type checking tags

["kenlohk" <kenlo@...>]

I found there is no type checking on tags even in core Lua.
Consider these codes,

      -- this will hang
      p = 123
      s = read(p,'*a')
      -- this will give error
      q = 'I love you'
      t = read(q,'*a')

What is the philosophy behind the use of tag?  Confusion on
them can give serious errors i.e. core dump.  For example I
implemented a userdata with

      w = myuserdata_init('something')
      x = myuserdata_query(w)

Where in myuserdata_query I need to access a data structure.
If I pass myuserdata_query an abritrary pointer i.e. a pointer
that has not been initialized with myuserdata_init, then I
can core dump.  I have tested that.  Someone in this list
has advised these codes,

      lua_pushusertag(L, 1, newtag())
      lua_setglobal(L, mytag)

So mytag becomes global and next time I can check if the
pointer passed is a valid myuserdata.  But wait a minute.
Someone can erase mytag or set it to something else.  So
what really is the philosophy behind tag?