Circular Instance Declarations

[Brandon Michael Moore]

On Thu, 11 Sep 2003, Simon Peyton-Jones wrote:

> OK, I yield!
>
> The HEAD now runs this program.  It turned out to be a case of
> interchanging two lines of code, which is the kind of fix I like.
>
> Simon

Cool! Yet another domain where haskell handles infinities quite happily.
Thanks.

Hopefully I'll have some code to contribute soon.

  Brandon

>
>
> | -----Original Message-----
> | From: haskell-admin at haskell.org [mailto:haskell-admin at haskell.org] On
> Behalf Of Ashley Yakeley
> | Sent: 07 September 2003 06:57
> | To: haskell at haskell.org
> | Subject: Circular Instance Declarations
> |
> | 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
> |
> | _______________________________________________
> | Haskell mailing list
> | Haskell at haskell.org
> | http://www.haskell.org/mailman/listinfo/haskell
>
>
> _______________________________________________
> Haskell mailing list
> Haskell at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell
>
>
>