Lua functions: how to use tables as function arguments and self-document the code

Question

When I have functions which take many arguments it's sometimes useful to pass a single table as an argument instead of many local variables.

function example_1(arg_one, arg_two, arg_three)
end

becomes

function example_2(arg_table)
    arg_table.arg_one, arg_table.arg_two, arg_table.arg_three
end

The problem is, when calling the function somewhere else in code, it's hard to remember what arg_table needs to consist of. There are plenty of code-completion plugins for many editors which will help you with remembering the arguments for the example_1 function, but not for example_2.

Is there any way to write the example_2 function with a table parameter in a way that is still a table, but also shows the necessary parameters for the function inside the ()?

something like this (which does not work):

function example_2(arg_table = {arg_one, arg_two, arg_three})

end

Start your function with assert(arg_table.arg_one ~= nil and arg_table.arg_two ~= nil and arg_table.arg_three ~= nil) — Egor Skriptunoff
– Egor Skriptunoff, Commented Sep 4, 2019 at 16:20
Why dont you simply unpack the table when calling the function and keep the function parameters? — Felix
– Felix, Commented Sep 4, 2019 at 16:57

Tom Blodget · Accepted Answer · 2019-09-05 11:34:07Z

1

Write your formal parameter list and documentation with separate parameters, as usual. Then document that if the first (and only) actual argument is a table, the effective arguments will be taken from the table using the formal parameter names as string keys.

answered Sep 5, 2019 at 11:34

Tom Blodget

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1 Comment

Tom Blodget Over a year ago

And, in the implementation, this is one of the few cases where I would ever change the value of an incoming parameter: arg_one, arg_two = arg_one.arg_one, arg_one.arg_two

Felix · Accepted Answer · 2019-09-05 13:12:57Z

0

You can keep the function parameters as they are and pass a table for convenience.

For example:

function example(arg_one, arg_two, arg_three)
    -- do stuff
end

local tbl = {"value for arg_one", "value for arg_two", "value for arg_three"}
example(table.unpack(tbl))

Note that this doesnt work for tables with named keys.

answered Sep 5, 2019 at 13:12

Felix

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