When I read "side-effects", I understand it as "unwanted effects", like "aliasing", and "effects depending on the order of execution". I'm not sure if my understanding here is correct. I hope Haskell does not allow "side-effects" but only "effects", meaning the monads do not allow you to write the typical ill-behaving code you get when doing real imperative programming, enforcing a single wiring of execution, not allowing the capture of the RealWorld object. In Concurrent Clean special compiler support is present to enforce "uniqueness typing", and in Haskell special compiler support is available to make the RealWorld object not available at runtime (so e.g. you can't put the RealWorld object in a list). Is this correct? BTW: What is the correct word in Haskell for "object"? I mean the (lazy) value you get when evaluating a data constructor? -----Original Message----- From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Kim-Ee Yeoh Sent: Monday, August 13, 2007 15:30 To: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] Explaining monads Ronald Guida wrote:
Given the question "What is a Monad", I would have to say "A Monad is a device for sequencing side-effects."
There are side-effects and there are side-effects. If the only monad you use is Maybe, the only side-effect you get is a slight warming of the CPU. Dave Menendez pointed to that fine Wadler link earlier. Please read it. To wit, in Section 2: "Explaining Monads" the "essence of an algorithm can become buried under the plumbing required to carry data from its point of creation to its point of use." Monads can help keep the clarity of your code untrammelled by providing implicit plumbing, "side-channels" if you prefer, when data is moved around. In fact if you follow Wadler all the way to his monadic expression evaluator, you see that you could modularize your code in awesomely cool ways. You get to see how the kernel of the expression evaluator could be written for a generic monad and compiled once-and-for-all. Any additional feature (the "variations") is coded by enriching the monad. Monads are powerful devices for modularizing code. Available for free. Only in Haskell (thanks to type classes!). Get them today. "Side-effects" is a piece of linguistic cruft played fast-and-loose by too many people in this game. "Sequencing" suffers the same disease. -- View this message in context: http://www.nabble.com/Explaining-monads-tf4244948.html#a12126170 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe No virus found in this incoming message. Checked by AVG Free Edition. Version: 7.5.476 / Virus Database: 269.11.15/949 - Release Date: 12/08/2007 11:03 No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.476 / Virus Database: 269.11.15/949 - Release Date: 12/08/2007 11:03