This is for discussion only at present.

With the switch to GHC 7.4, we can switch the "aggregator" Patch classes
to use constraint synonyms - kind of like replacing a newtype with a type
synonym.

I've done this in the attached patch, though I'm not sure it's actually
a good thing. We lose some orphan instances, but gain some
UndecidableInstances pragmas (I don't quite understand why they are
actually necessary, but GHC insisted).

In particular I think we lose the documentation benefits of the 
aggregator classes as separate entities. For example,

    instance PrimPatch prim => Patchy (Patch prim)
    instance PrimPatch prim => RepoPatch (Patch prim)

tells us clearly that a (V1) patch just depends on the PrimPatch class 
for its contained primitives, whereas some of the messier instances like

    instance (IsHunk p, PatchListFormat p, Patchy p) => Patchy (FL p)
    instance (IsHunk p, PatchListFormat p, Patchy p) => Patchy (RL p)

tell us that things aren't quiet as clean as we might like.

On the other hand making this change may make other changes nicer - for 
example I'm currently looking at using a type function to allow the 
inverse of a patch type p to be a different type.

Copy on darcs hub at 
http://hub.darcs.net/ganesh/darcs-screened-constraint-synonyms/patch/20140210213341-81bb2

1 patch for repository darcs-unstable@darcs.net:screened:

Mon Feb 10 21:33:41 GMT 2014  Ganesh Sittampalam <ganesh@earth.li>
  * Replace "aggregator" classes with constraint synonyms
  This applies to PrimPatch, Patchy and RepoPatch

Attachments

• patch-preview.txt
• replace-_aggregator_-classes-with-constraint-synonyms.dpatch
• unnamed

On reflection I think it's best not to do this. If anyone would like to 
revive it just re-open this ticket and we can discuss more.