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| author | Ryan Scott <ryan.gl.scott@gmail.com> | 2021-03-05 19:43:23 -0500 |
|---|---|---|
| committer | Marge Bot <ben+marge-bot@smart-cactus.org> | 2021-03-09 02:46:56 -0500 |
| commit | 376427ece3a6206b35a13837ec14c86673eb0fe4 (patch) | |
| tree | 04130654858b09d882067862a35744595675b248 | |
| parent | 0a709dd9876e40c19c934692415c437ac434318c (diff) | |
| download | haskell-376427ece3a6206b35a13837ec14c86673eb0fe4.tar.gz | |
Document operator sections' interaction with subsumption
This resolves #19457 by making a note of breaking changes (introduced in
GHC 9.2) to the way that GHC typechecks operator sections where the operator
has nested `forall`s or contexts in its type signature.
| -rw-r--r-- | docs/users_guide/9.2.1-notes.rst | 15 | ||||
| -rw-r--r-- | docs/users_guide/exts/rank_polymorphism.rst | 4 |
2 files changed, 19 insertions, 0 deletions
diff --git a/docs/users_guide/9.2.1-notes.rst b/docs/users_guide/9.2.1-notes.rst index 515dd4bbb2..de4a983001 100644 --- a/docs/users_guide/9.2.1-notes.rst +++ b/docs/users_guide/9.2.1-notes.rst @@ -71,6 +71,21 @@ Language record pattern synonyms. In particular, it is possible for a single module to define multiple pattern synonyms using the same field names. +* Because of simplifications to the way that GHC typechecks operator sections, + operators with nested ``forall``\ s or contexts in their type signatures might + not typecheck when used in a section. For instance, the ``g`` function below, + which was accepted in previous GHC releases, will no longer typecheck: :: + + f :: a -> forall b. b -> a + f x _ = x + + g :: a -> a + g = (`f` "hello") + + ``g`` can be made to typecheck once more by eta expanding it to + ``\x -> x \`f\` "hello"``. For more information, see + :ref:`simple-subsumption`. + Compiler ~~~~~~~~ diff --git a/docs/users_guide/exts/rank_polymorphism.rst b/docs/users_guide/exts/rank_polymorphism.rst index 648ce40a70..8e9f55dd5b 100644 --- a/docs/users_guide/exts/rank_polymorphism.rst +++ b/docs/users_guide/exts/rank_polymorphism.rst @@ -195,6 +195,10 @@ Similarly ``f4 g4`` is not well typed, because the constraints appear in a diffe These examples can be made to typecheck by eta-expansion. For example ``f3 (\x -> g3b x)`` is well typed, and similarly ``f3 (\x -> g3c x)`` and ``f4 (\x -> g4 x)``. +A similar phenomenon occurs for operator sections. For example, +``(\`g3a\` "hello")`` is not well typed, but it can be made to typecheck by eta +expanding it to ``\x -> x \`g3a\` "hello"``. + Historical note. Earlier versions of GHC allowed these now-rejected applications, by inserting automatic eta-expansions, as described in Section 4.6 of `Practical type inference for arbitrary-aank types <https://www.microsoft.com/en-us/research/publication/practical-type-inference-for-arbitrary-rank-types/>`__, where it is called "deep skolemisation". |