On Monday 13 April 2009 11:42:33 am John Smith wrote:
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?
I don't know the type of all these things, but here's a try.
My attempt:
removeMaybeHash x = case x of Just ref -> ref Nothing -> HashTable.new (==) (\key -> key)
'HashTable.new ...' has type IO (HashTable k v). This means that the type of removeMaybeHash must be: Maybe (IO (HashTable k v)) -> IO (HashTable k v) because in the Just case, you're just using 'ref' as the result of the function. This is probably not what you want, so you should use 'return ref'. This will make it have type: Maybe (HashTable k v) -> IO (HashTable k v)
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
I don't know what the type of removeMaybe is supposed to be, but as used in this expression, with the above note about the type of removeMaybeHash, it must have type: IO (HashTable k v) -> Int -> IO (something) which is an odd type for a function with name "removeMaybe".
return res
Any clues?
Hopefully those help. As an additional clue, I'd recommend you write some type signatures at the top level for what you expect the types of your functions to be. That might help point out more specifically where things aren't going as you expect. -- Dan