[Haskell-cafe] What am I missing? Cycle in type synonym declarations

[David Fox]

On Tue, Aug 20, 2013 at 2:35 PM, adam vogt <vogt.adam at gmail.com> wrote:
> On Tue, Aug 20, 2013 at 5:00 PM, David Fox <dsf at 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

Thanks Adam, this must be the answer.  And thanks for the equivalent expression.