Can Lua define anonymous member functions?

Question

In Lua, I can define a member function like this:

Foo = {}

function Foo:bar()
  ...
end

I realise this is just syntactic sugar for this:

Foo = {}

foo.bar = function(self)
  ...
end

Is there a way to write an anonymous function with an implicit self parameter or do I always have to spell out the self parameter on anonymous functions?

The motivation is to create coroutine member functions in a way most analogous to ordinary functions. I can do this:

Foo.bar = coroutine.wrap(function(self) ... end)

Or this:

function Foo:bar() ... end

Foo.bar = coroutine.wrap(Foo.bar)

But is there a way to do a one-line declaration with an implicit self parameter?

Oka · Accepted Answer · 2024-12-30 23:59:30Z

2

Short answer: not possible.

Longer:

As you've correctly identified, the syntactic sugar

exp ::= functiondef

functiondef ::= function funcbody
funcbody ::= ‘(’ [parlist] ‘)’ block end

stat ::= function funcname funcbody
stat ::= local function Name funcbody
funcname ::= Name {‘.’ Name} [‘:’ Name]

that enables the shorthand function name () end syntax (and thus the form with an implicit self variable) changes the function definition from an expression to a statement by introducing the assignment.

There is no mechanism to interrupt this syntactically, in order to insert an arbitrary expression in to the assignment (desired here: a functioncall, coroutine.wrap).

Similarly, the arguments to a function call must be expressions - so something like coroutine.wrap(function Foo:bar() end) is also not possible.

Short of modifying the language with a new keyword (think async function Foo:bar() end), your example of

Foo.bar = coroutine.wrap(function(self) ... end)

is as good as it gets. For what it is worth, this example is perfectly understandable, and self-documenting (IMO; pun intended).

^{Lua 5.4: 3 – The Language | 3.3.3 – Assignment | 3.4.10 – Function Calls | 3.4.11 – Function Definitions | 9 – The Complete Syntax of Lua}

answered Dec 30, 2024 at 23:59

Oka

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2 Comments

Tom Dec 31, 2024 at 17:38

Thanks for the answer. I assume that also means there's no direct language support for constructing a partially-bound method with a self reference? Like using local x = foo:bar; x(). Obviously not useful like that, but useful to pass a member function as a callback etc. Of course you can always construct a free function that does the job, capturing self in the closure.

Oka Dec 31, 2024 at 21:29

Correct again, as the functioncall self-insert is also purely syntactic sugar (foo:bar() -> foo.bar(foo), where foo is only evaluated once). Utilizing closures is definitely the approach for a bound function.

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