Lua: Can I use colon syntax here?

Question

I'm running this function a lot which shortens the name of a string:

function shorten(st) return string.sub(st,6) end function test(input) local h = shorten(input) print(""..h) end test("helloworld")

I want to write it with a colon like the other functions in this context, which all go like world():context():manager(), not manager(context(world())). I read that using the syntax with the colon passes the first as an argument into the latter, but this does not work:

function shorten(st) return string.sub(st,6) end function test(input) local h = input:shorten() print(""..h) end test("helloworld")

Is there a way to do this?

if it is ok that :shorten() can be used on every string in your program that's probably fine. just add it to the string library as shown in the answer below. I personally would simply use :sub(6) so I don't have to think about what :shorten() actually does. but if shortenning to 6 characters is what you always do, go ahead. — Piglet
– Piglet, Commented Feb 3, 2022 at 16:22

Nifim · Accepted Answer · 2022-02-03 16:09:24Z

depending on your flavor of lua, you can simply define shorten as string.shorten

This works because the metatable for all strings references the string library. That is why adding a function to string will add make it accessible to any defined strings.

function string.shorten(st) return string.sub(st,6) end function test(input) local h = input:shorten() print(h) end test("helloworld")

Output:

world

Your example of colon syntax is not exactly how it works. It is syntactic sugar

someClassInstance:method("arg")

is equivalent to

someClassInstance.method(someClassInstance, "arg")

You often see colon syntax used to improve readability of OO style lua, and is described here in Programing in Lua: 16 – Object-Oriented Programming

side note: you did print(""..h) which seems weird since it is functionally identical to just doing print(h) but is more expensive.

"This works because the metatable for strings is the string table. " no. string refers to Lua's string library. So does the __index field of the metatable all strings have in common. that's two different tables.
@Piglet fair point I did over simplify that. does this sound better? "This works because the metatable for all strings references the string library."

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