Re: tag method for dostring?

[Roberto Ierusalimschy <roberto@...>]

> I was perusing the lua 3.0 documentation, and I was looking for a tag
> method that could be overridden for tostring() and tonumber().  Do
> these exist?  If not, they might be a useful addition to the language
> so that userdatas could be more easily debugged.

What you can do is to override the functions "tostring" and "tonumber",
so they can handle your special cases. A simple way to do that is as
follows:


Howtoshow = {}  -- keep functions to show selected tags

old_tostring = tostring

function tostring (o)  -- redefine 'tostring'
  local f = Howtoshow[tag(o)]
  if f then return f(o)
  else return old_tostring(o)
  end
end


-- supose 'tt' is a new tag, and 'foo' is
-- the function you want to use to convert
-- such objects to strings:

Howtoshow[tt] = foo

----
Notice: in Lua 3.1, you could write the above
function as (there is no need of 'old_tostring'):

function tostring (o)
  local f = Howtoshow[tag(o)]
  if f then return f(o)
  else return %tostring(o)
  end
end


-- Roberto