I was running out of energy, and the post was running long, and hence the sketchy section. I think I’ll release a new library soon, and at the least, I’ll mention it with your example as a comment here.

For integral types, even including Integer (unbounded), one can use a structure like a Patricia tree. See Chris Okasaki and Andy Gill’s paper, Fast Mergeable Integer Maps or the Word instance of HasTrie in Elegant memoization with functional memo tries. For signed types (like Int and Integer), one has to handle negative and non-negative keys. You can find the details in Luke Palmer’s data-inttrie package.

Your remarks under “Beyond Bool” are a little sketchy. I was missing a demo of memoizing a function like and . take 10 on repeat True and False:undefined.

I guess it works as expected but it would be nice to be convinced by an example!

Let’s give it a try:

h4, s4,n4 :: Bool -> (Integer,Integer)
h4 = const (undefined, 4)
s4 = memoBoolStrict h4
n4 = memoBool h4

Testing:

*Main> snd (h4 undefined)    -- original
4
*Main> snd (s4 undefined)    -- strict
*** Exception: Prelude.undefined
*Main> snd (n4 undefined)    -- nonstrict
4

Looks good.

h3, s3 :: Bool -> Integer
h3 = const (undefined, 3)
s3 = memoBoolStrict h3

snd (s3 (undefined))