1. 22

This is the first pre-release for the next version of Lua.

The email announcement from Luiz Henrique de Figueiredo is not yet in the archive, so I reproduce it here:

Lua 5.4.0 (work1) is now available for testing at

The checksums are
        MD5     0ff232b8658884155a43cf72212edbd9  -
        SHA1    a8193b14ed3869917d1102cb0418cf9dfb0d9baf  -

This is a work version. All details may change in the final version.

An updated reference manual is included and also available at

The complete diffs from Lua 5.3 are too extensive to show.

A test suite is available at

All feedback welcome. Thanks.

  2. 3

    New features:

    new generational mode for garbage collection

    userdata can have multiple user values

    debug information about function arguments and returns

    new implementation for math.random

    Holy cow, 8.1 is a huge change:

    The coersion of strings to numbers in arithmetic and bitwise operations has been removed from the core language.


    1. 2

      I think the change isn’t quite as big as the wording might imply. The functionality is still there, it’s just implemented via metamethods instead of specified as part of the base language.

      It’s still a bigger change than you usually see in Lua, though.

    2. 2

      It is off by default, but a potentially immense change concerns the handling of nil and a new keyword undef.


      btw, I notice that there is a new keyword, “undef”, not mentions in manual, is that just a undocumented feature or the doc just not the latest?

      I am grad you asked :-)

      Lua 5.4 has an experimental change that is off by default (to keep compatibility). We plan to keep this off in Lua 5.4 final. You can turn it on by compiling this version with the option -DLUA_NILINTABLE. As the name hints, this option allows nils in tables:

       t = {nil, nil, nil}
       for k,v in pairs(t) do print(k, v) end
      1	nil
      2	nil
      3	nil
      t[#t + 1] = nil
      for k,v in pairs(t) do print(k, v) end
      1	nil
      2	nil
      3	nil
      4	nil

      Once you have nils in tables, t[i]=nil does not remove elements from a table anymore. For that, you need to write t[i]=undef.

      No, this is not like JavaScript! ‘undef’ is NOT a value.

      t[i]=undef is a special syntax form that means “remove the key ‘i’ from table ‘t’”. You can only use ‘undef’ in three specific forms:

        t[i] = undef     -- remove a key from a table
        t[i] == undef    -- test whether a table has a key
        t[i] ~= undef    -- test whether a table has a key

      Any other use of ‘undef’ gives a syntax error.

      You still can create tables with holes, but you must get out of your way to do it.

      The nice thing about this syntax is that it is compatible with previous versions of Lua (as long as you do not use ‘undef’ for other stuff). In Lua 5.0/1/2/3, if you write ‘t[i] = undef’, you remove the element from the table. Even if we never change Lua to this new mode, even if you never use Lua 5.4, we think this syntax is a nice way to document whether an assignment is being done to remove an element from a table.

      (A search for the pattern ‘][ =~]*nil’ finds most of the places in your code where you could/should use undef instead of nil.)

      – Roberto