[Haskell-cafe] Space questions about intern and sets

Bjorn Bringert bringert at cs.chalmers.se
Wed Jun 8 09:52:53 EDT 2005

Gracjan Polak wrote:
> Hello all,
> I've got two questions, both are space related so I'll put them in one 
> e-mail.
> 1. I'd like to have function intern, that behaves essentially like id, 
> but with the following constraint:
>    if x==y then makeStableName(intern x)==makeStableName(intern y)
> Reasoning behind this hack: objects equal (as in Eq) should occupy the 
> same place in memory. I've got to parse quite large file, most tokens 
> are the same. Haskell speed is very good, but I constantly run out of 
> memory. Here is something I wrote, but it doesn't work :(

The code below seems to work for strings, and should be generalizable to 
any type for which you have a hash function:

import Data.HashTable as H
import System.IO.Unsafe (unsafePerformIO)

{-# NOINLINE stringPool #-}
stringPool :: HashTable String String
stringPool = unsafePerformIO $ new (==) hashString

{-# NOINLINE shareString #-}
shareString :: String -> String
shareString s = unsafePerformIO $ do
     mv <- H.lookup stringPool s
     case mv of
             Just s' -> return s'
             Nothing -> do
                        H.insert stringPool s s
                        return s

It seems very similiar to your code, except that it uses HashTable 
instead of Map.


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