== Concise Anonymous Functions ==

 *'''Backwards compatible:''' yes
 *'''Lines changed/added/deleted:''' 3/26/0
 *'''Lua authors' position:''' ?
 *'''Author:''' Brian Palmer (luausers [at] brian <dot> codekitchen (dot) net)
 *'''Last update:''' 2007-Jun-10
 * [http://svn.codekitchen.net/lua_patches/lambda.patch] (Patched against Lua 5.1.2)

Some syntactic sugar for writing more concise anonymous functions, very useful when
passing arguments to higher-order functions. Completely backwards compatible and only
modifies the parser, emits the same bytecode as the old syntax. As an example, the old
syntax (assuming you've defined a 'filter' method, standard in functional programming
style):

open_exits = location.exits:filter(function (e) return not e:is_locked() end)

With new syntax:

open_exits = location.exits:filter( [(e) | not e:is_locked()] )

The patch makes three changes:

 - Most importantly, anonymous functions can be defined between square [] brackets,
   taking advantage of the fact that there is no ambiguity between when you are indexing
   a table and when you are defining a function. The argument list can be left off, in
   which case the function is implied vararg:

function pass_through(f)
  return function(...) do_something_useful(...); return f(...) end
end

becomes:

function pass_through(f)
  return [ do_something_useful(...); | f(...) ]
end

 - The pipe | character is short-hand for the 'return' keyword. If you'd prefer not to
   have this behavior, maybe because you've installed a bitwise operator patch, simply
   remove the one line from the patch that defines the operator.

 - Lastly, 'then' becomes optional in 'if' constructs. In the current version of Lua,
   'if' statements can be parsed just as well without a 'then' keyword, however if you
   take advantage of this keep in mind that the Lua authors may change this is the
   future (of course this applies to any power patch).

Quick example:

if a > 5 then return 'hi' else return 'bye' end

now can be written:

if a > 5 | 'hi' else | 'bye' end

or:

if a > 5 return 'hi' else return 'bye' end

Of course, the | and 'then' keyword additions are available to all lua code, not just
code inside [] functions. | looks kind of weird at first, but it's grown on me quickly.
Here's an example from a recent project:

function describe_floor(location, objs, obj_locs)
 | objs :iselect(curry(is_at, location, obj_locs))
        :map([ (o) | "you see a "..o.." on the floor." ])
        :concat' '
end