Re: Syntactic sugar for sets

[Asko Kauppi <askok@...>]

With the same reasons, there's bound to be an avalanche of "within the  
spirit of Lua" suggestions.

Why not just make a luaSub or MetaLua syntax mod out of it, and the  
ones alike?  Portability, you may say. There's no way all suggestions  
will end up in core, and that's not necessarily a bad thing. In fact,  
I'd see many _existing_ syntax sugar be removable from the Lua core,  
and added as syntax front end features instead. This would allow  
explicitly stating when they are needed to be used (s.a. ":" class  
mechanism, which is arguable and has some unwanted side effects).

This is how you'd make the below syntax work in luaSub:

<<
local function NO_EQUALS
    p:replace( -0,0, "=", "true" )	-- appends to the scope
end

--
--  .field = alt{ ["["]= { exp,"]","=",exp },
--                  -->
--                ["["]= { exp, "]", alt{ ["="]= exp },
--                                        NO_EQUALS },
--                { "<name>", "=", exp },   -- must not be made keyed  
since
--                exp                       -- 'exp' may also start  
with "<name>"
--              }
--
return function( syntax )

    local t= syntax.field
    assert( t[0]=="alt" )   -- just checking

    local exp= ref"exp"

    t["["]= { exp, "]", alt{ ["="]= exp,
                             NO_EQUALS } }
end
<<

Then you'd save this file in -say- 'lookup.luas' and refer to using it  
in your files by starting them with "#!luas -slookup".

Over and out.

-asko



John Hind kirjoitti 24.2.2008 kello 14:24:

> About a year ago, Peter Jacobi suggested a syntactic sugar for sets:
>
> planet = {["mercury"], ["venus"], ["earth"]} -- In Lua 5.1, Gives  
> Error '='
> expected
>
> Would be compiled as:
>
> planet = {["mercury"]=true, ["venus"]=true, ["earth"]=true}
>
> http://lua-users.org/lists/lua-l/2007-03/msg00767.html
>
> Of course, Peter's syntax could be further simplified to:
>
> planet = {[mercury], [venus], [earth]} -- In Lua 5.1, Gives Error '='
> expected
>
> Then you could write:
>
> local input = "pluto"
>
> if planet[input] then
> 	print(input, " is a planet")
> else
> 	print(input, " is not a planet")
> end
>
> I think this is a really nice idiom and entirely within the spirit  
> of Lua.
> It expresses sets using the standard table mechanism + syntax sugar in
> exactly the same spirit that arrays and objects are implemented in  
> Lua.
>
> Of course, responses focussed on pointing out that you can already  
> do this
> and various ways of expressing it within the existing language. But  
> that's
> not the point. Having it in the basic language definition makes Lua  
> better
> and more expressive at negligible cost. It also allows mixed idioms  
> in data
> description that are difficult to implement generically using factory
> functions:
>
> local mark = {name="Mark Jones", age=34, [male]}
>
> I definitely vote this excellent suggestion be seriously considered  
> for a
> future version of Lua!
>
> - John Hind