[Haskell-cafe] A tale of Project Euler

[Bulat Ziganshin]

Hello Andrew,

Thursday, November 29, 2007, 9:43:48 PM, you wrote:

>> Fifth thing: better use an STUArray, don't drag IO in if it's not necessary.
>>   

> I don't understand the ST monad.

it's just a subset of IO monad, with renamed operations

the subset is selected in the way that guarantees reference
transparency, i.e. compiler is able to do more checks that your
program is correct. so, instead of writing

main = do a <- newArray
          writeArray a 1 3.14
          x <- readArray a 1
          print x

you can write

main = do x <- runST (do a <- newArray
                         writeArray a 1 3.14
                         x <- readArray a 1
                         return x)
          print x


-- 
Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com