Re: Forward References - a Useful Pattern

[Majic <majic.one@...>]

Very nice, good post :)

On Wed, Feb 17, 2010 at 1:31 AM, steve donovan
<steve.j.donovan@gmail.com> wrote:
> Hi all,
>
> This hack du jour is for when you need forward references to a set of functions.
>
> E.g.
>
> mod = Forward()
>
> funs = {mod.copy, mod.print, mod.eval}
>
> function mod.print()
>        print 'printing!'
> end
>
> function mod._not_implemented (name)
>        print(name..' is not implemented')
> end
>
> for _,f in ipairs(funs) do f() end
>
> ==>
> copy is not implemented
> printing!
> eval is not implemented
>
> This is useful if say you are mocking up a user interface and want to
> fill in all the functions; it also makes it easy to automatically
> check whether all references have been satisfied.
>
> This is done by making the __index function of our table create
> references; when these are called, they forward to the actual
> function, if available, otherwise produce a result which can be
> customized by defining _not_implemented.
>
> function Forward ()
>        return setmetatable({},forward_MT)
> end
>
> forward_MT = {
>        __index = function(mod,name)
>                local ref = forward_ref(mod,name)
>                mod[name] = ref
>                return ref
>        end
> }
>
> function forward_ref (mod,name)
>        return setmetatable({mod=mod,name=name},forward_ref_MT)
> end
>
>
> forward_ref_MT = {
>        __call = function(ref,...)
>                local fun = rawget(ref.mod,ref.name)
>                if type(fun) == 'function' then -- forward the call to the implementation
>                        fun(...)
>                elseif rawget(mod,'_not_implemented') then -- wants to handle a miss...
>                        mod._not_implemented(ref.name)
>                else
>                        error("'"..ref.name .. "' is not implemented",2)
>                end
>        end
> }
>
>
> steve d.
>