#7477: reifyInstances can't deal with polykinded type families
-----------------------------+----------------------------------------------
Reporter: goldfire | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.7 | Keywords: TemplateHaskell TypeFamilies
PolyKinds
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Blockedby:
Blocking: | Related:
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When I run the following code
{{{
{-# LANGUAGE DataKinds, KindSignatures, PolyKinds, TypeFamilies,
TemplateHaskell #-}
import Language.Haskell.TH
type family F (a :: k)
type instance F Int = Bool
$( do { info <- reifyInstances ''F [ConT ''Int]
; reportWarning (pprint info)
; return [] })
}}}
I get this error:
{{{
Wrong number of types (expected 2)
In the argument of reifyInstances: Main.F GHC.Types.Int
}}}
I assumed that TH wanted me to supply the kind parameter, so I tried this:
{{{
...
$( do { info <- reifyInstances ''F [StarT, ConT ''Int]
...
}}}
I got this response:
{{{
`F' is applied to too many type arguments
In the argument of reifyInstances: Main.F * GHC.Types.Int
}}}
I poked around in the code to see what might be causing it. I found a
couple of interesting things:
* {{{reifyInstances}}} uses {{{tyConArity}}} to get the arity of a type
family. For my {{{F}}}, {{{tyConArity}}} reported 3. So, I wrote some code
to go through the kind and count only non-superkind arguments. This didn't
fix the problem, because ...
* {{{reifyInstances}}} passes control off, through a handful of other
functions, to {{{matchExpectedFunKind}}}, which works only with
{{{FunTy}}}, not {{{ForAllTy}}}. So, my {{{F}}}, whose kind is headed by
{{{ForAllTy}}}, looks like it takes no arguments at all.
I could try to fix this, but I'm worried about upsetting the apple cart
here. If a knowledgeable individual could give some pointers about what's
safe and what's not safe to change in this code, I'm happy to write the
fix.
This was all tested on 7.7.20121130.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7477>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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