Re: declaring minimal complete instance definition

On 23/05/13 22:20, Shachaf Ben-Kiki wrote:

On Thu, May 23, 2013 at 4:17 PM, Herbert Valerio Riedel wrote:

...

As this is a general problem which comes up often with mutually recursive default implementations for typeclass methods, I have been wondering if somebody came up with a solution for that, which allows the compiler to emit warnings or even errors.

Often the documentation for a typeclass already specifies the "minimal complete definition", if this could be meta-annotated with some pragma for the compiler to pick up, we could have the compiler help us avoid invalid instance definition. Has some ghc-feature/extension to this effect been discussed in the past?

Btw, even the simple 'Eq' and 'Ord' classes have mutually depending default implementations:

- Eq: "Minimal complete definition: either '==' or '/='."

- Ord: "Minimal complete definition: either 'compare' or '<='. Using 'compare' can be more efficient for complex types."

cheers, hvr

See my proposal along these lines from a few months ago to implement this as a pragma in GHC:

http://hackage.haskell.org/trac/ghc/ticket/7633

Shachaf

I have made an implementation, the patch is attached to that ticket. Twan