Re: [Haskell-cafe] Troubles understanding memoization in SOE

Mon, 24 Sep 2007 04:44:05 -0700 


On Sat, 22 Sep 2007, Peter Verswyvelen wrote:


Hi,

in SOE, the following memoization function is implemented:

memo1 :: (a->b) -> (a->b)
memo1 f = unsafePerformIO $ do
cache <- newIORef []
return $ \x -> unsafePerformIO $ do
            vals <- readIORef cache
            case x `inCache` vals of
              Nothing -> do let y = f x
writeIORef cache [(x,y)] -- ((x,y) : -- if null vals then [] else [head vals]) 
                            return y
              Just y  -> do return y


Hm, why the unsafePerformIO hacks? It should be possible without:
  http://www.haskell.org/haskellwiki/Memoization
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