What is the usage of parentheses for strings when calling binding methods in Lua ？

Question

I am a beginner of Lua, and I was confused with the usage of ().

Here are some examples on Lua5.3.

a = '%s %s'
a:format("Hello", "World")             -- Hello World
'%s %s':format("Hello", "World")       -- stdin:1: unexpected symbol near ''%s %s''
('%s %s'):format("Hello", "World")     -- Hello World

type(a) == type('%s %s')               -- true
getmetatable(a)                        -- table: 0x1f20bf0
getmetatable('%s %s')                  -- table: 0x1f20bf0 for any other hard coded strings 

Question:

1. What did the () do to the hard coded string?
2. Otherwise, only syntax error? :D

Answer 1

The mandatory usage of parenthesis (or a variable) in the first example with the function call is due to Lua grammar rules: this is simply how the language grammar is defined, and not following the defined grammar results in syntax/parse errors.

See Section 2.5.8 – Function Calls, of the Lua Reference.

A function call in Lua has the following syntax:

functioncall ::= prefixexp args

In a function call, first prefixexp and args are evaluated. If the value of prefixexp has type function, then this function is called with the given arguments. Otherwise, the prefixexp "call" metamethod is called, having as first parameter the value of prefixexp, followed by the original call arguments (see §2.8).

The form

functioncall ::= prefixexp `:´ Name args

..

Where prefixexp is defined as:

prefixexp ::= var | functioncall | ( exp )

That is, prefixexp cannot be a String (or any other) literal, but it can be a variable (var); or any expression inside parentheses (( expr )); or even chained function call (functioncall)..

Using a String (or any other) literal in the args position is permitted due to the following productions:

args ::=  `(´ [explist] `)´ | tableconstructor | String
explist ::= {exp `,´} exp

As a complimentary case to the examples, note that the args production has a special-case that doesn't require parenthesis if taking a single String literal. Thus the following is valid, even if seemingly odd:

('Hello %s'):format "World!"