lua-users home
lua-l archive

[Date Prev][Date Next][Thread Prev][Thread Next] [Date Index] [Thread Index]

Suppose that I have an immutable userdata type which I would like to be able to intern, but which I don't always want to intern. Something like bignums, perhaps: I'd like to intern them for efficiency, but I don't want to intern intermediate results which are going to disappear right away. In particular, if I'm going to use the objects as table keys, I really need to intern them so that the table lookup will work.

What would be necessary for Lua to help me do this? It occurs to me that it wouldn't require much. If I'm willing to handle the interning, in some manner (perhaps by constructing a canonical representation as a Lua string), then I only need to be "informed" when Lua is about to use the userdata as a table key. (I could handle equality myself in an __eq metamethod, possibly by interning both comparands and then doing object equality.)

For example: the settable operation currently calls the table's __newindex metamethod if the key is not present. But it could also call the *key*'s __newkey metamethod (if the key were a userdata and had a metatable with __newkey....)

So the settable loop might look something like:

function set(table, key, value)
  if rawget(table, key) ~= nil then
    return rawset(table, key, value)
  elseif type(key) == "userdata" then
    local func = getmeta(key, "__newkey")
    if func then
      local newkey = func(key)
      if newkey ~= key then
        if rawget(table, newkey) then
          return rawset(table, newkey, value)
      key = newkey
  -- Now continue with the current algorithm:
  return internedset(table, key, value)

where internedset is the current algorithm:

function internedset(table, key, value)
  local meta = getmeta(table, "__newindex")
  if meta == nil then
    return rawset(table, key, value)
  elseif type(meta) == "table" then
    return internedset(meta, key, value)
    return meta(table, key, value)


I don't think this is much overhead. Is there anyone other than me who thinks it might be useful?