[GHC] #8426: one-shot compilation + TH doesn't see instances that is seen in batch mode

[GHC]

#8426: one-shot compilation + TH doesn't see instances that is seen in batch mode
----------------------------+----------------------------------------------
        Reporter:  errge    |            Owner:
            Type:  bug      |           Status:  new
        Priority:  normal   |        Milestone:
       Component:           |          Version:  7.7
  Template Haskell          |         Keywords:
      Resolution:           |     Architecture:  Unknown/Multiple
Operating System:           |       Difficulty:  Moderate (less than a day)
  Unknown/Multiple          |       Blocked By:
 Type of failure:  Other    |  Related Tickets:  #7867
       Test Case:           |
        Blocking:           |
----------------------------+----------------------------------------------

Comment (by errge):

 Yes, the exhaustive list can be big.

 I agree with your plan of just removing [InstanceDec] from ClassI and
 FamilyI, fixing the ambiguity.

 But actually I need the functionality of looking up instances without
 types.

 So what if we somehow expose that functionality directly?  Meaning that
 only those users will pay the cost who explicitly want to do that.

 I see two options:
   - (clean) adding a new Q monad method for this,
   - (a bit hacky) specifying that qReifyInstances with an *empty* list
 parameter does that.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8426#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler