Template Haskell: Reification of type family instances
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 +22I'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 = DecAlternatively, 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 |
| Test case | |
| Differential revisions | |
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| Related | |
| Blocking | |
| CC | |
| Operating system | |
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