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 <gcross@phys.washington.edu> wrote:

From: Gregory Crosswhite <gcross@phys.washington.edu>
Subject: Re: [Haskell-cafe] Simple hash table creation
To: "michael rice" <nowgate@yahoo.com>
Cc: haskell-cafe@haskell.org, "Daniel Fischer" <daniel.is.fischer@web.de>
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 <daniel.is.fischer@web.de> wrote:

From: Daniel Fischer <daniel.is.fischer@web.de>
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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