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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--
Andreas Abel <>< Du bist der geliebte Mensch.
Theoretical Computer Science, University of Munich
Oettingenstr. 67, D-80538 Munich, GERMANY
andreas.a...@ifi.lmu.de
http://www2.tcs.ifi.lmu.de/~abel/
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