And it seems that
--------------------------------------
data Experiment p
= Experiment {
e_ext :: (XEOverLit p),
e_val :: Int }
| XExperiment (XXOverLit p)
deriving instance (Data GhcPs) => Data (Experiment GhcPs)
deriving instance (Data GhcRn) => Data (Experiment GhcRn)
deriving instance (Data GhcTc) => Data (Experiment GhcTc)
----------------------------------------
works, but only for GHC 8.2.1, not GHC 8.0.2
Alan
On 13 November 2017 at 11:23, Alan & Kim Zimmerman
At the moment the Trees that Grow implementation in GHC master makes use of massive constraint types to provide Data instances for the hsSyn AST.
I am trying to remove the need for this, and have hit a problem.
The example I have reduced it to is here [1]
The essence of the problem is
------------------- data Experiment p = Experiment { e_ext :: (XEOverLit p), e_val :: Int } | XExperiment (XXOverLit p) deriving instance (Data (GhcPass p)) => Data (Experiment (GhcPass p))
type family XEOverLit x type family XXOverLit x
-- The following line works -- type instance XEOverLit (GhcPass _) = PlaceHolder
-- The following 3 lines do noe type instance XEOverLit (GhcPass 'Parsed ) = PlaceHolder type instance XEOverLit (GhcPass 'Renamed ) = PlaceHolder type instance XEOverLit (GhcPass 'Typechecked) = PlaceHolder
type instance XXOverLit (GhcPass _) = PlaceHolder -------------------
Where specifying the type instance with a wild card keeps GHC happy (the XXOverLit case), but specifying for each of the three constructors for pass does not (the XEOverLit case)
The exact error message is
------------------------------ Experiment.hs:34:1: error: • Could not deduce (Data (XEOverLit (GhcPass p))) arising from a use of ‘k’ from the context: Data (GhcPass p) bound by the instance declaration at Experiment.hs:34:1-69 • In the first argument of ‘k’, namely ‘(z Experiment `k` a1)’ In the expression: ((z Experiment `k` a1) `k` a2) In an equation for ‘gfoldl’: gfoldl k z Experiment a1 a2 = ((z Experiment `k` a1) `k` a2) When typechecking the code for ‘gfoldl’ in a derived instance for ‘Data (Experiment (GhcPass p))’: To see the code I am typechecking, use -ddump-deriv | 34 | deriving instance (Data (GhcPass p)) => Data (Experiment (GhcPass p)) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ------------------------------
Alan