On 14 Aug 2011, at 21:26, Brandon Allbery wrote:
Speaking of which, has anyone formally proposed the class/instance alias stuff for GHC? If so, has anyone stepped in to implement it? Having the tools to actually make this kind of change would be helpful.
*ahem* http://hackage.haskell.org/trac/ghc/wiki/DefaultSuperclassInstances I don't know if it's likely to be implemented in GHC anytime soon, but SHE has something rather like it (ie, it's so simple, a preprocessor can do it). A key motivation is exactly to minimise (but the minimum is not zero) the damage done in the situation envisaged here. In particular, it should be possible to add (<>) as a method to Monoid, then, at a later date, split off Semigroup, taking (<>) with it, leaving existing Monoid instances still valid (as they would implicity spawn Semigroup instances). The nasty cases are when you want to define liftings where Monoid x => Monoid (Lift x), but Semigroup x => Semigroup (Lift x). The proposal as it stands would leave Monoid x => Semigroup (Lift x) which might sometimes (but not all the time) be weaker than one has the right to. Other bits of class alias stuff (constraint synonyms) seem to be happening round about now, somewhere in the vicinity of Cambridge. So things are looking up. It should soon be technically feasible to separate the issues of whether the Monoid operator should be (<>) and whether it should actually live in a Semigroup superclass, pretty much allowing neither, either or both-in-either-order adoption strategies. Hoping this helps Conor