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authorRyan Scott <ryan.gl.scott@gmail.com>2021-03-05 19:43:23 -0500
committerRyan Scott <ryan.gl.scott@gmail.com>2021-03-05 19:45:25 -0500
commit7ee44d589a94c9cf7d352cab9424f2e78504f0ee (patch)
tree0a6cda5819e65693dd3d6e2d8e2326e63d3136ab
parent31e265c1df948d1bcc82d08affe995fd1d1c1438 (diff)
downloadhaskell-wip/T19457.tar.gz
Document operator sections' interaction with subsumptionwip/T19457
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.rst15
-rw-r--r--docs/users_guide/exts/rank_polymorphism.rst4
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 5e0bf1c317..521427b4d9 100644
--- a/docs/users_guide/9.2.1-notes.rst
+++ b/docs/users_guide/9.2.1-notes.rst
@@ -65,6 +65,21 @@ 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".