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

On Tue, Aug 20, 2013 at 2:35 PM, adam vogt wrote:

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

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