[Haskell-cafe] style question: Writer monad or unsafeIOToST?

Gregory Wright gwright at comcast.net
Thu Aug 24 19:08:09 EDT 2006

Hi Bulat!

On Aug 24, 2006, at 1:17 PM, Bulat Ziganshin wrote:

> Hello Gregory,
> Thursday, August 24, 2006, 7:29:47 PM, you wrote:
>> it seems that unsafeIOToST is safe in this case, in the sense that
> why you are stuck to ST monad? isn't it better to use just IO monad?

The IO monad may be more appropriate.  The simulation evolved out
of a different (and simpler) simulate for a 6502 microcontroller which
used the ST monad.  I had thought at the time that there may be multiple
threads in the full simulation, so using state threads seemed a good
idea at the time.  (The full simulation may still need multiple threads;
I don't know yet.)

As it stands, the code I had written was almost correct.  I needed a
lazy version of the WriterT monad to make it work.  Chris Kuklewicz
pointed this out to me. The toy model now works with both the lazy  
(called LogT here) and the unsafe* operation.

Some performance data:  using unsafeIOToST to write log messages
directly to the output, the simulation does 10^7 state updates in  
about 45 seconds
on my 1.5 GHz ppc G4.  Using LogT, with a list of strings as the monoid,
it takes about 7 minutes to do the same, and the system swaps heavily
during the last few minutes.  Not surprising, given that the mappend
operation is not very efficient for the list monoid.

Is there a simple monoid structure I could use instead of a list to  
the log string incrementally?  I don't care if the order of the  
output is

> and about total style - again, you can use my lib or write this
> yourself so that all you reference operations will work independent on
> Monad used and you can freely experiment with different monads without
> rewriting whole code:
> class Ref m r | m->r where
>   newRef
>   readRef
>   writeRef
> instance Ref IO IORef
>   writeRef r x = writeIORef r $! x
> instance (Ref m r) => Ref (WriterT m) r where
>   writeRef = lift . writeRef
> and so on...

The code snippet above looks like a very good idea.  The monad
dependent operations combined with "lift" seem more complicated
than necessary.  "lift" in particular often seems like plumbing that
should not be necessary.

Best Wishes,

> ps to Brian: it is why i was so interested in your idea. writing
> monad-independent code, including code that can be applied to any
> monad lifted from ST or IO, looks for me very promising idea, somewhat
> that will be widely used in future
> -- 
> Best regards,
>  Bulat                            mailto:Bulat.Ziganshin at gmail.com
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