Re: ipairs in Lua 5.3.0-alpha

[Jan Behrens <jbe-lua-l@...>]

On Mon, 18 Aug 2014 19:28:29 +0200
Dirk Laurie <dirk.laurie@gmail.com> wrote:

> 2014-08-18 16:35 GMT+02:00 Jan Behrens <jbe-lua-l@public-software-group.org>:
> 
> > This just got me to the idea that if ipairs is made to accept
> > functions (or even iterator triplets), we could also implement
> > a generic concat:
> >
> > ==================================================
> > function string.concat(sep, ...)
> >   local t = {}
> >   for i, v in ipairs(...) do
> >     t[i] = tostring(v)
> >   end
> >   return table.concat(t, sep)
> > end
> > ==================================================
> ...
> > Can anyone come up with a real con?
> 
> Only that this was proposed two years ago [1] but the Lua team did not bite.
> 
> [1] http://lua-users.org/lists/lua-l/2012-07/msg00220.htm
> 

I assume you are referring to:
http://lua-users.org/lists/lua-l/2012-07/msg00220.html ("l" was missing)

My proposal goes beyond applying "tostring" to every value. That would
just be some extra-convenience. My main proposal is to allow ipairs to
be used as a general interface for (ordinal) iteration. Consider this
alternate form of string.concat:

==================================================
function string.concat(sep, ...)
  local t = {}
  for i, v in ipairs(...) do
    t[i] = v
  end
  return table.concat(t, sep)
end
==================================================


Here, there is no call to "tostring". So that's not my point. My
point is:

If ipairs(...) accepted a function or an iterator triplet (which it
currently doesn't do in Lua 5.3-alpha), then the following code
would work:

do
  local letter = nil
  local function alphabet()  -- some example iterator
    if letter == nil then
      letter = "a"
    elseif letter == "z" then
      return nil
    else
      letter = string.char(string.byte(letter) + 1)
    end
    return letter
  end
  print(string.concat(",", alphabet))
  -- prints:
  -- a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z
end

print(string.concat(",", {"a", "b", "c"}))
-- prints:
-- a,b,c


The same would work for file:lines() and other iterators.


Regards
Jan