How to make Faster "TYPE" function

[dvd_video]

Please inspect the code below to see some strange behavior :

  local t = {1,2,3,4,5}
  local f = function() return 5 end
  local n = 5
  local s = "five"
  
  local B = {
    ["NUM"] = getmetatable(n),
    ["STR"] = getmetatable(s),
    ["TAB"] = getmetatable(t),
    ["FUN"] = getmetatable(f)
  }
  
  local function IsMType(anyValue,sType)
    if(getmetatable(anyValue) == B[sType]) then
      return true 
    end
    return false
  end
  
  print("MT Number   : "..tostring(IsMType(n,"NUM")),"NUM")
  print("MT Number   : "..tostring(IsMType(n,"STR")),"STR")
  print("MT Number   : "..tostring(IsMType(n,"FUN")),"FUN")
  print("MT Number   : "..tostring(IsMType(n,"TAB")),"TAB")
                                                    
  print("MT String   : "..tostring(IsMType(s,"NUM")),"NUM")
  print("MT String   : "..tostring(IsMType(s,"STR")),"STR")
  print("MT String   : "..tostring(IsMType(s,"FUN")),"FUN")
  print("MT String   : "..tostring(IsMType(s,"TAB")),"TAB")
                                                    
  print("MT Function : "..tostring(IsMType(f,"NUM")),"NUM")
  print("MT Function : "..tostring(IsMType(f,"STR")),"STR")
  print("MT Function : "..tostring(IsMType(f,"FUN")),"FUN")
  print("MT Function : "..tostring(IsMType(f,"TAB")),"TAB")
                                                    
  print("MT Table    : "..tostring(IsMType(t,"NUM")),"NUM")
  print("MT Table    : "..tostring(IsMType(t,"STR")),"STR")
  print("MT Table    : "..tostring(IsMType(t,"FUN")),"FUN")
  print("MT Table    : "..tostring(IsMType(t,"TAB")),"TAB")

Output:

MT Number : true
MT Number : false
MT Number : true
MT Number : true

MT String : false
MT String : true
MT String : false
MT String : false

MT Function : true
MT Function : false
MT Function : true
MT Function : true

MT Table : true
MT Table : false
MT Table : true
MT Table : true

As you see here the so called types "number", "function", "table" have the same meta table... Is this normal ?

[Bo98]

Because getmetatable on numbers, functions and standard tables return nil.

String does not because it has a metatable (eg. "SomeString":lower())