Global variable question

Joe Van Dyk

30 Dec 2009 30 Dec '09

8:49 p.m.

What's the appropriate way to write this in Haskell? # Ruby code, ignore the thread-unsafety for now. class Ticker @@tick = 0 def self.update! @@tick += 1 end def self.current return @@tick end end Ticker.update! # 1 Ticker.update! # 2 Ticker.current # 2 -- Joe Van Dyk http://fixieconsulting.com

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Brent Yorgey

30 Dec 30 Dec

8:59 p.m.

On Wed, Dec 30, 2009 at 05:49:12PM -0800, Joe Van Dyk wrote:

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What's the appropriate way to write this in Haskell?

# Ruby code, ignore the thread-unsafety for now. class Ticker @@tick = 0 def self.update! @@tick += 1 end def self.current return @@tick end end

Ticker.update! # 1 Ticker.update! # 2 Ticker.current # 2

If you really want a global ticker, use the State monad: tick :: State Int () tick = modify (+1) -- increment the counter twice then return its new value mainStuff :: State Int Int mainStuff = tick >> tick >> get -Brent

Edward Z. Yang

9:05 p.m.

Excerpts from Joe Van Dyk's message of Wed Dec 30 20:49:12 -0500 2009:

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What's the appropriate way to write this in Haskell?

Hi Joe, The Ticker class is an example of stateful computation, which we try to avoid in Haskell. What are you really trying to do? * If you a number, and you want to add to it, (+1) works plenty well. * If you are going to be making ticks, but you don't care about reading the ticks until the end of the computation, the Writer monad is of interest. * If you need to update the tick count and read it, the State monad is a good bet. * If you're interested in curiosities from imperative land, STRef, IORef and other similar creatures may be helpful. Cheers, Edward

Joe Van Dyk

31 Dec 31 Dec

12:37 p.m.

On Wed, Dec 30, 2009 at 6:05 PM, Edward Z. Yang wrote:

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Excerpts from Joe Van Dyk's message of Wed Dec 30 20:49:12 -0500 2009:

...
What's the appropriate way to write this in Haskell?

Hi Joe,

The Ticker class is an example of stateful computation, which we try to avoid in Haskell. What are you really trying to do?

* If you a number, and you want to add to it, (+1) works plenty well.

* If you are going to be making ticks, but you don't care about reading the ticks until the end of the computation, the Writer monad is of interest.

* If you need to update the tick count and read it, the State monad is a good bet.

* If you're interested in curiosities from imperative land, STRef, IORef and other similar creatures may be helpful.

It's like haskell is on a personal vendetta against me to make me feel dumb. -- Joe Van Dyk http://fixieconsulting.com

John Velman

6:38 p.m.

On Thu, Dec 31, 2009 at 09:37:10AM -0800, Joe Van Dyk wrote:

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On Wed, Dec 30, 2009 at 6:05 PM, Edward Z. Yang wrote:

...
Excerpts from Joe Van Dyk's message of Wed Dec 30 20:49:12 -0500 2009:

...
What's the appropriate way to write this in Haskell?

Hi Joe,

The Ticker class is an example of stateful computation, which we try to avoid in Haskell. What are you really trying to do?

* If you a number, and you want to add to it, (+1) works plenty well.

* If you are going to be making ticks, but you don't care about reading the ticks until the end of the computation, the Writer monad is of interest.

* If you need to update the tick count and read it, the State monad is a good bet.

* If you're interested in curiosities from imperative land, STRef, IORef and other similar creatures may be helpful.

It's like haskell is on a personal vendetta against me to make me feel dumb.

It's not personal. John Velman.

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-- Joe Van Dyk http://fixieconsulting.com _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

Joe Van Dyk

6:43 p.m.

On Thu, Dec 31, 2009 at 3:38 PM, John Velman wrote:

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On Thu, Dec 31, 2009 at 09:37:10AM -0800, Joe Van Dyk wrote:

...
On Wed, Dec 30, 2009 at 6:05 PM, Edward Z. Yang wrote:

...
Excerpts from Joe Van Dyk's message of Wed Dec 30 20:49:12 -0500 2009:

...
What's the appropriate way to write this in Haskell?

Hi Joe,

The Ticker class is an example of stateful computation, which we try to avoid in Haskell. What are you really trying to do?

* If you a number, and you want to add to it, (+1) works plenty well.

* If you are going to be making ticks, but you don't care about reading the ticks until the end of the computation, the Writer monad is of interest.

* If you need to update the tick count and read it, the State monad is a good bet.

* If you're interested in curiosities from imperative land, STRef, IORef and other similar creatures may be helpful.

It's like haskell is on a personal vendetta against me to make me feel dumb.

It's not personal.

Feels like it though. :(

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Brent Yorgey
Edward Z. Yang
Joe Van Dyk
John Velman