Add LambdaCase proposal

texts/0000-lambdacase.rst

@@ -0,0 +1,116 @@
+- Feature Name: LambdaCase
+- Start Date: 2016-07-23
+- RFC PR:
+- Haskell Report Issue:
+
+
+
+#######
+Summary
+#######
+
+For convenience, add the shorthand
+
+.. code-block:: haskell
+
+    \case            >>>   \x -> case x of
+        Foo a -> …   >>>       Foo a -> …
+        …            >>>       …
+
+where ``x`` is a fresh variable.
+
+This is currently implemented via the ``LambdaCase`` GHC extension.
+
+
+
+##########
+Motivation
+##########
+
+
+``LambdaCase`` is a syntactic convenience feature whose sole purpose is making
+code a bit cleaner.
+
+
+Example 1: Multiple pattern matches
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The following example repeats the function name multiple times:
+
+.. code-block:: haskell
+
+    myFunction (Foo (Just a)) = …
+    myFunction (Bar b)        = …
+    myFunction (Qux (a,b))    = …
+
+Using lambda case, this could be refactored to
+
+.. code-block:: haskell
+
+    myFunction = \case
+        Foo (Just a) -> …
+        Bar b        -> …
+        Qux (a,b)    -> …
+
+This lacks one level of parentheses, indents the patterns to make the top-level
+definition stand out more, and avoids repeating the function name multiple
+times.
+
+
+Example 2: Monadic binding
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To pass a value through a monadic chain of binds, pattern matching on an
+intermediate result requires creating a new variable whose only purpose is to be
+scrutinized by a ``case`` again in the next step:
+
+.. code-block:: haskell
+
+    main = foo >>= \x -> case x of Just bar -> …
+                                   Nothing  -> …
+
+Lambda case would clean this up by getting rid of the intermediate variable,
+
+.. code-block:: haskell
+
+    main = foo >>= \case Just bar -> …
+                         Nothing  -> …
+
+
+
+###############
+Detailed design
+###############
+
+The Summary_ already describes this to sufficient detail.
+
+
+
+#########
+Drawbacks
+#########
+
+There are no technical drawbacks. Since ``case`` is a reserved keyword in
+Haskell, it cannot be used as a variable name. Therefore, in standard Haskell,
+``\case`` is invalid. Thus, this proposal gives meaning to a previously
+non-existing construct, resulting in no breakage.
+
+Feature-wise, it is worth noting that lambda case does not support lambdas with
+multiple arguments well. In this case, explicitly naming the `case` scrutinee,
+as is current (standard Haskell) practice, explicitly.
+
+
+
+############
+Alternatives
+############
+
+No alternatives tackling the same issue are known (to me).
+
+
+
+####################
+Unresolved questions
+####################
+
+(none)