Re: [Haskell-cafe] Monads

2012-09-30 Thread Tillmann Rendel 

Vasili I. Galchin wrote:

I would an examples of monads that are pure, i.e. no side-effects.


One view of programming in monadic style is: You call return and >>= all 
the time. (Either you call it directly, or do notation calls it for 
you). So if you want to understand whether a monad "has side-effects", 
you should look at the implementation of return and >>=. If the 
implementation of return and >>= is written in pure Haskell (without 
unsafePerformIO or calling C code etc.), the monad is pure.


  Tillmann

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Re: [Haskell-cafe] Monads

2012-09-30 Thread Albert Y. C. Lai 

On 12-09-29 09:57 PM, Vasili I. Galchin wrote:

 I would an examples of monads that are pure, i.e. no side-effects.


What does "side effect" mean, to you? Definition?

Because some people say "State has no side effect", and some other 
people say "State has side effects". The two groups use different 
definitions.


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Re: [Haskell-cafe] One of the new buzz phrases is "Event-Sourcing"; is Haskell suitable for this?

2012-09-30 Thread Marcelo Sousa 
Hi,

On Sun, Sep 30, 2012 at 4:22 AM, Alberto G. Corona  wrote:
> It´´s a very iteresting concept.
>
> The Workflow Monad transformer [1], in Control.Workflow perform
> logging and recovery of application istate from the log created.
> It has no implementation of roll-back or limited recovery upto a
> point, but this is easy to implement.

Is Control.Workflow similar with acid-state with respect to the way
you recovery the current state?

> It also has many inspection and synchronization primitives. It has
> been used also for translating the log of a program and recovering the
> state in another machine. The log  can be pretty-printed for
> debugging.

Can you "somehow" recover impure (IO) computations?

> [1] http://hackage.haskell.org/package/Workflow

Regards,
Marcelo

> 2012/9/30 KC :
>> http://martinfowler.com/eaaDev/EventSourcing.html
>>
>> http://martinfowler.com/articles/lmax.html
>>
>>
>> --
>> --
>> Regards,
>> KC
>>
>> ___
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>
>
>
> --
> Alberto.
>
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Re: [Haskell-cafe] One of the new buzz phrases is "Event-Sourcing"; is Haskell suitable for this?

2012-09-30 Thread Alberto G. Corona 
Hi,Marcelo,
No. .Acid state is  explcitly managed by the process by means of state
management primitives

In Control.Workflow the state is managed in a implicit way.

It is a monad transformer mainly  is designed for wrapping IO computations.

the lifting primitive, step, store the intermediate result and recover the
application state.

in acid state the process choose what to write in the state

in workflow the state written is the complete state of the process.

See the example in the documentation. the process ,

http://hackage.haskell.org/packages/archive/Workflow/0.7.0.7/doc/html/Control-Workflow.html


import Control.Workflow
import Control.Concurrent(threadDelay)
import System.IO (hFlush,stdout)

mcount n= do step

$  do
   putStr (show n ++ " ")
   hFlush stdout
   threadDelay 100
 mcount (n+1)
 return () -- to disambiguate the return type

main= exec1 

 "count"  $ mcount (0 :: Int)

>>> *runghc demos\sequence.hs*>0 1 2 3
>CTRL-C Pressed>>> *runghc demos\sequence.hs*>3 4 5 6 7
>CTRL-C Pressed>>> *runghc demos\sequence.hs*>7 8 9 10 11
...

in subsequent executions the process start to execute IO computations from
the last point logged:

As the documentation says  some side effect can be re-executed after
recovery if the log is not complete. This may happen after an unexpected
shutdown (in this case Contro-C has been pressed) or due to an asynchronous
log writing policy. (see
syncWrite
) (writing is cached).

Althoug this is not event sourcing, The logging and recovery facilities can
be used for even sourcing.

Alberto

2012/9/30 Marcelo Sousa 

> Hi,
>
> On Sun, Sep 30, 2012 at 4:22 AM, Alberto G. Corona 
> wrote:
> > It´´s a very iteresting concept.
> >
> > The Workflow Monad transformer [1], in Control.Workflow perform
> > logging and recovery of application istate from the log created.
> > It has no implementation of roll-back or limited recovery upto a
> > point, but this is easy to implement.
>
> Is Control.Workflow similar with acid-state with respect to the way
> you recovery the current state?
>
> > It also has many inspection and synchronization primitives. It has
> > been used also for translating the log of a program and recovering the
> > state in another machine. The log  can be pretty-printed for
> > debugging.
>
> Can you "somehow" recover impure (IO) computations?
>
> > [1] http://hackage.haskell.org/package/Workflow
>
> Regards,
> Marcelo
>
> > 2012/9/30 KC :
> >> http://martinfowler.com/eaaDev/EventSourcing.html
> >>
> >> http://martinfowler.com/articles/lmax.html
> >>
> >>
> >> --
> >> --
> >> Regards,
> >> KC
> >>
> >> ___
> >> Haskell-Cafe mailing list
> >> Haskell-Cafe@haskell.org
> >> http://www.haskell.org/mailman/listinfo/haskell-cafe
> >
> >
> >
> > --
> > Alberto.
> >
> > ___
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> > http://www.haskell.org/mailman/listinfo/haskell-cafe
>



-- 
Alberto.
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Re: [Haskell-cafe] One of the new buzz phrases is "Event-Sourcing"; is Haskell suitable for this?

2012-09-30 Thread Bardur Arantsson 
On 09/30/2012 02:46 AM, KC wrote:
> http://martinfowler.com/eaaDev/EventSourcing.html
> 
> http://martinfowler.com/articles/lmax.html
> 
> 

Sure, why not? See

http://hackage.haskell.org/package/cqrs-0.8.0
and http://hackage.haskell.org/package/cqrs-example-0.8.0

for an example application.

I should note that the "cqrs" package API is by no means finalized;
there are some limitations(*) to the current implementation, but I've
not had time to actually get rid of those limitations.

(*) The major ones being the requirement for a global version number and
lack of streaming event sourcing.




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Re: [Haskell-cafe] Monads

2012-09-30 Thread wren ng thornton 

On 9/30/12 7:00 AM, Tillmann Rendel wrote:

Vasili I. Galchin wrote:

I would an examples of monads that are pure, i.e. no side-effects.


One view of programming in monadic style is: You call return and >>= all
the time. (Either you call it directly, or do notation calls it for
you). So if you want to understand whether a monad "has side-effects",
you should look at the implementation of return and >>=. If the
implementation of return and >>= is written in pure Haskell (without
unsafePerformIO or calling C code etc.), the monad is pure.


I'm not sure return and bind will give you the information you seek, 
however. In order to obey the monad laws, return and bind must be (for 
all intents and purposes) pure.


The place to look for "impurities" is the primitive operations of the 
monad; i.e., those which cannot be implemented using return and bind. 
These are the operations which take you out of a free monad and the 
operations which must be given some special semantic interpretation.



--
Live well,
~wren

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Re: [Haskell-cafe] Possible bug in Criterion or Statistics package

2012-09-30 Thread Ben Gamari 
Aleksey Khudyakov  writes:

> On 13.08.2012 19:43, Ryan Newton wrote:
>> Terrible!  Quite sorry that this seems to be a bug in the monad-par library.
>>
>> I'm copying some of the other monad-par authors and we hopefully can get
>> to the bottom of this.  If it's not possible to create a smaller
>> reproducer, is it possible to share the original test that triggers this
>> problem?  In the meantime, it's good that you can at least run without
>> parallelism.
>
> Here is slightly simplified original test case. By itself program is 
> very small but there is statistics and criterion on top of the monad-par
> Failure occurs in the function 
> Statistics.Resampling.Bootstrap.bootstrapBCA. However I couldn't trigger 
> bug with mock data.
>
Has there been any progress or an official bug report on this?

Cheers,

- Ben


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Re: [Haskell-cafe] Monads

2012-09-30 Thread Jake McArthur 
On Sep 30, 2012 10:56 AM, "Albert Y. C. Lai"  wrote:
>
> On 12-09-29 09:57 PM, Vasili I. Galchin wrote:
>>
>>  I would an examples of monads that are pure, i.e. no
side-effects.
>
>
> What does "side effect" mean, to you? Definition?

When discussing monads, at least, a side effect is an effect that is
triggered by merely evaluating an expression. A monad is an interface that
decouples effects from evaluation.

>
> Because some people say "State has no side effect", and some other people
say "State has side effects". The two groups use different definitions.
>
>
> ___
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Re: [Haskell-cafe] Monads

2012-09-30 Thread Kristopher Micinski 
On Sun, Sep 30, 2012 at 6:33 PM, Jake McArthur  wrote:
>
> On Sep 30, 2012 10:56 AM, "Albert Y. C. Lai"  wrote:
>>
>> On 12-09-29 09:57 PM, Vasili I. Galchin wrote:
>>>
>>>  I would an examples of monads that are pure, i.e. no
>>> side-effects.
>>
>>
>> What does "side effect" mean, to you? Definition?
>
> When discussing monads, at least, a side effect is an effect that is
> triggered by merely evaluating an expression. A monad is an interface that
> decouples effects from evaluation.
>

Ohh, I like that quote.., that's another good one..

kris

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Re: [Haskell-cafe] One of the new buzz phrases is "Event-Sourcing"; is Haskell suitable for this?

2012-09-30 Thread Joey Adams 
On Sat, Sep 29, 2012 at 8:46 PM, KC  wrote:
> http://martinfowler.com/eaaDev/EventSourcing.html
>
> http://martinfowler.com/articles/lmax.html

This notion of "Capture all changes to an application state as a
sequence of events" sounds a lot like what John Carmack did in Quake 3
[1]:

> I settled on combining all forms of input into a single system event queue, 
> similar to the windows message queue. My original intention was to just 
> rigorously define where certain functions were called and cut down the number 
> of required system entry points, but it turned out to have much stronger 
> benefits.

 [1]: http://www.team5150.com/~andrew/carmack/johnc_plan_1998.html#d19981014

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Re: [Haskell-cafe] Monads

2012-09-30 Thread Albert Y. C. Lai 

On 12-09-30 06:33 PM, Jake McArthur wrote:

When discussing monads, at least, a side effect is an effect that is
triggered by merely evaluating an expression. A monad is an interface
that decouples effects from evaluation.


I don't understand that definition. Or maybe I do subconsciously.

I have

s :: State Int ()
s = do { x <- get; put (x+1) }

Is there an effect triggered by merely evaluating s?

I have

m :: IO ()
m = if True then putStrLn "x" else putChar 'y'

Is there an effect triggered by merely evaluating m?

What counts as "evaluate"?

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