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

adam vogt vogt.adam at gmail.com
Tue Aug 20 23:35:17 CEST 2013

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")))


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