Question

Being rather new to pure functional programming idiom, I can’t get how to implement this case of dynamic programming. I have a function f :: String -> [String] which is calculated recursively and want to memoize it. Input Strings can be arbitrary, so I guess that something like a lazy Map is needed, but couldn’t find any. How to implement such case in Haskell?

Answer 1

Use a memoizer library:

import qualified Data.MemoCombinators as Memo

f :: String -> [String]
f = Memo.list Memo.char memof  -- because String = [Char]
    where
    memof x = ... f ...          -- call *f* recusively (not memof)

See the documentation for more. Also see MemoTrie