[Haskell-cafe] Types and hashes of hashes, trouble for a Java-programmer...

John Smith smithsnorth at gmail.com
Mon Apr 13 11:42:33 EDT 2009

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?

In java this would be as simple as (pseudocode): h.lookup(3).lookup(1000)


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