id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
5406	Template Haskell: Reification of type family instances	reinerp		"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."	feature request	closed	normal		Template Haskell	7.0.4	fixed		alfonso.acosta@… illissius@… Ashley Yakeley SamAnklesaria	Unknown/Multiple	Unknown/Multiple	None/Unknown