Passing A Function to Lua

You can give host functions to the guest state. Guest code can call them.

local lua = require("lua-sys")

local state = lua.new()

state:globals().adder = function(a, b)
	print("Called by guest!")
	return a + b
end

print(state:eval("return adder(5, 5)")) -- 10

The bridge converts the arguments for the basic types: numbers, strings and booleans. Refer to How values cross the boundary.

A guest function that the host receives becomes a normal host function. This event occurs when a guest function is a result of a call, or a value in a guest table:

local double = state:eval("return function(x) return x * 2 end")
print(double(21))  -- 42

A guest function as an argument of a host callback is different. An argument of the type function, userdata or thread gives this error:

bridge: cannot pass function across independent states

Put the value in a guest table and pass the table instead.

Tables

A table is different from a function. The bridge does not copy a table into a host value, for three reasons:

1. A table can change at any time. Fields can come and go.
2. Because of reason 1, a copy is not correct.
3. A copy is slow.

Therefore, the host keeps a table as a reference and reads fields as necessary.

The call state:globals() shows this design. It returns a lua.Table for the guest global table. The assignment state:globals().adder = ... above uses that proxy.

To identify a lua.Table, use type(x) == "table". A guest table always arrives as a lua.Table, never as a plain host table.

Tip

For more information, refer to the API Reference.