Re: params to __tostring

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

On 10/11/2012 18:53, Kevin Martin wrote:
> On 10 Nov 2012, at 12:25, spir wrote:
>
> > Is there a way to pass additional params to the function pointed by __tostring? (Would be practicle for instance to pass the set of already seen tables in stringing functions specialised for arrays, maps, objects, trees…)
>
> I can't envisage an example of where this is necessary or useful. However, can you not just use an upvalue in your tostring to store previously passed values?
>
> Thanks,
> Kev

To print an array, object, whatever... in "tree view" with indentation, we need 
to pass a nesting/indent level, and also the set of previously visited tables to 
avoid inf recursion. If the function performing tostring for each type accepts 
these parameters and deals with them, but __tostring does not forward them, then 
we need to check the type of each sub-item to explicitely call this function, 
instead of relying on the magic __tostring mechanism.
Or I miss a point.
For instance, if the top-level table is an array, to get the list of item 
strings I could do something like:
   local strings = {}
   for _, item in ipairs(arr) do
      strings[#strings + 1] = tostring(item, level, tables)
   end
For now, I must specialise for every case, eg:
   local strings = {}
   local s
   for _, item in ipairs(arr) do
      if     item.is_array then s = array_notation(item, level, tables)
      elseif ...
      elseif ...
      elseif ...
      end
      strings[#strings + 1] = s
   end


Denis