diff options
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/users_guide/9.4.1-notes.rst | 6 | ||||
| -rw-r--r-- | docs/users_guide/exts/empty_case.rst | 5 | ||||
| -rw-r--r-- | docs/users_guide/exts/lambda_case.rst | 19 |
3 files changed, 25 insertions, 5 deletions
diff --git a/docs/users_guide/9.4.1-notes.rst b/docs/users_guide/9.4.1-notes.rst index 39ab943356..0bb8b21eee 100644 --- a/docs/users_guide/9.4.1-notes.rst +++ b/docs/users_guide/9.4.1-notes.rst @@ -41,6 +41,12 @@ Language re-exported from ``Prelude``. When ``(~)`` is not in scope, its use results in a warning (:ghc-flag:`-Wtype-equality-out-of-scope`). +- GHC Proposal `#302 <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0302-cases.rst>`_ has been implemented. + This means under ``-XLambdaCase``, a new expression heralded by ``\cases`` is + available, which works like ``\case`` but can match on multiple patterns. + This means constructor patterns with arguments have to parenthesized here, + just like in lambda expressions. + - There were previously cases around functional dependencies and injective type families where the result of type inference would depend on the order of constraints, as written in a source file. These cases are fundamentally ambiguous. diff --git a/docs/users_guide/exts/empty_case.rst b/docs/users_guide/exts/empty_case.rst index c42de22e9e..7a32f609e6 100644 --- a/docs/users_guide/exts/empty_case.rst +++ b/docs/users_guide/exts/empty_case.rst @@ -19,6 +19,9 @@ or :: \case { } -- -XLambdaCase is also required +Note that it is not allowed for ``\cases``, since it would be unclear how many +patterns are being matched. + This can be useful when you know that the expression being scrutinised has no non-bottom values. For example: @@ -45,5 +48,3 @@ We much prefer (B). Why? Because GHC can figure out that is able to compile with :ghc-flag:`-Wincomplete-patterns` and :ghc-flag:`-Werror`. On the other hand (A) looks dangerous, and GHC doesn't check to make sure that, in fact, the function can never get called. - - diff --git a/docs/users_guide/exts/lambda_case.rst b/docs/users_guide/exts/lambda_case.rst index 74bc84e164..6872101698 100644 --- a/docs/users_guide/exts/lambda_case.rst +++ b/docs/users_guide/exts/lambda_case.rst @@ -18,7 +18,20 @@ which is equivalent to :: \freshName -> case freshName of { p1 -> e1; ...; pN -> eN } -Note that ``\case`` starts a layout, so you can write :: +Since GHC 9.4.1, it also allow expressions with multiple scrutinees (see GHC +proposal `#302 <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0302-cases.rst>`_) +of the form :: + + \cases { p11 ... pM1 -> e1; ...; p1N ... pMN -> eN } + +which is equivalent to a function defined as + + f p11 ... pM1 -> e1 + ... + f p1N ... pMN -> eN + + +Note that both ``\case`` and ``\cases`` start a layout, so you can write :: \case p1 -> e1 @@ -26,8 +39,8 @@ Note that ``\case`` starts a layout, so you can write :: pN -> eN Additionally, since GHC 9.0.1, combining :extension:`LambdaCase` with -:extension:`Arrows` allows ``\case`` syntax to be used as a command in -``proc`` notation: :: +:extension:`Arrows` allows ``\case`` (and since GHC 9.4.1 ``\cases``) +syntax to be used as a command in ``proc`` notation: :: proc x -> (f -< x) `catchA` \case p1 -> cmd1 |