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| author | Ryan Scott <ryan.gl.scott@gmail.com> | 2021-04-13 18:17:19 -0400 |
|---|---|---|
| committer | Marge Bot <ben+marge-bot@smart-cactus.org> | 2021-04-29 17:27:54 -0400 |
| commit | 5981ac7d83810d26d74a07bbc452a0be278de66c (patch) | |
| tree | 6681ca9ff678352669f763666e7734ed22143fc6 /docs | |
| parent | 7d18e1bace3f3a85eae177654690d91b688c0e8f (diff) | |
| download | haskell-5981ac7d83810d26d74a07bbc452a0be278de66c.tar.gz | |
Redesign withDict (formerly magicDict)
This gives a more precise type signature to `magicDict` as proposed in #16646.
In addition, this replaces the constant-folding rule for `magicDict` in
`GHC.Core.Opt.ConstantFold` with a special case in the desugarer in
`GHC.HsToCore.Expr.dsHsWrapped`. I have also renamed `magicDict` to `withDict`
in light of the discussion in
https://mail.haskell.org/pipermail/ghc-devs/2021-April/019833.html.
All of this has the following benefits:
* `withDict` is now more type safe than before. Moreover, if a user applies
`withDict` at an incorrect type, the special-casing in `dsHsWrapped` will
now throw an error message indicating what the user did incorrectly.
* `withDict` can now work with classes that have multiple type arguments, such
as `Typeable @k a`. This means that `Data.Typeable.Internal.withTypeable` can
now be implemented in terms of `withDict`.
* Since the special-casing for `withDict` no longer needs to match on the
structure of the expression passed as an argument to `withDict`, it no
longer cares about the presence or absence of `Tick`s. In effect, this
obsoletes the fix for #19667.
The new `T16646` test case demonstrates the new version of `withDict` in
action, both in terms of `base` functions defined in terms of `withDict`
as well as in terms of functions from the `reflection` and `singletons`
libraries. The `T16646Fail` test case demonstrates the error message that GHC
throws when `withDict` is applied incorrectly.
This fixes #16646. By adding more tests for `withDict`, this also
fixes #19673 as a side effect.
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/users_guide/9.4.1-notes.rst | 23 | ||||
| -rw-r--r-- | docs/users_guide/release-notes.rst | 1 |
2 files changed, 24 insertions, 0 deletions
diff --git a/docs/users_guide/9.4.1-notes.rst b/docs/users_guide/9.4.1-notes.rst new file mode 100644 index 0000000000..8d92d57eec --- /dev/null +++ b/docs/users_guide/9.4.1-notes.rst @@ -0,0 +1,23 @@ +.. _release-9-4-1: + +Version 9.4.1 +============== + +``base`` library +~~~~~~~~~~~~~~~~ + +- ``GHC.Exts.magicDict`` has been renamed to ``withDict`` and given a more + specific type: :: + + withDict :: forall {rr :: RuntimeRep} st dt (r :: TYPE rr). st -> (dt => r) -> r + + Unlike ``magicDict``, ``withDict`` can be used without defining an + intermediate data type. For example, the ``withTypeable`` function from the + ``Data.Typeable`` module can now be defined as: :: + + withTypeable :: forall k (a :: k) rep (r :: TYPE rep). () + => TypeRep a -> (Typeable a => r) -> r + withTypeable rep k = withDict @(TypeRep a) @(Typeable a) rep k + + Note that the explicit type applications are required, as the call to + ``withDict`` would be ambiguous otherwise. diff --git a/docs/users_guide/release-notes.rst b/docs/users_guide/release-notes.rst index 92d4277a67..8b1313d8fe 100644 --- a/docs/users_guide/release-notes.rst +++ b/docs/users_guide/release-notes.rst @@ -6,3 +6,4 @@ Release notes 9.0.1-notes 9.2.1-notes + 9.4.1-notes |