| |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| |
Summary:
After 8136a5cbfcd24647f897a2fae9fcbda0b1624035 (#11450), if you have
a class with an associated type:
```
class C a where
type T a b
```
And you try to create an instance of `C` like this:
```
instance C Int where
type T Int Char = Bool
```
Then it is rejected, since you're trying to instantiate the variable ``b`` with
something other than a type variable. But this restriction proves quite
onerous in practice, as it prevents you from doing things like this:
```
class C a where
type T a (b :: Identity c) :: c
instance C Int where
type T Int ('Identity x) = x
```
You have to resort to an auxiliary type family in order to define this now,
which becomes extremely tiring. This lifts this restriction and fixes #13398,
in which it was discovered that adding this restriction broke code in the wild.
Test Plan: ./validate
Reviewers: simonpj, bgamari, austin
Reviewed By: simonpj
Subscribers: rwbarton, thomie
Differential Revision: https://phabricator.haskell.org/D3302
|
| |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| |
This patch finishes off Trac #11450. Following debate on that ticket,
the patch tightens up the rules for what the instances of an
associated type can look like. Now they must match the instance
header exactly. Eg
class C a b where
type T a x b
With this class decl, if we have an instance decl
instance C ty1 ty2 where ...
then the type instance must look like
type T ty1 v ty2 = ...
with exactly
- 'ty1' for 'a'
- 'ty2' for 'b', and
- a variable for 'x'
For example:
instance C [p] Int
type T [p] y Int = (p,y,y)
Previously we allowed multiple instance equations and now, in effect,
we don't since they would all overlap. If you want multiple cases,
use an auxiliary type family.
This is consistent with the treatment of generic-default instances,
and the user manual always said "WARNING: this facility (multiple
instance equations may be withdrawn in the future".
I also improved error messages, and did other minor refactoring.
|
