John Smith
Hi, a java-programmer running into trouble while trying to learn Haskell.
I try to make a hash containing hashes but can not getting a value out of the innermost hash - I keep running into type-trouble where IO and Maybe monad is getting mixed?
My attempt:
removeMaybeHash x = case x of Just ref -> ref Nothing -> HashTable.new (==) (\key -> key)
test = do h <- HashTable.new (==) (\key -> key) h1 <- HashTable.new (==) (\key -> key) HashTable.insert h 3 h1 HashTable.insert h1 1 1000 maybeOuterHash <- HashTable.lookup h 3 res <- removeMaybe (removeMaybeHash maybeOuterHash) 1000 return res
Any clues?
Just don't use it, someone deserves to be disemboweled for implementing it via IO. Most likely, you don't want to have a hash in specific, but a key->value mapping in general, and the Haskell libs offer more than one algorithm to do that, the most basic one being Data.Map, and the rest having more or less the same interface (e.g. IntMap, or Trie). There are two possibilities to "nest" two maps: The first being indexing the map by a tuple of both lookup keys, the second one using two maps, one inside the other. Personally, I always use Data.Map unless I identify it as a bottleneck after completing the program. Using tuples should be straight forward, if you want to nest Maps, you will want to chain two lookup's together. The most straight-forward way to do this is using Maybe's monad instance, which looks like this: instance Monad Maybe where (Just x) >>= k = k x Nothing >>= _ = Nothing It "bypasses" the second argument of >>= in case the first argument is Nothing. Filling in the types, we get (>>=) :: Maybe a -> (a -> Maybe b) -> Maybe b . "a" is going to be the inner map, which the second lookup will take as second parameter, so you can write this: import Data.Map as M lookup' :: (Ord k, Ord l) => Map k (Map l v) -> k -> l -> Maybe v lookup' m k l = M.lookup k m >>= M.lookup l ...Somewhere, I once read that monads can be seen as some fancy way to do OOP's foo.bar().baz(), but I'm not supposed to tell you that: You should try your best to forget anything you know about programming if you come from an imperative language and want to learn haskell :) -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited.