Lua Academy · Lesson

Memoization with Closures

Cache expensive function results inside a closure-held table.

Lesson 4 of 413 steps

Memoization with Closures is a free Lua Academy lesson on CoddyKit. This is lesson 4 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Lua Academy learning path, and your progress syncs across the web and the CoddyKit app. The Lua Academy course includes 4 lessons in total.

What Is Memoization?

Memoization caches the results of expensive function calls so that repeated calls with the same arguments return the cached result instantly.

Basic Memoize Wrapper

A closure holds a cache table. On first call, compute and store. On repeat calls, return the stored value.

local function memoize(fn)
  local cache = {}
  return function(x)
    if cache[x] == nil then
      cache[x] = fn(x)
    end
    return cache[x]
  end
end

Memoized Fibonacci

Memoizing the recursive Fibonacci function reduces its time complexity from exponential to linear.

local fib
fib = memoize(function(n)
  if n <= 1 then return n end
  return fib(n-1) + fib(n-2)
end)
print(fib(30))  -- 832040 (fast!)

Multi-Argument Memoization

For functions with multiple arguments, compose a cache key string from all arguments.

local function memoize2(fn)
  local cache = {}
  return function(a, b)
    local key = a .. "," .. b
    if cache[key] == nil then cache[key] = fn(a, b) end
    return cache[key]
  end
end

Nested Cache Tables

For mixed argument types, use nested tables keyed by each argument for O(1) lookup without string serialization.

local function nestedMemo(fn)
  local cache = {}
  return function(a, b)
    cache[a] = cache[a] or {}
    if cache[a][b] == nil then cache[a][b] = fn(a, b) end
    return cache[a][b]
  end
end

Cache Invalidation

Memoization assumes pure functions (no side effects, same output for same input). Functions with side effects should not be memoized without careful cache invalidation.

TTL-Based Memoization

Add time-to-live logic using os.time() to expire cached entries.

local function memoizeTTL(fn, ttl)
  local cache = {}
  return function(x)
    local entry = cache[x]
    if entry and os.time() - entry.time < ttl then
      return entry.value
    end
    cache[x] = {value = fn(x), time = os.time()}
    return cache[x].value
  end
end

Weak Value Cache

Use weak values so cached results can be garbage collected when memory is tight.

local function memoizeWeak(fn)
  local cache = setmetatable({}, {__mode = "v"})
  return function(x)
    if cache[x] == nil then cache[x] = fn(x) end
    return cache[x]
  end
end

Recursive Memoization Pattern

For mutually recursive functions, assign the memoized wrapper before the recursive body uses it, so recursion hits the cache.

Bounded Cache (LRU sketch)

For memory control, implement an LRU cache that evicts the least-recently-used entry when the cache exceeds a maximum size.

Memoization in Practice

Common uses: expensive mathematical computations, database query result caching, Lua pattern compilations, and config file parsing.

Memoization Question

Why should memoization only be applied to pure functions?

Answer it in the interactive lesson

Recap: Memoization

Memoization uses a closure-held cache to store results by argument key, eliminating redundant computations. Use weak tables for memory safety and add TTL for time-sensitive caches.

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Frequently Asked Questions

Is the “Memoization with Closures” lesson free?

Yes — any 3 lessons of the Lua Academy learning path, including “Memoization with Closures”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Lua Academy course includes 4 lessons in total.

What will I learn in “Memoization with Closures”?

Cache expensive function results inside a closure-held table. You practise Lua Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Lua Academy?

No prior experience is required. Lua Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 4 of 4.

How long does the “Memoization with Closures” lesson take?

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