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

Thanks, I was close, but the I was trying to use (something like) this statement without the return: maybe (return Nothing) (flip HashTable.lookup 1000) More or less like this: maybe (Nothing) (flip HashTable.lookup 1000) Which did't work... Guess the return is needed because we use a new monad (Maybe) inside another monad (IO). Petter On Mon, Apr 13, 2009 at 7:45 PM, Lennart Augustsson wrote:

res <- HashTable.lookup h 3 >>= maybe (return Nothing) (flip HashTable.lookup 1000)

On Mon, Apr 13, 2009 at 6:59 PM, John Smith wrote:

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Yes, agreed. Got any clue on the original problem (except to use Data.Map)?

On Mon, Apr 13, 2009 at 6:55 PM, Jason Dagit wrote:

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Others have provided help to answer your question but I'd like to provide a little bit different feedback.

On Mon, Apr 13, 2009 at 8:42 AM, John Smith wrote:

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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)

When you see yourself writing a function like this, you could write it like this instead: removeMaybeHash (Just ref) = ref removeMaybeHash Nothing = HashTable.new (==) (\key -> key)

Hopefully you agree this 2 line version is more clear.  You could go further of course and use the function 'maybe' from the prelude, and pass the function 'id' instead of \key -> key, but I don't want to overwhelm you.

Good luck, Jason

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