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Due to the way `DerivEnv` is currently structured, there is an
invariant that every derived instance must consist of a class applied
to a non-empty list of argument types, where the last argument *must*
be an application of a type constructor to some arguments. This works
for many cases, but there are also some design patterns in standalone
`anyclass`/`via` deriving that are made impossible due to enforcing
this invariant, as documented in #13154.
This fixes #13154 by refactoring `TcDeriv` and friends to perform
fewer validity checks when using the `anyclass` or `via` strategies.
The highlights are as followed:
* Five fields of `DerivEnv` have been factored out into a new
`DerivInstTys` data type. These fields only make sense for
instances that satisfy the invariant mentioned above, so
`DerivInstTys` is now only used in `stock` and `newtype` deriving,
but not in other deriving strategies.
* There is now a `Note [DerivEnv and DerivSpecMechanism]` describing
the bullet point above in more detail, as well as explaining the
exact requirements that each deriving strategy imposes.
* I've refactored `mkEqnHelp`'s call graph to be slightly less
complicated. Instead of the previous `mkDataTypeEqn`/`mkNewTypeEqn`
dichotomy, there is now a single entrypoint `mk_eqn`.
* Various bits of code were tweaked so as not to use fields that are
specific to `DerivInstTys` so that they may be used by all deriving
strategies, since not all deriving strategies use `DerivInstTys`.
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Test Plan: Corrected a few tests to include the new message.
Reviewers: goldfire, austin, bgamari
Reviewed By: bgamari
Subscribers: goldfire, thomie
Differential Revision: https://phabricator.haskell.org/D2484
GHC Trac Issues: #12546
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This implements the ideas originally put forward in
"System FC with Explicit Kind Equality" (ICFP'13).
There are several noteworthy changes with this patch:
* We now have casts in types. These change the kind
of a type. See new constructor `CastTy`.
* All types and all constructors can be promoted.
This includes GADT constructors. GADT pattern matches
take place in type family equations. In Core,
types can now be applied to coercions via the
`CoercionTy` constructor.
* Coercions can now be heterogeneous, relating types
of different kinds. A coercion proving `t1 :: k1 ~ t2 :: k2`
proves both that `t1` and `t2` are the same and also that
`k1` and `k2` are the same.
* The `Coercion` type has been significantly enhanced.
The documentation in `docs/core-spec/core-spec.pdf` reflects
the new reality.
* The type of `*` is now `*`. No more `BOX`.
* Users can write explicit kind variables in their code,
anywhere they can write type variables. For backward compatibility,
automatic inference of kind-variable binding is still permitted.
* The new extension `TypeInType` turns on the new user-facing
features.
* Type families and synonyms are now promoted to kinds. This causes
trouble with parsing `*`, leading to the somewhat awkward new
`HsAppsTy` constructor for `HsType`. This is dispatched with in
the renamer, where the kind `*` can be told apart from a
type-level multiplication operator. Without `-XTypeInType` the
old behavior persists. With `-XTypeInType`, you need to import
`Data.Kind` to get `*`, also known as `Type`.
* The kind-checking algorithms in TcHsType have been significantly
rewritten to allow for enhanced kinds.
* The new features are still quite experimental and may be in flux.
* TODO: Several open tickets: #11195, #11196, #11197, #11198, #11203.
* TODO: Update user manual.
Tickets addressed: #9017, #9173, #7961, #10524, #8566, #11142.
Updates Haddock submodule.
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This patch makes the code a bit simpler if anything.
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This matches GCC's choice of Unicode quotation marks (i.e. U+2018 and U+2019)
and therefore looks more familiar on the console. This addresses #2507.
Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
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