[Haskell-cafe] Help using Groundhog Template Haskell.

[Andrew]

I seem to have worked around this by hand rolling instances
for TransactionType as below.  I would still like to understand
how to make this work with mkPersist though.

instance G.PersistField TransactionType where
  persistName _ = "TransactionType"
  toPersistValues = GG.primToPersistValue
  fromPersistValues = GG.primFromPersistValue
  dbType _ = G.DbTypePrimitive G.DbString False Nothing Nothing

instance G.PrimitivePersistField TransactionType where
  toPrimitivePersistValue _ a = G.PersistString (show a)
  fromPrimitivePersistValue _ (G.PersistString s) = read s
  fromPrimitivePersistValue _ _ = error "Expected PersistString"