Trying to refactor DeriveAnyClass, stuck on typechecker-related code

[Matthew Pickering]

Ryan and I chatted about this on IRC and he has a plan to fix this problem.

On Sat, Aug 27, 2016 at 5:07 PM, Ryan Scott <ryan.gl.scott at gmail.com> wrote:
> Simon,
>
> I'm currently working on a solution to #12144, which changes the way
> inferred contexts for DeriveAnyClass works, as per your suggestion
> here [1]. I've made some good progress in that instances derived using
> DeriveAnyClass now emit contexts that are gathered from the default
> signatures of the derived class itself.
>
> However, I've come to an impasse. This approach currently fails to
> work with most GHC Generics-style classes, for reasons that I'll
> explain below. Consider this example (abridged from here [2]):
>
>     data StrictMaybe a = StrictNothing | StrictJust !a
>       deriving (FullyStrict, Generic)
>
>     class FullyStrict a where
>       fullyStrict :: proxy a -> Bool
>       default fullyStrict :: (GFullyStrict (Rep a)) => proxy a -> Bool
>       fullyStrict _ = gfullyStrict (Proxy :: Proxy (Rep a p))
>
> With the new approach, the derived FullyStrict instance for
> StrictMaybe a will emit the context (GFullyStrict (Rep (StrictMaybe
> a)), but then fail with the error:
>
>     No instance for (GFullyStrict (Rep (StrictMaybe a))
>
> I think I understand why this is happening: if you look at the
> definition of tcDeriving in TcDeriv [3], it calls
> simplifyInstanceContexts, which would normally reduce type families
> like Rep. But the problem is that the Rep (StrictMaybe a) instance is
> also being derived at the same time (via the derived Generic
> instance)! What's worse, the call to simplifyInstanceContexts happens
> before the code that adds Rep (StrictMaybe a) to the type family
> instance environment (tcExtendLocalFamInstEnv (bagToList famInsts)).
> So simplifyInstanceContexts is unable to reduce Rep (StrictMaybe a),
> and as a result the context fails to simplify, resulting in the above
> error.
>
> This leads me to think we need to be adding Rep (StrictMaybe a) to the
> instance environment before calling simplifyInstanceContexts. But with
> the way the code is structured currently, I don't see an easy way to
> do this. The problem is that the genInst function [4] generates all of
> the following at the same time in one large bundle:
>
> * The instance code itself (InstInfo RdrName)
> * The associated type family instances (BagDerivStuff)
> * Auxiliary top-level bindings (BagDerivStuff)
>
> And within tcDeriving, the call to genInst takes as input the output
> of simplifyInstanceContexts, making it impossible to get the type
> family instances before calling simplifyInstanceContexts.
>
> Here are my questions: is it possible to take type family instances
> out of BagDerivStuff and instead add them to the instance environment
> before simplifyInstanceContexts? Would there be any typechecking
> issues involved with having the associated family instances in the
> local environment before the actual class instances themselves?
>
> Ryan S.
> -----
> [1] https://mail.haskell.org/pipermail/ghc-devs/2016-June/012276.html
> [2] http://git.haskell.org/ghc.git/blob/0050aff22ba04baca732bf5124002417ab667f8a:/testsuite/tests/generics/GFullyStrict.hs
> [3] http://git.haskell.org/ghc.git/blob/0050aff22ba04baca732bf5124002417ab667f8a:/compiler/typecheck/TcDeriv.hs#l364
> [4] http://git.haskell.org/ghc.git/blob/0050aff22ba04baca732bf5124002417ab667f8a:/compiler/typecheck/TcDeriv.hs#l2269
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