Trees that Grow and constraints

[Alan & Kim Zimmerman]

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 <alan.zimm at gmail.com>
wrote:

> 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
>
> [1] http://lpaste.net/360017
>
>
>
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