Are you making the case for an actual change in the behaviour of the current TH operations? The current reifyInstances is deliberately
#7066: isInstance does not work for compound types -------------------------------------+------------------------------------ Reporter: edsko | Owner: simonpj Type: bug | Status: new Priority: normal | Milestone: 7.8.3 Component: Template Haskell | Version: 7.4.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by mojojojo): primitive (a one-level lookup) so that other stuff can be built on top. Or are you asking for some new functions in the Quasi monad; or new functions in the template-haskell library? Well. I consider the current behaviour of `reifyInstances` counterintuitive, to say at very least. It took me quite some time of debugging the library I was developing at the time, before I narrowed the issue I was experiencing down to the unexpected behaviour of `reifyInstances`. Only then did I get to googling and finally here to find out that it's actually an intended behaviour, though misleadingly documented. Let's consider the use cases for that function. All I can imagine is finding the instances, which then can be relied on as applicable to the tested types. I can imagine no scenario, in which one would want to get instances, which the typechecker would deny. Can you imagine any examples of such a case? Considering the above, I can only see the current behaviour of that function as a bug. However I am not strongly opinionated about changing that, especially since I've already published a solution. The documentation however definitely should be updated to make the behaviour of this function clear.
It would help a lot to be clear precisely what the behaviour is supposed to be when the types concerned contain type variables. Currently reifyInstances returns all the instance whose heads unify with the specified constraint. I've looked at your code and it's not clear to me exactly what it does. Could you write a specification?
I've [https://github.com/nikita-volkov/th-instance- reification/blob/master/src/THInstanceReification.hs#L36-L87 updated the code] by covering it with extensive comments. [https://github.com/nikita- volkov/th-instance-reification/blob/master/src/HTFTestSuite.hs The test suite] should also be useful for clarifying the expected behaviour.
I think it's arguable that what you really want is something like {{{isInstance :: Cxt -> Name -> [Type] -> Q Bool}}} return True if the constraint (C tys) is provable from the specifed context. For example, you could ask, say isIntance [Show a, Num a] Foo [Maybe a], to ask whether you can prove (Foo (Maybe a)) from (Show a, Num a). For ground types you could give the empty Cxt.
Yes, this looks like it. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7066#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler