declaring minimal complete instance definition (was: 2014 Applicative => Monad proposal)

[Herbert Valerio Riedel]

On 2013-05-23 21:56:30 +0200, Henning Thielemann wrote:

[...]

> Currently, when I forget to implement the two essential Monad methods
> return' and '>>=' in a Monad instance, I get a warning. However with
> the mutual implementations of 'join' and '>>=' I do not get a warning
> but an infinite loop. Similarly, I already had nasty bugs with (a-b)
> defaulting to (a + negate b) and (negate a) defaulting to (0-a).

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