No parentheses function "call" in Lua example from official docs

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The official documentation for metatables in Lua shows the following code:

Set = {}
    
    function Set.new (t)
      local set = {}
      for _, l in ipairs(t) do set[l] = true end
      return set
    end
    
    function Set.union (a,b)
      local res = Set.new{}
      for k in pairs(a) do res[k] = true end
      for k in pairs(b) do res[k] = true end
      return res
    end

The syntax Set.new{} is unclear to me. new is a function and typically functions are called with parentheses (). What is this syntax and where is it documented in lua.org?

lua
1 Answers

Lua allows the syntax of passing a string literal or table constructor into a function as its only param without needing to wrap it with parentheses, which in my opinion is hideous.

You will find it covered in Programming in Lua: 5 – Functions.

If the function has one single argument and this argument is either a literal string or a table constructor, then the parentheses are optional:

   print "Hello World"     <-->     print("Hello World")
   dofile 'a.lua'          <-->     dofile ('a.lua')
   print [[a multi-line    <-->     print([[a multi-line
    message]]                        message]])
   f{x=10, y=20}           <-->     f({x=10, y=20})
   type{}                  <-->     type({})

You can also find it in the refrence manaul Lua 5.4 Reference Manual: 3.4.10 – Function Calls

A call of the form f{fields} is syntactic sugar for f({fields}); that is, the argument list is a single new table. A call of the form f'string' (or f"string" or f[[string]]) is syntactic sugar for f('string'); that is, the argument list is a single literal string.

user_73961480
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