Hello David, I do agree, Monads as *such* may not be super complicated. That said I see monads constructed by some of the Haskell big brains and used that would never ever occur to me. And then I read their papers, blog posts and everything I can get my hands on and still not get it. A shining example are Dan Piponis blog posts. Not his fault, mind. All I see is that there is something powerful. I also notice that the big brains construct monads in many different ways and thus giving them entirely different capabilities. An example of this is some techniques turn CBV to CBN or CBL while other techniques null this. I just cannot find my way through all this, in part because the information is scattered through many papers from many different authors on what does seem quite similar subjects. It's bloody confusing. So, no book, eh? Günther Am 01.03.10 16:41, schrieb David Leimbach:

I don't think a Haskell-monad book would be terribly interesting. A book on taking the pieces of category theory, with a little bit more of the math, to apply to Haskell would be greatly interesting to me.

Also a book on learning what to look for for measuring Haskell performance in space and time + optimization seems like it'd be a good thing to have as well.

Monad in itself is really simple. Some of the implementations of Monad can be a little mind bending at times, but the Monad itself is not really that complicated.

Dave

2010/3/1 Günther Schmidt mailto:gue.schmidt@web.de>

Hi all,

there seems to be a huge number of things that monads can be used for. And there are lots of papers, blog posts, etc. describing that, some more or less accessible.

Apart from monads there are of course also Applicative Functors, Monoids, Arrows and what have you. But in short the Monad thingy seems to be the most "powerful" one of them all.

Is there a book that specializes on Monads? A Haskell-Monad book?

Günther

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2010/3/4 Matthias Görgens

...

A shining example are Dan Piponis blog posts. Not his fault, mind. All I see is that there is something powerful. I also notice that the big brains construct monads in many different ways and thus giving them entirely different capabilities. An example of this is some techniques turn CBV to CBN or CBL while other techniques null this.

What are CBV, CBN and CBL?

It's a series of 3 TLAs designed to make people who know what they are feel more in-the-know than those who don't. :-) Dave

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2010/3/5 Michael Vanier :

CBV = Call By Value, essentially strict evaluation CBN = Call By Name, essentially lazy evaluation CBL = I don't know.

Commercial Bill of Lading? :p http://www.google.com.au/search?q=define%3ACBL -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com

I assumed that CBL meant Call By Location

Am 04.03.10 23:19, schrieb Matthias Görgens:

...

A shining example are Dan Piponis blog posts. Not his fault, mind. All I see is that there is something powerful. I also notice that the big brains construct monads in many different ways and thus giving them entirely different capabilities. An example of this is some techniques turn CBV to CBN or CBL while other techniques null this.

What are CBV, CBN and CBL?

Hi Matthias, sry for using the abbreviations. CBV = Call by Value, CBN = Call by Name, CBL = Call by Need (Lazy) Günther

Hello Dan, sorry, I did not mean to single you out, I love your stuff. But I can't find the thread through it, I pretty much lack basics / fundamentals to digest it. But I do see *power* and it bugs the crap out of me that I can't grasp it yet. One example was your response to my DSL posts on this list, where you said that in a way ever/any? monad can be considered a DSL. At that time I was totally unable to understand the sentence as such nor its significance. I kinda was focused on the "finally tagless" approach from Oleg for DSLs. Now that I do understand the significance of that statement I try to really figure it out. Mostly it finally occurred to me because of Heinrich Apfelmus's "Operational monad tutorial". Which I have no means of comprehending either because he's using GADTs. From your blog I got the impression that you focus on monads, but I may have gotten that wrong. You seem to be able to take this a very long way, without using much fancy type level arithmetic or elaborate class schemes. That gives me hope in a way because so far I'm not happy with serious type-level programming. Right now I'm reading J. Hughes '95 paper "The design of a pretty printing library" which I believe I can grasp, may even before the years end. Would you consider creating a "guide" through your blog, something like "read this first, then that"? Günther Am 06.03.10 21:14, schrieb Dan Piponi:

Günther,

...

A shining example are Dan Piponis blog posts.

When you get stuck, post a comment saying where so that I can learn what people find difficult.

On the other hand, I understand how intangible not-understanding can be, so it can be hard to point to where the problem is. -- Dan

Hi All What is the state-of-the-practice in type-level programming? I know Günther started this thread about monads, but I seem to remember him having a long running problem with "typeful" database programming, and I wonder if some of his problems are really in the later area. Compared to monads, type-level programming seems much more a wild frontier - scattered docs, fewer definitive examples, no de-facto standard library. The HList paper provides an implementation of extensible records - have people found it palatable in practice? Roel van Dijk's Hackage rev-dependency page indicates only 1 package depends on it, and HaskellDB appears, at least from a cursory look, to support its own re-implementation of of HList. Best wishes Stephen

2010/3/5 Günther Schmidt :

When you say that you use monads to the left and right, do you mean using pre-defined monad instances, or do you construct your own, ie. define something to be an instance of a monad and then write codes using that instance?

Hi Günther One view of Monads (in the context of Haskell) is that they provide effects: - State Monad - state - Maybe monad - partiality (answer or failure) - Error monad - partiality with some answer type for the failure (variation of Maybe) - Environment monad aka Reader - read only access to "context" - Writer monad - tracing - write only access to "context" - List monad - non-determinism - Resumption monad - interleaving for (simulating) concurrency Because a monad (in the context of Haskell) is an implementation of the Monad type class and the obligation to satisfy the monad laws, there are quite few 'variations on a theme' monads e.g.: Jerzy Karczmarczuk's time travel State Monad http://users.info.unicaen.fr/~karczma/arpap/revpearl.pdf Ralf Hinze's - partial CPS monad "Typed Quote/Antiquote" -- paper currently off-line unfortunately The codensity monad - variation on the CPS monad http://www.cs.nott.ac.uk/~gmh/wrapper2.pdf Plus unique but currently rare monads, e.g: Probability http://web.engr.oregonstate.edu/~erwig/papers/PFP_JFP06.pdf These latter monads still represent effects, and one should be able to define them as transformers (I think, I haven't actually tried), but they less common presumably because people have found less need for the effects they provide. There are also monads where I'd struggle to claim any analogous 'effect' (maybe they have one and I don't understand the presentation, or maybe they don't actually have one) - generally these monads don't seem to have escaped from the mathematical world to the Haskell world. E.g. the monad of locales, power locales http://www.cs.bham.ac.uk/~mhe/papers/pinjective.pdf To work with monads in Haskell its not ignoble simple to decide what effect or combination of effects you want and use the relevant monad (for a single effect) or build a transformer (for multiple effects). Best wishes Stephen

On 04/03/2010, at 8:28 PM, Curt Sampson wrote:

... I recommend reading "The Typeclassopedia,"[1], which will introduce you to all of the monad's friends and family.

[1]: http://byorgey.wordpress.com/2009/03/16/monadreader-13-is-out/

I'd love to read a book-length version of the Typeclassopedia, with more examples of applications of the various type classes, and pitched at a more ignorant reader (like myself). Tom