Trees that Grow and constraints

[Simon Peyton Jones]

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)
Well, of course!  The derived data instance looks something like

instance (Data (GhcPass p)) => Data (Experiment (GhcPass p)) where
   gfoldl = ….Needs (Data (XEOverLit (GhcPass p)))…

How can GHC solve the wanted constraint
            Data (XEOverlit (GhcPass p))

With three equations, it can’t. With one, it can.

So why not use one?

Simon

From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Alan & Kim Zimmerman
Sent: 13 November 2017 09:24
To: ghc-devs at haskell.org
Subject: Trees that Grow and constraints

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<https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Flpaste.net%2F360017&data=02%7C01%7Csimonpj%40microsoft.com%7C805caf6c0aac49f030ee08d52a785574%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636461618682611590&sdata=VxK0zeEmVsEahWRYMJ6xAG2gZviqGCd%2FHS4DrRXC0dM%3D&reserved=0>


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