Generics in Lua

Lua doesn’t have built-in support for generics, but we can simulate some generic-like behavior using tables and functions. Here’s an example that demonstrates concepts similar to the original code:

-- As an example of a function that works with different types,
-- tableIndex takes a table and a value, and returns the index
-- of the first occurrence of v in t, or -1 if not present.
function tableIndex(t, v)
    for i, value in ipairs(t) do
        if value == v then
            return i
        end
    end
    return -1
end

-- As an example of a "generic" type, List is a
-- singly-linked list that can hold values of any type.
List = {}
List.__index = List

function List.new()
    return setmetatable({head = nil, tail = nil}, List)
end

function List:push(v)
    local newElement = {value = v, next = nil}
    if self.tail == nil then
        self.head = newElement
        self.tail = newElement
    else
        self.tail.next = newElement
        self.tail = newElement
    end
end

-- AllElements returns all the List elements as a table.
function List:allElements()
    local elements = {}
    local current = self.head
    while current do
        table.insert(elements, current.value)
        current = current.next
    end
    return elements
end

-- Main function to demonstrate usage
function main()
    local s = {"foo", "bar", "zoo"}
    
    -- When using tableIndex, we don't need to specify types
    -- as Lua is dynamically typed.
    print("index of zoo:", tableIndex(s, "zoo"))
    
    local lst = List.new()
    lst:push(10)
    lst:push(13)
    lst:push(23)
    print("list:", table.concat(lst:allElements(), " "))
end

main()

To run the program, save it as generics_example.lua and use the Lua interpreter:

$ lua generics_example.lua
index of zoo: 3
list: 10 13 23

This Lua code demonstrates concepts similar to generics in the original example:

The tableIndex function works with tables of any type, similar to the generic SlicesIndex function in the original.
The List type can hold elements of any type, simulating a generic data structure.
Methods like push and allElements work with any type of data stored in the list.
The main function demonstrates using these “generic-like” constructs with both strings and numbers.

While Lua doesn’t have explicit generic syntax, its dynamic typing and flexible table structure allow for creating code that works with multiple types in a similar way to generics in statically-typed languages.

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Generics in Lua