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

jabolopes at google.com jabolopes at google.com
Tue Aug 20 23:16:57 CEST 2013


In this case, you have two 'FooT' names: one is the Type and the other
is the Constructor.  Perhaps Template Haskell is capturing the wrong
one inside the quote (probably the constructor).  When you have name
shadowing, you should always use a lookup function.  You can find
these lookup functions in the Template Haskell library.

In the meantime, just for a quick test, you can try to change the name
of the constructor to something else to avoid capturing, and you can
see if the rest of the code works.


On Tue, Aug 20, 2013 at 02:00:29PM -0700, 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 |]
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