Just today, my student asked me why the following program does nothing: {-# LANGUAGE CPP, GeneralizedNewtypeDeriving, BangPatterns #-} import Control.Monad import System.IO.Unsafe import Data.Array.IO import Data.IORef import Debug.Trace type LinearArray a = (Int, IORef Int, IOArray Int a) initLinearArray :: Int -> a -> LinearArray a initLinearArray l a = trace "init" ( unsafePerformIO (do version <- newIORef 0 array <- newArray (0, l - 1) a return (0, version, array))) readLinearArray :: Int -> (LinearArray a) -> a readLinearArray l !(ver, realver, arr) = trace "read" ( unsafePerformIO (do version <- readIORef realver element <- readArray arr l if (version == ver) then return element else error "Non-Linear read of linear Array")) writeLinearArray :: Int -> a -> LinearArray a -> LinearArray a writeLinearArray l e !(ver, realver, arr) = trace "write" ( unsafePerformIO (do version <- readIORef realver if (version == ver) then do writeIORef realver $ ver + 1 writeArray arr l e return (ver + 1, realver, arr) else error "Non-Linear write of linear Array")) linearArrayToList :: Int -> Int -> (LinearArray a) -> [a] linearArrayToList c m !a = trace "toList" ( if (c >= m) then [] else (readLinearArray c a) : (linearArrayToList (c + 1) m a)) eratostenesTest :: Int -> [Bool] eratostenesTest length = let strikeMult :: Int -> Int -> Int -> (LinearArray Bool) -> (LinearArray Bool) strikeMult div cur len arr = trace "smStart" ( if (cur >= len) then trace "arr" arr else let arr = trace "write" $ writeLinearArray cur False arr in trace "strikeMult2" $ strikeMult div (cur + div) len arr) nextPrime :: Int -> Int -> (LinearArray Bool) -> (LinearArray Bool) nextPrime cur len !arr = if (cur >= len) then arr else if (readLinearArray cur arr) then let arr = trace "strikeMult" $ strikeMult cur (cur + cur) len arr in trace "nextPrime" $ nextPrime (cur + 1) len arr else nextPrime (cur + 1) len arr ini = trace "ini" (initLinearArray length True) theArray = trace "nextPrimeCall" $ nextPrime 2 length ini in linearArrayToList 0 length theArray On 22.07.13 9:01 AM, Richard A. O'Keefe wrote:
On 21/07/2013, at 7:36 AM, Evan Laforge wrote:
Just by coincidence, I recently wrote this:
This is a BEAUTIFUL example. I think we may disagree about what it's an example OF, however. I found the code a little difficult to follow, but when that's fixed up, there's no longer any reason to want non-recursive let, OR a monad.
I've run out of time tonight, but hope to say more tomorrow.
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