Re: [Haskell-cafe] Simple hash table creation

Hi Gregory, I was wondering about that, because of the following: [1 of 1] Compiling Main             ( hash1.hs, interpreted ) Ok, modules loaded: Main. *Main> ht <- new (==) dummy :: IO MyHashTable *Main> dummy "mike" 7 *Main> dummy "michael" 7 *Main> insert ht "mike" 1 *Main> toList ht [("mike",1)] *Main> insert ht "michael" 2 *Main> toList ht [("michael",2),("mike",1)] *Main> insert ht "miguel" 3 *Main> toList ht [("miguel",3),("michael",2),("mike",1)] *Main> :t dummy "miguel" dummy "miguel" :: Int32 *Main> It seems my dummy function is being ignored. I figured I would only be able to store a single value with a hash function that always returns 7. Why ask for a hash function and not use it? Also, it's said that it is good programming practice  to include type information in function definitions, so I always try to do that, even though it usually leads to my code being rejected. I need to step back and use the :t  and module functions defs to figure out what types are returned and required as arguments, instead of trying to puzzle it out myself. Like juggling, there's a lot of balls in the air w/Haskell, lots of things to remember, but it's the most intriguing computer language I've looked at in a long time. Thanks for your input. Michael --- On Tue, 11/17/09, Gregory Crosswhite wrote: From: Gregory Crosswhite Subject: Re: [Haskell-cafe] Simple hash table creation To: "michael rice" Cc: haskell-cafe@haskell.org, "Daniel Fischer" Date: Tuesday, November 17, 2009, 3:30 PM Look in Data.Int for a list of the Int types. Basically you generally only use Int unless you are working at a lower-level which has an explicit requirement for the number of bits.  In this case, HashTable has such a requirement, so you have two choices:  you can either change the type annotation on dummy: import Data.Int dummy:: String -> Int32 dummy s = 7 or you can just leave it off entirely, and GHC will automatically infer the correct type (without you needing to import Data.Int): dummy s = 7 On Nov 17, 2009, at 12:09 PM, michael rice wrote: Hi Daniel, Thanks for the IO monad reminder. What is GHC.Int.Int32? Can't find it on Hoogle. Michael ================== *Main> ht <- new (==) dummy :: IO MyHashTable <interactive>:1:15:     Couldn't match expected type `GHC.Int.Int32'            against inferred type `Int'     In the second argument of `new', namely `dummy'     In a stmt of a 'do' expression:         ht <- new (==) dummy :: IO MyHashTable *Main> :t dummy dummy :: String -> Int *Main> --- On Tue, 11/17/09, Daniel Fischer wrote: From: Daniel Fischer Subject: Re: [Haskell-cafe] Simple hash table creation To: haskell-cafe@haskell.org Date: Tuesday, November 17, 2009, 2:45 PM Am Dienstag 17 November 2009 20:36:46 schrieb Daniel Fischer:

What you probably wanted was

type MyHashTable = HashTable String Int -- not data MyHashTable

Just in case it's not clear:

ht <- new (==) dummy :: IO MyHashTable

only works at the prompt or in an IO do-block, not at the top level of the module.

then ht is a hashtable of type MyHashTable.

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