(I'm using GHC 6.2.2 on SPARC/Solaris.) I've got a class: class (Arrow arrow) => ArrowComb arrow b where or2 :: arrow (b, b) b ... and want to make an instance for it: data DCin b = MkDCin { field :: b } instance ArrowComb (->) b => ArrowComb (->) (DCin b) where or2 = ... using Template Haskell. Trying to use "arrowT" as a constructor in this context leads to the runtime error: *** Exception: hsSyn/Convert.lhs:317: Non-exhaustive patterns in function trans I can provide a test case if you need one. I don't have access to a CVS version, so this may already have been fixed. regards, Peter.