What is happening when you have a function as a parameter?

user_5106912 0
Viewed 76

I'm still ramping up in lua, and I am not quite familiar with this syntax. What is happening when you pass in a function as a parameter like below?

Comm.setRouting(function(url)
        for i = 1,4 do
            local portIndex = "Path"..i
            if url:match(portConfig[portIndex]) ~= nil then
                return Comm.slots()[1], Comm.groups()[i]
            end
        end
    end)
lua
3 Answers

The other answers are correct, but it might help you if you wrote your own function that calls another function:

function CallAFunction(func_to_call)
   func_to_call()
end

You could pass a named function (an anonymous function assigned to a variable) or an anonymous function written on the fly.

function SayHello()
   print("Hello!")
end
--[[
^This is equivalent to:
SayHello = function()
   print("Hello!")
end
]]--

CallAFunction(SayHello)
CallAFunction(function()
                  print("Goodbye!")
              end)

output:

Hello!
Goodbye!

and this can be done with parameters

function CallAFunction(func)
   func("Bonjour")
end

CallAFunction(function(parameter)
                  print(parameter)
              end)

Here, func is the anonymous function, which accepts 1 parameter parameter.
When you call func("Bonjour") you are passing Bonjour as parameter, like a normal function call.

user_125149970

A couple of things are happening in this example:

  1. You're passing a function as a parameter. The callee (e.g. setRouting()) can invoke that function. This is often referred to as a callback function.

  2. You're defining the function itself, on-the-fly. This is an example of an "anonymous function", or a lambda function.

user_4211950

Here you're passing an argument to setRouting that is an "anonymous function"

Functions are first-class values in Lua and can be stored in local variables, global variables and table fields. Here the function is being passed anonymously on the call stack to the setRouting function.

setRouting would be called a "higher-order function" because it accepts a function as its input.

The following page has some more information about functions in Lua:

https://www.lua.org/pil/6.html

user_24862990
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