etc/setfallback.lua

   1 --------------------------------------------------------------
   2 -- Definition of "setfallback" using tag methods
   3 -- (for compatibility with old code)
   4 --------------------------------------------------------------
   5
   6
   7 -- default fallbacks for each event:
   8 local defaults = {
   9   gettable = function () error('indexed expression not a table') end,
  10   settable = function () error('indexed expression not a table') end,
  11   index = function () return nil end,
  12   getglobal = function () return nil end,
  13   arith = function () error('number expected in arithmetic operation') end,
  14   order = function () error('incompatible types in comparison') end,
  15   concat = function () error('string expected in concatenation') end,
  16   gc = function () return nil end,
  17   ['function'] = function () error('called expression not a function') end,
  18   error = function (s) write(_STDERR, s, '\n') end,
  19 }
  20
  21
  22 function setfallback (name, func)
  23
  24   -- set the given function as the tag method for all "standard" tags
  25   -- (since some combinations may cause errors, use call to avoid messages)
  26   local fillvalids = function (n, func)
  27     call(settagmethod, {0, n, func}, 'x', nil)
  28     call(settagmethod, {tag(0), n, func}, 'x', nil)
  29     call(settagmethod, {tag(''), n, func}, 'x', nil)
  30     call(settagmethod, {tag{}, n, func}, 'x', nil)
  31     call(settagmethod, {tag(function () end), n, func}, 'x', nil)
  32     call(settagmethod, {tag(settagmethod), n, func}, 'x', nil)
  33     call(settagmethod, {tag(nil), n, func}, 'x', nil)
  34   end
  35
  36   assert(type(func) == 'function')
  37   local oldfunc
  38   if name == 'error' then
  39     oldfunc = seterrormethod(func)
  40   elseif name == 'getglobal' then
  41     oldfunc = settagmethod(tag(nil), 'getglobal', func)
  42   elseif name == 'arith' then
  43     oldfunc = gettagmethod(tag(0), 'pow')
  44     foreach({"add", "sub", "mul", "div", "unm", "pow"},
  45             function(_, n) %fillvalids(n, %func) end)
  46   elseif name == 'order' then
  47     oldfunc = gettagmethod(tag(nil), 'lt')
  48     foreach({"lt", "gt", "le", "ge"},
  49             function(_, n) %fillvalids(n, %func) end)
  50   else
  51     oldfunc = gettagmethod(tag(nil), name)
  52     fillvalids(name, func)
  53   end
  54   return oldfunc or rawgettable(%defaults, name)
  55 end