Re: Forward References - a Useful Pattern

[spir <denis.spir@...>]

On Wed, 17 Feb 2010 11:31:13 +0200
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.

Hello,

I find this similar in principle to the case of recursive patterns in a grammar/parser: a pattern that needs to be refered to before it is actually defined. A usual trick is to build it as a symbolic reference (e.g. by name) to a yet-to-be-defined pattern. However it is done in practice, the binding to an actual pattern will be defined later.

Denis
________________________________

la vita e estrany

http://spir.wikidot.com/