Re: Self-awareness of functions?

[Lorenzo Donati <lorenzodonatibz@...>]

Dirk Laurie wrote:
> On Wed, Dec 29, 2010 at 09:30:28PM +0200, Lorenzo Donati wrote:
> > Dirk Laurie wrote:
> > > On Wed, Dec 29, 2010 at 09:22:36AM +0200, Dirk Laurie wrote:
> > > > Because of the need for a local name for the function, you can't define
> > > > an anonymous recursive function, e.g. for use in an argument list.  
> > Wouldn't this idiom suffice?
> >
> > fac = (function()
> > 	local function fact(x) if x<2 then return 1 else return x*fact(x-1) end end
> > 	return fact
> > end)()
> > fact = "Napoleon escaped from Elba."
> > print(fac(5)) --> 120
>
> It took me a minute to figure out why it works, but many idioms are 
> like that.
>
> It's got this over the "Y constructor" that 'fact' remains a function
> of one argument.  But is it easier to read?
>
> Dirk
Well, probably its main advantage is that it is a fairly "standard" Lua 
idiom (and quite a general one) to turn a statement into an expression .

As for its readability, it could be improved by suitable 
indenting/spacing, but _it is_ verbose indeed.

After a while one gets accustomed to spot it if it is suitably indented:


fac = (function() 	-- begin of closure factory
   local function fact(x)
      if x<2 then return 1 else return x*fact(x-1) end
   end
   return fact		-- returns the closure
end)()	-- end of closure factory + closure creation


of course it depends on the context whether it is readable enough.


Other typical uses:
local hasNormallyEnded, result1_or_errMsg, result2 = 	-- add more 
results if necessary
   pcall( function() 	-- 'try'
         -- Protected block ('try' block).
         -- If the protected block terminates normally,
         -- all its results are returned from pcall (result1, result2, 
etc.).
      end
   )
if not hasNormallyEnded then	-- 'catch'
   -- error handler ('catch' block)
end


Regards

--
Lorenzo