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In Template Haskell, there's no way to find out all instances of a type or data family. Manuel commented on this in an old patch (third dot-point):

Tue Mar 24 20:34:47 PDT 2009  Manuel M T Chakravarty <chak at cse.unsw.edu.au>
  * Template Haskell: make reify aware of type families
  Ignore-this: 4b9c2d626e7c506a74331bb91d0fcff7
  - Reifying a type family returns a TH family declaration
  - Reifying a data constructor from a data instance attributes that
    constructor to the family (not the representation tycon)
  - Ideally, we should have facilities to reify all type/data instances of a 
    given family (and the same for instances of a class).  I haven't added that
    here as it involves some API design.

    M ./compiler/basicTypes/DataCon.lhs -2 +12
    M ./compiler/typecheck/TcSplice.lhs -5 +22

I'm not particularly concerned about the specific API, but the following seems reasonable:

data Info
 ...
 | DataFamI Dec [DataFamInstance]
 | TypeFamI Dec [TypeFamInstance]
 ...

-- | Always a 'DataInstD' or 'NewtypeInstD' constructor
type DataFamInstance = Dec
-- | Always a 'TySynInstD' constructor
type TypeFamInstance = Dec

Alternatively, perhaps the DataFamI and TypeFamI constructors should be discarded in favour of a single FamI constructor.

Trac metadata

Trac field	Value
Version	7.0.4
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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