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| author | Ryan Scott <ryan.gl.scott@gmail.com> | 2021-04-13 19:50:50 -0400 |
|---|---|---|
| committer | Ryan Scott <ryan.gl.scott@gmail.com> | 2021-04-14 08:41:32 -0400 |
| commit | 78369f354faf4667d576d9762c6a85dd753fc6da (patch) | |
| tree | f0980e72efb2898386b84dfb33c2a46763cf9843 /docs/users_guide | |
| parent | e63cdccabe0bfbd73a475c669404b9604cf859f5 (diff) | |
| download | haskell-wip/T16646-take-two.tar.gz | |
Redesign the type of magicDictwip/T16646-take-two
This gives a more precise type signature to `magicDict` as proposed in #16646.
In addition to making it more type safe:
* This allows the `magicDict` constant-folding rule to apply to classes with
multiple type arguments, such as `Typeable @k a`. This means that
`withTypeable` can now be implemented in terms of `magicDict`.
* Moreover, since the constant-folding rule no longer needs to case on the
structure of the expression passed as an argument to `magicDict`, 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 `magicDict` in
action, both in terms of `base` functions defined in terms of `magicDict`
as well as in terms of functions from the `reflection` and `singletons`
libraries.
This fixes #16646. By adding more tests for `magicDict`, this also
fixes #19673 as a side effect.
Diffstat (limited to 'docs/users_guide')
| -rw-r--r-- | docs/users_guide/9.4.1-notes.rst | 22 | ||||
| -rw-r--r-- | docs/users_guide/release-notes.rst | 1 |
2 files changed, 23 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..188593872e --- /dev/null +++ b/docs/users_guide/9.4.1-notes.rst @@ -0,0 +1,22 @@ +.. _release-9-4-1: + +Version 9.4.1 +============== + +``base`` library +~~~~~~~~~~~~~~~~ + +- ``GHC.Exts.magicDict`` now has the type: :: + + magicDict :: forall {rr :: RuntimeRep} dt st (r :: TYPE rr). (dt => r) -> st -> r + + ``magicDict`` can now 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 = magicDict @(Typeable a) k rep + + Note that the explicit type application is required, as the call to + ``magicDict`` 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 |