[Haskell-cafe] Simple hash table creation

[Gregory Crosswhite]

You don't need to create a new type for a String -> Int hashtable, you  
already get it for free because HashTable is a parameterized type.

Also, although you are apparently trying to make life simpler for  
yourself using a HashTable, you are actually making like more  
complicated because the Data.HashTable implementation can only be  
worked with inside the IO monad.  So what you'd need is something like:


====================

import Prelude hiding (lookup)
import Data.HashTable

dummy s = 7

main = do
	ht <- new (==) dummy
	insert ht "Foo" 12
         value <- lookup ht "Foo"
         putStrLn . show $ value

====================

Note that I didn't have to label any of the types;  Haskell is smart  
enough to mostly infer all of them automatically.  The main reason to  
include types is if there is some ambiguity.  So for example, if you  
actually wanted to map Strings to Floats, then you would need to  
explicitly tell it somewhere that the values it is storing are  
floats.  You could do this by either pinning down explicitly the  
HashTable type:

====================

import Prelude hiding (lookup)
import Data.HashTable

dummy s = 7

main = do
	ht <- new (==) dummy  :: IO (HashTable String Float)
	insert ht "Foo" 12
         value <- lookup ht "Foo"
         putStrLn . show $ value

====================

Or just by pinning down the type of a value that you insert into it:

====================

import Prelude hiding (lookup)
import Data.HashTable

dummy s = 7

main = do
	ht <- new (==) dummy
	insert ht "Foo" (12 :: Float)
         value <- lookup ht "Foo"
         putStrLn . show $ value

====================

Again, the downside though is that you can't work with HashTable  
outside of the IO monad.  If what you want is just a map from strings  
to values, then you probably are better off using Data.Map:

====================

import Prelude hiding (lookup)
import Data.Map

my_map = empty :: Map String Int

my_map_after_adding_key = insert "Foo" 12 my_map

value_associated_with_Foo = lookup "Foo" my_map_after_adding_key

main = putStrLn . show $ value_associated_with_Foo

====================

Cheers,
Greg

On Nov 17, 2009, at 11:16 AM, michael rice wrote:

> I'm trying to create a hash table. Yeah, I know, don't use hash  
> tables, but I need to create something I'm familiar with, not  
> something I've never worked with before. What's wrong with this code?
>
> Michael
>
> ====================
>
> import Prelude hiding (lookup)
> import Data.HashTable
>
> data MyHashTable = HashTable String Int
>
> dummy:: String -> Int
> dummy s = 7
>
> ht = MyHashTable.new (==) dummy
>
> ====================
>
> [michael at localhost ~]$ ghci hash1
> GHCi, version 6.10.3: http://www.haskell.org/ghc/  :? for help
> Loading package ghc-prim ... linking ... done.
> Loading package integer ... linking ... done.
> Loading package base ... linking ... done.
> [1 of 1] Compiling Main             ( hash1.hs, interpreted )
>
> hash1.hs:9:5: Not in scope: `MyHashTable.new'
> Failed, modules loaded: none.
> Prelude>
>
>
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