#4135: Can't Quote Instance Associated Types in Template Haskell
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Reporter: Ashley Yakeley | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Template Haskell | Version: 6.12.1
Keywords: | Difficulty:
Os: Linux | Testcase:
Architecture: x86_64 (amd64) | Failure: GHC rejects valid program
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Comment(by simonpj):
Actually the email cited has a different problem
{{{
{-# LANGUAGE TemplateHaskell, MultiParamTypeClasses, TypeFamilies,
FlexibleInstances, OverlappingInstances #-}
module Sample where
import Control.Monad
import Language.Haskell.TH
class Foo a where
type FooType a
createInstance' :: Q Type -> Q Dec
createInstance' t = liftM head [d|
instance Foo $t where
type FooType $t = String |]
}}}
This fails with a similar error:
{{{
Sample.lhs:22:10:
Type indexes must match class instance head
Found `t_aMn' but expected `t_aMl'
}}}
Here it's plain that we can't really do full type-checking of the quoted
instance until `createInstance'` is applied, so that we can run the splice
`$t`. Somehow TH needs to be less picky about the consistency checks on
types when typechecking quotes.
The earlier example, though, should be fine. I'll look into it.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4135#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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