runmemo

A simple memoization helper library.

This library encourages you to do memoization in three separate steps:

1. Create a memoizable function

2. Create or select an appropriate memoizer

3. Run the memoizer on the memoizable function

Let's start with the first. When you create a memoizable function, you should use the self convention, which is that the first input to the function is self, and all recursive calls are replaced with self. One common convention that goes well with the self convention is using a helper function go, like so:

fib :: Memoizable (Integer -> Integer)
fib self = go
  where go 0 = 1
        go 1 = 1
        go n = self (n-1) + self (n-2)

Now for the second. For this example, we need a Memoizer that can handle an Integer input, and an Integer output. Data.MemoCombinators provides integral, which handles any Integral input, and any output. Data.MemoUgly provides memo, which can memoize any function a -> b, given an Ord instance for a.

Third, let's run our memoizers! Since we have decoupled the definition of the memoized function from its actual memoization, we can create multiple memoized versions of the same function if we so desire.

import qualified Data.MemoUgly as Ugly
import qualified Data.MemoCombinators as MC

fibUgly :: Integer -> Integer
fibUgly = runMemo Ugly.memo fib

fibMC :: Integer -> Integer
fibMC = runMemo MC.integral fib

You could easily do the same with Data.MemoTrie.memo, Data.Function.Memoize.memoize, etc.

Using this technique, you can create local memoized functions whose memo tables are garbage collected as soon as they are no longer needed.