On Tue, Aug 20, 2013 at 5:00 PM, David Fox <d...@seereason.com> wrote:
> This file gives me the error "Cycle in type synonym declarations" Can
> anyone tell me why? I'm just trying to write a function to create a
> type that is a FooT with the type parameter fixed.
>
> {-# LANGUAGE TemplateHaskell #-}
> import Language.Haskell.TH (Q, Dec, TypeQ)
>
> data FooT a = FooT a
>
> foo :: TypeQ -> Q [Dec]
> foo t = [d| type Bar = FooT $t |]
Hi David,
That's strange considering you can accomplish the same thing with:
foo t = fmap (:[]) $ tySynD (mkName "Bar") [] [t| FooT $t |]
Bugs like <http://ghc.haskell.org/trac/ghc/ticket/4230> are a similar
problem. In your case it seems that GHC is too eager to prevent the
cycle you could make with foo (conT (mkName "Bar")))
Regards,
Adam
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