[Haskell-cafe] Progress indications

[Thomas Hartman]

Obviously heaps better than what I initially proposed.

However, I would argue to go boldly with unsafePerformIO, which is the 
same thing Debug.Trace uses

http://darcs.haskell.org/ghc-6.6/packages/base/Debug/Trace.hs

since we are after debug.trace -like behavior.

In particular, you wouldn't be able to use the unsafeInterleaveIO version 
to do a progress indicator for the function I initially proposed

> t = foldr (+) 0 [1..10000]

since your lift would wind up being lifted into IO. But you would be able 
to use the unsafePerformIO version, just like in what I initially proposed 
you could use trace.

t = foldr (+) 0 ( lessSafeMonitoryProgress f [1..10000] )
  where f i | i mod 1000 == 0 = (putStrLn . show ) i
                   | otherwise = return ()
 
Make sense?

thomas.





David Roundy <droundy at darcs.net> 
Sent by: haskell-cafe-bounces at haskell.org
11/28/2007 06:16 PM

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haskell-cafe at haskell.org
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Subject
Re: [Haskell-cafe] Progress indications






On Wed, Nov 28, 2007 at 05:58:07PM -0500, Thomas Hartman wrote:
> maybe Debug.Trace? like...
> 
> import Debug.Trace
> 
> t = foldr debugf 0 [1..10000]
> 
> f :: Int -> Int -> Int
> f = (+)
> 
> -- same typesig as f
> debugf :: Int -> Int -> Int
> debugf x y | y `mod` 1000 == 0 = x + (trace (show y) y)
> debugf x y = x + y

Or, more flexibly:

import System.IO.Unsafe ( unsafeInterleaveIO )

monitorProgress :: (Int -> IO ()) -> [a] -> IO [a]
monitorProgress f xs = mapM f' $ zip [0..] xs
   where f' (n,x) = unsafeInterleaveIO (f n >> return x)

You could, of course, make this a function

lessSafeMonitoryProgress :: (Int -> IO ()) -> [a] -> [a]

by using unsafePerformIO instead of unsafeInterleaveIO, but that seems
slightly scary to me.

In any case, you can stick this on whichever of the lists you want to
monitor the progress of.
-- 
David Roundy
Department of Physics
Oregon State University
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