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 <[email protected]> wrote:
From: Gregory Crosswhite <[email protected]>
Subject: Re: [Haskell-cafe] Simple hash table creation
To: "michael rice" <[email protected]>
Cc: [email protected], "Daniel Fischer" <[email protected]>
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 <[email protected]> wrote:
From: Daniel Fischer <[email protected]>
Subject: Re: [Haskell-cafe] Simple hash table creation
To: [email protected]
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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