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, 1 insertions, 0 deletions

diff --git a/compiler/parser/ApiAnnotation.hs b/compiler/parser/ApiAnnotation.hs
index 4d1758ff3c..6ae01d6fe0 100644
--- a/compiler/parser/ApiAnnotation.hs
+++ b/compiler/parser/ApiAnnotation.hs
@@ -286,6 +286,7 @@ data AnnKeywordId
     | AnnVal  -- ^ e.g. INTEGER
     | AnnValStr  -- ^ String value, will need quotes when output
     | AnnVbar -- ^ '|'
+    | AnnVia -- ^ 'via'
     | AnnWhere
     | Annlarrowtail -- ^ '-<'
     | AnnlarrowtailU -- ^ '-<', unicode variant