RE: Iteration and nils

["Jerome Vuarand" <jerome.vuarand@...>]

Luis Carvalho wrote:
>> The recent thread about lists and nil got me thinking about the ways
>> in which nil isn't a first-class data type in Lua.  Of course, the
>> focus of that thread was on sparse arrays and vararg lists.  I've
>> been thinking about another limitation of nil -- that the generic
>> for loop doesn't allow an iterator to provide nil as a value.
> 
> <snip>
> 
> Another approach -- already mentioned several times here -- is to use
> tuples. 
> You could then pass and return tuples to/from vararg functions. It
> would be easy to iterate over them (similarly to ipairs but checking
> for the number of arguments instead of a nil value), and the
> iteration would respect nils. Moreover, tuples can be implemented as
> closures and no change to the language would be necessary (a new
> datatype is a big change, btw).    

I thought that a new iterator on ... would solve the problem without
having to modify the language. I found out while looking for 'apairs'
that it was proposed last year by Mark Hamburg [1], with the name
varargs(), Mike Pall [2] proposing a more elegant apairs() name.

Unfortunately neither of them proposed an implementation. Since it's
trivial here is a simple (tested) implementation:

function apairs(...)
  local n = select('#', ...)
  return function(a, i)
    if i < n then
      return i+1,a[i+1]
    end
  end, {...}, 0 
end

function f(...)
  for i,a in apairs(...) do
    print(i, a)
  end
end

f("foo", nil, 32, nil)

[1] http://lua-users.org/lists/lua-l/2006-04/msg00108.html
[2] http://lua-users.org/lists/lua-l/2006-04/msg00114.html