Introduce DerivingVia

This implements the `DerivingVia` proposal put forth in
https://github.com/ghc-proposals/ghc-proposals/pull/120.

This introduces the `DerivingVia` deriving strategy. This is a
generalization of `GeneralizedNewtypeDeriving` that permits the user
to specify the type to `coerce` from.

The major change in this patch is the introduction of the
`ViaStrategy` constructor to `DerivStrategy`, which takes a type
as a field. As a result, `DerivStrategy` is no longer a simple
enumeration type, but rather something that must be renamed and
typechecked. The process by which this is done is explained more
thoroughly in section 3 of this paper
( https://www.kosmikus.org/DerivingVia/deriving-via-paper.pdf ),
although I have inlined the relevant parts into Notes where possible.

There are some knock-on changes as well. I took the opportunity to
do some refactoring of code in `TcDeriv`, especially the
`mkNewTypeEqn` function, since it was bundling all of the logic for
(1) deriving instances for newtypes and
(2) `GeneralizedNewtypeDeriving`
into one huge broth. `DerivingVia` reuses much of part (2), so that
was factored out as much as possible.

Bumps the Haddock submodule.

Test Plan: ./validate

Reviewers: simonpj, bgamari, goldfire, alanz

Subscribers: alanz, goldfire, rwbarton, thomie, mpickering, carter

GHC Trac Issues: #15178

Differential Revision: https://phabricator.haskell.org/D4684

1 files changed, 21 insertions, 2 deletions

diff --git a/libraries/template-haskell/Language/Haskell/TH/Lib.hs b/libraries/template-haskell/Language/Haskell/TH/Lib.hs
index dbf01f11df..b7966cefac 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Lib.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Lib.hs
@@ -21,7 +21,7 @@ module Language.Haskell.TH.Lib (
         StmtQ, RangeQ, SourceStrictnessQ, SourceUnpackednessQ, BangQ,
         BangTypeQ, VarBangTypeQ, StrictTypeQ, VarStrictTypeQ, FieldExpQ, PatQ,
         FieldPatQ, RuleBndrQ, TySynEqnQ, PatSynDirQ, PatSynArgsQ,
-        FamilyResultSigQ,
+        FamilyResultSigQ, DerivStrategyQ,
 
     -- ** Constructors lifted to 'Q'
     -- *** Literals
@@ -79,7 +79,9 @@ module Language.Haskell.TH.Lib (
     -- *** Top Level Declarations
     -- **** Data
     valD, funD, tySynD, dataD, newtypeD,
-    derivClause, DerivClause(..), DerivStrategy(..),
+    derivClause, DerivClause(..),
+    stockStrategy, anyclassStrategy, newtypeStrategy,
+    viaStrategy, DerivStrategy(..),
     -- **** Class
     classD, instanceD, instanceWithOverlapD, Overlap(..),
     sigD, standaloneDerivD, standaloneDerivWithStrategyD, defaultSigD,
@@ -140,6 +142,9 @@ import Language.Haskell.TH.Lib.Internal hiding
   , kindSig
   , tyVarSig
 
+  , derivClause
+  , standaloneDerivWithStrategyD
+
   , Role
   , InjectivityAnn
   )
@@ -262,3 +267,17 @@ kindSig = KindSig
 
 tyVarSig :: TyVarBndr -> FamilyResultSig
 tyVarSig = TyVarSig
+
+-------------------------------------------------------------------------------
+-- * Top Level Declarations
+
+derivClause :: Maybe DerivStrategy -> [PredQ] -> DerivClauseQ
+derivClause mds p = do
+  p' <- cxt p
+  return $ DerivClause mds p'
+
+standaloneDerivWithStrategyD :: Maybe DerivStrategy -> CxtQ -> TypeQ -> DecQ
+standaloneDerivWithStrategyD mds ctxt ty = do
+  ctxt' <- ctxt
+  ty'   <- ty
+  return $ StandaloneDerivD mds ctxt' ty'