Some Lua pattern in Teal.

[nagolove]

In my project I have next pattern.

local commands = {}

commands["do3"] = function()
    print("do3")
end

function commands.do1() print("do1") end
function commands.do2() print("do2") end

function process(cmd)
    if cmd then
        local command = commands[cmd]
        if command then
            command()
        end
    end
end

How I can rewrite such code?
First attempt

local commands: {string: function} = {}

commands["do3"] = function()
    print("do3")
end

function commands.do1() print("do1") end
function commands.do2() print("do2") end

function process(cmd: string)
    if cmd then
        local command = commands[cmd]
        if command then
            command()
        end
    end
end

It has not a module: {string : function(<any type>):<any type>} error in function commands.do1() print("do1") end and next function. But it has no error in commands["do3"] = function(). Why?
Next attempt. More type declarations.

local type Commands = record
    do1: function()
    do2: function()
    do3: function()
end

local type Cmds = enum
    "do1"
    "do2"
    "do3"
end

local commands: Commands = {}

commands["do3"] = function()
    print("do3")
end

function commands.do1() print("do1") end
function commands.do2() print("do2") end

function process(cmd: Cmds)
    if cmd then
        local command: function() = commands[cmd]
        if command then
            command()
        end
    end
end

It compiles but has more duplication. Also I am embarrassed with types naming for record and enum. What is right approach for such patterns?

[lenscas]

every function that can be executed using that index need to have the same type, so what you can do is something like

local type CommandNames = enum
    "do1"
    "do2"
end

local commands : {CommandNames : function()} = { 
 do1 = function() print("do1")end,
 do2 = function() print("do2")end
}

function process(cmd: CommandNames)
    if cmd then
        local command: function() = commands[cmd]
        if command then
            command()
        end
    end
end

Basically, stop using a record and start using a map of commandName : yourFunctionType

[hishamhm]

@lenscas's response is correct, this pattern calls for a map, not a record.

Which I noticed was the first thing you tried to do, but stumbled into this:

It has not a module: {string : function():} error in function commands.do1() print("do1") end and next function. But it has no error in commands["do3"] = function(). Why?

That's a good question. Lua supports different syntaxes for some equivalent operations on tables, because the table type takes on many duties (maps, records, arrays, etc.) — in Teal, I have reserved the function r.f() syntax for records and the assignment syntax r.f = function() for maps.

The reason is inference: a typical pattern is to do

local mymodule = {}

function mymodule.myfunction()
   -- end
end

so Teal infers that mymodule is a record and function mymodule.myfunction() is declaring a field to that record.

Since you have declared in your first attempt that you wanted commands to be a map of strings to functions, I guess Teal could make an exception and accept that syntax (but noting that the function signature would have to match that of the map value type exactly).

But a slight variation of your first attempt does, in fact, work:

local commands: {string: function} = {}

commands["do3"] = function()
    print("do3")
end
commands["do1"] = function()
    print("do1")
end
commands["do2"] = function()
    print("do2")
end

function process(cmd: string)
    if cmd then
        local command = commands[cmd]
        if command then
            command()
        end
    end
end

[lenscas]

might be useful to note that using an enum gives you better type safety, as it will be impossible to give process a cmd key that isn't valid :)