#5406: Template Haskell: Reification of type family instances
---------------------------------+------------------------------------------
Reporter: reinerp | Owner:
Type: feature request | Status: new
Priority: normal | Component: Template Haskell
Version: 7.0.4 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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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.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5406>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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