[commit: ghc] master: Update docs for partial type signatures (#12365) (627c767)
git at git.haskell.org
git at git.haskell.org
Wed Jul 20 13:18:13 UTC 2016
Repository : ssh://git@git.haskell.org/ghc
On branch : master
Link : http://ghc.haskell.org/trac/ghc/changeset/627c767b8e5587de52086d8891d7f7aabf6fa49f/ghc
>---------------------------------------------------------------
commit 627c767b8e5587de52086d8891d7f7aabf6fa49f
Author: Thomas Winant <thomas.winant at cs.kuleuven.be>
Date: Wed Jul 20 09:57:04 2016 +0200
Update docs for partial type signatures (#12365)
* Update the sample error messages. The messages have been reworded and
reformatted since GHC 7.10.
* Mention `TypeApplications` in "Where can they occur?"
* The name of a named wild card is no longer used in the name of a
resulting type variable. Before: `_foo` => `w_foo`, now: `_foo` => `t`
or `a`.
Test Plan: generate the users guide
Reviewers: goldfire, austin, bgamari
Reviewed By: bgamari
Subscribers: thomie
Differential Revision: https://phabricator.haskell.org/D2413
GHC Trac Issues: #12365
>---------------------------------------------------------------
627c767b8e5587de52086d8891d7f7aabf6fa49f
docs/users_guide/glasgow_exts.rst | 79 ++++++++++++++++++++++++---------------
1 file changed, 48 insertions(+), 31 deletions(-)
diff --git a/docs/users_guide/glasgow_exts.rst b/docs/users_guide/glasgow_exts.rst
index 8a35899..56bf3f8 100644
--- a/docs/users_guide/glasgow_exts.rst
+++ b/docs/users_guide/glasgow_exts.rst
@@ -9522,22 +9522,25 @@ types like ``(Int -> Bool)`` or ``Maybe``.
For instance, the first wildcard in the type signature ``not'`` would
produce the following error message:
-::
+.. code-block:: none
+
+ Test.hs:4:17: error:
+ • Found type wildcard ‘_’ standing for ‘Bool’
+ To use the inferred type, enable PartialTypeSignatures
+ • In the type signature:
+ not' :: Bool -> _
+ • Relevant bindings include
+ not' :: Bool -> Bool (bound at Test.hs:5:1)
- Test.hs:4:17:
- Found hole ‘_’ with type: Bool
- To use the inferred type, enable PartialTypeSignatures
- In the type signature for ‘not'’: Bool -> _
When a wildcard is not instantiated to a monotype, it will be
-generalised over, i.e. replaced by a fresh type variable (of which the
-name will often start with ``w_``), e.g.
+generalised over, i.e. replaced by a fresh type variable, e.g.
::
foo :: _ -> _
foo x = x
- -- Inferred: forall w_. w_ -> w_
+ -- Inferred: forall t. t -> t
filter' :: _
filter' = filter -- has type forall a. (a -> Bool) -> [a] -> [a]
@@ -9571,7 +9574,7 @@ of the type signature to make sure that it unifies with something: ::
somethingShowable :: Show _x => _x -> _
somethingShowable x = show x
- -- Inferred type: Show w_x => w_x -> String
+ -- Inferred type: Show a => a -> String
somethingShowable' :: Show _x => _x -> _
somethingShowable' x = show (not x)
@@ -9586,7 +9589,7 @@ though syntactically similar, named wildcards can unify with monotypes
as well as be generalised over (and behave as type variables).
In the first example above, ``_x`` is generalised over (and is
-effectively replaced by a fresh type variable ``w_x``). In the second
+effectively replaced by a fresh type variable ``a``). In the second
example, ``_x`` is unified with the ``Bool`` type, and as ``Bool``
implements the ``Show`` type class, the constraint ``Show Bool`` can be
simplified away.
@@ -9605,23 +9608,29 @@ no matching the actual type ``Bool``.
.. code-block:: none
- Test.hs:5:9:
- Couldn't match expected type ‘_a’ with actual type ‘Bool’
+ Test.hs:5:9: error:
+ • Couldn't match expected type ‘_a’ with actual type ‘Bool’
‘_a’ is a rigid type variable bound by
- the type signature for foo :: _a -> _a at Test.hs:4:8
- Relevant bindings include foo :: _a -> _a (bound at Test.hs:4:1)
- In the expression: False
- In an equation for ‘foo’: foo _ = False
+ the type signature for:
+ foo :: forall _a. _a -> _a
+ at Test.hs:4:8
+ • In the expression: False
+ In an equation for ‘foo’: foo _ = False
+ • Relevant bindings include foo :: _a -> _a (bound at Test.hs:5:1)
-Compiling this program with :ghc-flag:`-XNamedWildCards` enabled produces the
-following error message reporting the inferred type of the named
-wildcard ``_a``.
+Compiling this program with :ghc-flag:`-XNamedWildCards` (as well as
+:ghc-flag:`-XPartialTypeSignatures`) enabled produces the following error
+message reporting the inferred type of the named wildcard ``_a``.
.. code-block:: none
- Test.hs:4:8: Warning:
- Found hole ‘_a’ with type: Bool
- In the type signature for ‘foo’: _a -> _a
+ Test.hs:4:8: warning: [-Wpartial-type-signatures]
+ • Found type wildcard ‘_a’ standing for ‘Bool’
+ • In the type signature:
+ foo :: _a -> _a
+ • Relevant bindings include
+ foo :: Bool -> Bool (bound at Test.hs:5:1)
+
.. _extra-constraints-wildcard:
@@ -9641,10 +9650,11 @@ extra-constraints wildcard is used to infer three extra constraints.
-- Inferred:
-- forall a. (Enum a, Eq a, Show a) => a -> String
-- Error:
- Test.hs:5:12:
- Found hole ‘_’ with inferred constraints: (Enum a, Eq a, Show a)
+ Test.hs:5:12: error:
+ Found constraint wildcard ‘_’ standing for ‘(Show a, Eq a, Enum a)’
To use the inferred type, enable PartialTypeSignatures
- In the type signature for ‘arbitCs’: _ => a -> String
+ In the type signature:
+ arbitCs :: _ => a -> String
An extra-constraints wildcard shouldn't prevent the programmer from
already listing the constraints he knows or wants to annotate, e.g.
@@ -9657,10 +9667,11 @@ already listing the constraints he knows or wants to annotate, e.g.
-- Inferred:
-- forall a. (Enum a, Show a, Eq a) => a -> String
-- Error:
- Test.hs:9:22:
- Found hole ‘_’ with inferred constraints: (Eq a, Show a)
+ Test.hs:9:22: error:
+ Found constraint wildcard ‘_’ standing for ‘()’
To use the inferred type, enable PartialTypeSignatures
- In the type signature for ‘arbitCs'’: (Enum a, _) => a -> String
+ In the type signature:
+ arbitCs' :: (Enum a, _) => a -> String
An extra-constraints wildcard can also lead to zero extra constraints to
be inferred, e.g.
@@ -9671,10 +9682,11 @@ be inferred, e.g.
noCs = "noCs"
-- Inferred: String
-- Error:
- Test.hs:13:9:
- Found hole ‘_’ with inferred constraints: ()
+ Test.hs:13:9: error:
+ Found constraint wildcard ‘_’ standing for ‘()’
To use the inferred type, enable PartialTypeSignatures
- In the type signature for ‘noCs’: _ => String
+ In the type signature:
+ noCs :: _ => String
As a single extra-constraints wildcard is enough to infer any number of
constraints, only one is allowed in a type signature and it should come
@@ -9702,6 +9714,11 @@ Anonymous and named wildcards *can* occur on the left hand side of a
type or data instance declaration;
see :ref:`type-wildcards-lhs`.
+Anonymous wildcards are also allowed in visible type applications
+(:ref:`visible-type-application`). If you want to specify only the second type
+argument to ``wurble``, then you can say ``wurble @_ @Int`` where the first
+argument is a wildcard.
+
In all other contexts, type wildcards are disallowed, and a named wildcard is treated
as an ordinary type variable. For example: ::
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