#15178: Implement DerivingVia
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Reporter: RyanGlScott | Owner: RyanGlScott
Type: feature request | Status: patch
Priority: normal | Milestone: 8.6.1
Component: Compiler | Version: 8.5
Resolution: | Keywords: deriving,
| GHCProposal
Operating System: Unknown/Multiple | Architecture:
| Unknown/Multiple
Type of failure: None/Unknown | Test Case:
Blocked By: | Blocking:
Related Tickets: | Differential Rev(s): Phab:D4684
Wiki Page: |
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Comment (by Ben Gamari ):
In [changeset:"8ed8b037fee9611b1c4ef49adb6cf50bbd929a27/ghc" 8ed8b03/ghc]:
{{{
#!CommitTicketReference repository="ghc"
revision="8ed8b037fee9611b1c4ef49adb6cf50bbd929a27"
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
}}}
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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15178#comment:4
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