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

20 Aug 2013

      On Tue, Aug 20, 2013 at 5:00 PM, David Fox  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

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

adam vogt