >
> >  module CircularInsts where
> >     {
> >     data D r = ZeroD | SuccD (r (D r));
> >
> >     instance (Eq (r (D r))) => Eq (D r) where
> >         {
> >         ZeroD == ZeroD = True;
> >         (SuccD a) == (SuccD b) = a == b;
> >         _ == _ = False;
> >         };
> >
> >     newtype C a = MkC a deriving Eq;
> >
> >     equalDC :: D C -> D C -> Bool;
> >     equalDC = (==);
> >     }
> >
> > When I compile this, I get this:
> >
> >  $ ghc -fglasgow-exts -fallow-undecidable-instances -c CircularInsts.hs
> >  CircularInsts.hs:2:
> >     Context reduction stack overflow; size = 21
> >     Use -fcontext-stack20 to increase stack size to (e.g.) 20
> >         `Eq (C (D C))' arising from use of `==' at CircularInsts.hs:16
> >         `Eq (D C)' arising from use of `==' at CircularInsts.hs:16
> >         `Eq (C (D C))' arising from use of `==' at CircularInsts.hs:16
> >         `Eq (D C)' arising from use of `==' at CircularInsts.hs:16
> >
> > Would it be reasonable for the compiler to check back through the stack
> > and allow the circularity? It will just create an ordinary recursive
> > function.
> >
> > --
> > Ashley Yakeley, Seattle WA
> >
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> >
> >
> 
> 
> 
> 
> 
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-- 
--
 Hal Daume III                                   | hdaume@isi.edu
 "Arrest this man, he talks in maths."           | www.isi.edu/~hdaume