Learn Lua in Neovim: First-Class Functions — Store, Pass & Filter with Functions | Episode 15

Learn Lua in Neovim: First-Class Functions — Store, Pass & Filter with Functions

In Lua, functions are first-class values, meaning you can store them in variables, pass them as arguments, and return them from other functions. This flexibility enables powerful patterns like callbacks, anonymous functions, and higher-order functions such as filters that select items from lists.

Code

-- Store a function in a variable that squares a number
local square = function(x)
  return x * x
end

print("5 squared is", square(5))  -- Output: 5 squared is 25

-- Define a function that takes another function and a value, then applies the function to the value
local function apply(func, value)
  return func(value)
end

print("Square of 6 is", apply(square, 6))       -- Output: Square of 6 is 36
print("Square root of 25 is", apply(math.sqrt, 25))  -- Output: Square root of 25 is 5

-- Use an anonymous function to filter even numbers from a list
local function filter(items, test)
  local result = {}
  for _, item in ipairs(items) do
    if test(item) then
      table.insert(result, item)
    end
  end
  return result
end

local numbers = {1, 2, 3, 4, 5, 6}
local evens = filter(numbers, function(n) return n % 2 == 0 end)

print("Even numbers:")
for _, v in ipairs(evens) do
  print(v)  -- Outputs: 2, 4, 6
end

Key Points

• Functions can be stored in variables just like any other data type in Lua.
• Passing functions as arguments (callbacks) allows flexible and reusable code.
• Anonymous functions let you define behavior inline without naming the function.
• Higher-order functions like filter demonstrate the power of first-class functions.
• These patterns form the foundation for closures and functional programming in Lua.