Circular Instance Declarations

[Brandon Michael Moore]

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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