#5406: Template Haskell: Reification of type family instances
---------------------------------+------------------------------------------
Reporter: reinerp | Owner:
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Template Haskell | Version: 7.0.4
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
---------------------------------+------------------------------------------
Changes (by simonpj):
* cc: alfonso.acosta@…, illissius@…, Ashley, Yakeley, SamAnklesaria
(added)
Comment:
Reasonable request -- but what ''should'' the API be? Currently for
classes we have
{{{
classInstances :: Name -> [Type] -> Q [ClassInstance]
data Info = ClassI Dec ClassInstance | ...
data ClassInstance = ...a record of stuff about the instance...
}}}
So doing `(reify c)` gets you all the instance of class `c` in its
`ClassI`; or you can look up an instance more selectively with
`classInstances` (which returns all instances matching the type template).
Thoughts:
* I like your idea of returning the instances as a Dec, rather than as a
new data type like `ClassInstance`.
* I'm slightly dubious about always returning all the instances of a
class in its `ClassI` constructor; there could be a lot of them. But only
slightly.
Here's a proposal
* Generalise `classInstances` thus:
{{{
reifyInstances :: Name -> [Type] -> Q [Dec]
}}}
this would replace `classInstances` (or at least `classInstances` could
be implemented in terms of it).
* `reifyInstances` would work on a `Name` of a class, type family, or
data family, returning a `Dec` that was a `InstanceD`, `DataInstD`,
`NewtypeInstD`, or `TySynInstD` respectively.
* For uniformity with `ClassI`, I suppose that `TyConI` should have a
`[Dec]` that was the list of instances in the case of data or type
families. But it's less nice, because most `TyCon`s are not families, so
the list would always be empty for them. I suppose we could split
`TyConI` thus:
{{{
data Info = ... | TyConI Dec
| FamilyConI Dec [Dec]
...
}}}
where family type constructors come ot as `FamilyTyConI`.
I'm ccing other people who were involved in #1835.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5406#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
_______________________________________________
Glasgow-haskell-bugs mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
