Re: Lua 5.2 custom array demo
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- Subject: Re: Lua 5.2 custom array demo
- From: David Manura <dm.lua@...>
- Date: Sat, 9 Jan 2010 11:27:10 -0500
On Sat, Jan 9, 2010 at 8:25 AM, steve donovan wrote:
> local array = {}
> local rawset,rawget,ipairs = rawset,rawget,ipairs
> in array do
> function __len(t) return #t.store end
> function __newindex(t,i,val) rawset(t.store,i,val) end
> function __index(t,i) return rawget(t.store,i) end
> function __ipairs(t) return ipairs(t.store) end
> end
> function new_array ()
> return setmetatable({store={}},array)
> end
> t = new_array()
Concerning this use case, it could be written more compactly as
local array = {}
in class(array) do
function __len(t) return #t.store end
function __newindex(t,i,val) rawset(t.store,i,val) end
function __index(t,i) return rawget(t.store,i) end
function __ipairs(t) return ipairs(t.store) end
function __init() return object{store={}} end
end
t = array()
But the following approach in Lua 5.1 is conceptually more flexible
(e.g. what if array is userdata?):
local array = class(function()
function __len(t) return #t.store end
function __newindex(t,i,val) rawset(t.store,i,val) end
function __index(t,i) return rawget(t.store,i) end
function __ipairs(t) return ipairs(t.store) end
function __init() return object{store={}} end
end)
t = array()
or if you like to avoid globals, we can introduce lexicals as function
arguments:
local array = class(function(s)
function s.__len(t) return #t.store end
function s.__newindex(t,i,val) rawset(t.store,i,val) end
function s.__index(t,i) return rawget(t.store,i) end
function s.__ipairs(t) return ipairs(t.store) end
function s.__init() return setmetatable({store={}},array) end
end)
t = array()
The "end)" syntax was always a little odd, but there are ways it could
be avoided:
local array = class ::
function __len(t) return #t.store end
function __newindex(t,i,val) rawset(t.store,i,val) end
function __index(t,i) return rawget(t.store,i) end
function __ipairs(t) return ipairs(t.store) end
function __init() return setmetatable({store={}},array) end
end
where "::" is basically like a "function()" but allows omitting
parenthesis in the call.