Re: [Haskell-cafe] Re: Books for advanced Haskell
On Sun, Mar 7, 2010 at 10:53 PM, Stephen Tetley stephen.tet...@gmail.com wrote: 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. So I want the book describes how to make type-level programming up to the practice. I think this would cover how to provide simple interface for types and contexts with full benefits, how to produce friendly error message in type-errors, when to use UndecidableInstances, and how to get rid of such itchy things. -nwn ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Books for advanced Haskell
Dan Piponi wrote: 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. I'd wager that due to wildly different backgrounds, the difficulties are also different for each person. Lately, I've been dreaming about some kind of (online-) book that covers the missing (Haskell) prerequisites in a modular fashion. I.e. when writing a blog post, you no longer have to implicitly assume / guess something about your readers' background, you can specify it explicitly, something like this module MyBlogPost where import EverythingYouKnowAboutMonoids import MonadsAsComputation import IntuitionAboutDifferentialForms import UnderstandingCurryHowards The book itself would track internal dependencies in the same fashion. Now, tracking each and every prerequisite is impossible, but some useful approximation ought to be possible. I remember a comment about people having trouble understanding monads on #haskell that might be solved by such a book thing. Namely, the main problem was that (a subset of) people simply lacked required Haskell knowledge, like having a good grasp on the distinction between type and type constructor, or being unfamiliar with Maybe. Likewise, it appears that me that a substantial fraction of the comments on RWH are actually about missing or just recently understood prerequisites. Hence, I think that tracking prerequisites explicitly has potential. Regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Books for advanced Haskell
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Books for advanced Haskell
Hi, oh boy it took me a while to find this post again. Perhaps not exactly. I build monads left and right, but that's because I don't understand much else. :-) Before you get all hung up on them, though, I recommend reading The Typeclassopedia,[1], which will introduce you to all of the monad's friends and family. 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? Günther ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Books for advanced Haskell
2010/3/5 Günther Schmidt gue.schm...@web.de: 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Books for advanced Haskell
Stephen Tetley wrote: 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). ... or use the free term algebra approach outlined in http://apfelmus.nfshost.com/articles/operational-monad.html when the semantics of the effects are a bit tricky to fit into existing transformers. My package operational http://projects.haskell.org/operational/ contains a bunch of examples. Regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
