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• is this a simple monad example?

  - by zcaudate

  This is my attempt to grok monadic functions after watching http://channel9.msdn.com/Shows/Going+Deep/Brian-Beckman-Dont-fear-the-Monads. h uses bind to compose together two arbitrary functions f and g. What is the unit operator in this case? ;; f :: int - [str] ;; g :: str = [keyword] ;; bind :: [str] - (str - [keyword]) - [keyword] ;; h :: int…

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• Missing something with Reader monad - passing the damn thing around everywhere

  - by Richard Huxton

  Learning Haskell, managing syntax, have a rough grasp of what monads etc are about but I'm clearly missing something. In main I can read my config file, and supply it as runReader (somefunc) myEnv just fine. But somefunc doesn't need access to the myEnv the reader supplies, nor do the next couple in the chain. The function that needs something…

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• Haskell Monad bind currying

  - by Chime

  I am currently in need of a bit of brain training and I found this article on Haskell and Monads I'm having trouble with exercise 7 re. Randomised function bind. To make the problem even simpler to experiment, I replaced the StdGen type with an unspecified type. So instead of... bind :: (a -> StdGen -> (b,StdGen)) -> (StdGen ->…

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• stm monad problem

  - by Alex

  This is just a hypothetical scenario to illustrate my question. Suppose that there are two threads and one TVar shared between them. In one thread there is an atomically block that reads the TVar and takes 10s to complete. In another thread is an atomically block that modifies the TVar every second. Will the first atomically block ever…

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• Threading extra state through a parser in Scala

  - by Travis Brown

  I'll give you the tl;dr up front I'm trying to use the state monad transformer in Scalaz 7 to thread extra state through a parser, and I'm having trouble doing anything useful without writing a lot of t m a -> t m b versions of m a -> m b methods. An example parsing problem Suppose I have a string containing nested parentheses…

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• Why are side-effects modeled as monads in Haskell?

  - by bodacydo

  Could anyone give some pointers on why the unpure computations in Haskell are modeled as monads? I mean monad is just an interface with 4 operations, so what was the reasoning as modeling side-effects in it?

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• Haskell IO Passes to Another Function

  - by peterwkc

  This question here is related to http://stackoverflow.com/questions/3066956/haskell-input-return-tuple I wonder how we can passes the input from monad IO to another function in order to do some computation. Actually what i want is something like -- First Example test = savefile investinput -- Second Example maxinvest :: a…

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• The reason for MonadState get and put?

  - by CiscoIPPhone

  I'm reading the Monads chapter in Real World Haskell (chapter 14). A function is defined as follows: type RandomState a = State StdGen a getRandom :: Random a => RandomState a getRandom = get >>= \gen -> let (val, gen')= random gen in put gen' >> return val I don't really understand the…

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• What advantage does Monad give us over an Applicative?

  - by arrowdodger

  I've read this article, but didn't understand last section. The author says that Monad gives us context sensitivity, but it's possible to achieve the same result using only an Applicative instance: let maybeAge = (\futureYear birthYear -> if futureYear < birthYear then yearDiff birthYear futureYear else…

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• Monad in plain English? (For the OOP programmer with no FP background)

  - by fig-gnuton

  In terms that an OOP programmer would understand (without any functional programming background), what is a monad? What problem does it solve and what are the most common places it's used? EDIT: To clarify the kind of understanding I was looking for, let's say you were converting an FP application that had monads…

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• Help a C# developer understand: What is a monad?

  - by Charlie Flowers

  There is a lot of talk about monads these days. I have read a few articles / blog posts, but I can't go far enough with their examples to fully grasp the concept. The reason is that monads are a functional language concept, and thus the examples are in languages I haven't worked with (since I haven't used a…

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• Common practice for higher-order-polymorphism in scala

  - by raichoo

  Hi, I'm trying to grasp higher-order-polymophism in scala by implementing a very basic interface that describes a monad but I come across a problem that I don't really understand. I implemented the same with C++ and the code looks like this: #include <iostream> template <typename T> class…

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• Must a Language that Implements Monads be Statically Typed?

  - by Morgan Cheng

  I am learning functional programming style. From this link http://channel9.msdn.com/shows/Going+Deep/Brian-Beckman-Dont-fear-the-Monads/, Brian Beckman gave a brilliant introduction about Monad. He mentioned that Monad is about composition of functions so as to address complexity. A Monad includes a…

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• Overloading generic implicit conversions

  - by raichoo

  Hi I'm having a little scala (version 2.8.0RC1) problem with implicit conversions. Whenever importing more than one implicit conversion the first one gets shadowed. Here is the code where the problem shows up: // containers class Maybe[T] case class Nothing[T]() extends Maybe[T] case class…

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• Very slow guards in my monadic random implementation (haskell)

  - by danpriduha

  Hi! I was tried to write one random number generator implementation, based on number class. I also add there Monad and MonadPlus instance. What mean "MonadPlus" and why I add this instance? Because of I want to use guards like here: -- test.hs -- import RandomMonad import…

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• Monads with Join() instead of Bind()

  - by MathematicalOrchid

  Monads are usually explained in turns of return and bind. However, I gather you can also implement bind in terms of join (and fmap?) In programming languages lacking first-class functions, bind is excruciatingly awkward to use. join, on the other hand, looks quite easy. I'm not…

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• Scala: "Parameter type in structural refinement may not refer to an abstract type defined outside th

  - by raichoo

  Hi, I'm having a problem with scala generics. While the first function I defined here seems to be perfectly ok, the compiler complains about the second definition with: error: Parameter type in structural refinement may not refer to an abstract type defined outside that…

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• Is there a good way to QuickCheck Happstack.State methods?

  - by Paul Kuliniewicz

  I have a set of Happstack.State MACID methods that I want to test using QuickCheck, but I'm having trouble figuring out the most elegant way to accomplish that. The problems I'm running into are: The only way to evaluate an Ev monad computation is in the IO monad via…

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• An unusual type signature

  - by Travis Brown

  In Monads for natural language semantics, Chung-Chieh Shan shows how monads can be used to give a nicely uniform restatement of the standard accounts of some different kinds of natural language phenomena (interrogatives, focus, intensionality, and quantification). He…

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• Haskell: monadic takeWhile?

  - by Mark Rushakoff

  I have some functions written in C that I call from Haskell. These functions return IO (CInt). Sometimes I want to run all of the functions regardless of what any of them return, and this is easy. For sake of example code, this is the general idea of what's…

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• Seeking Functional Programming Lexicon

  - by Randall Schulz

  Hi, Knowing the argot of a field helps me a lot, especially since it allows me to converse intelligently with those who know a lot more than I, so I would like to find a good lexicon of Functional Programming terms. E.g., I repeatedly encounter these:…

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• Performance of looping over an Unboxed array in Haskell

  - by Joey Adams

  First of all, it's great. However, I came across a situation where my benchmarks turned up weird results. I am new to Haskell, and this is first time I've gotten my hands dirty with mutable arrays and Monads. The code below is based on this example.…

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• How and why is ap defined as liftM2 id in Haskell

  - by luke_randall

  Whilst trying to better understand Applicative, I looked at the definition of <*, which tends to be defined as ap, which in turn is defined as: ap :: (Monad m) => m (a -> b) -> m a -> m b ap = liftM2 id …

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• Linking/Combining Type Classes in Haskell

  - by thegravian

  Say I have two type classes defined as follows that are identical in function but different in names: class Monad m where (>>=) :: m a -> (a -> m b) -> m b return :: a -> m a class PhantomMonad p where pbind :: p a…

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• Haskell: 'No instance for' arising from a trivial usage of Regex library

  - by artemave

  Following the (accepted) answer from this question, I am expecting the following to work: Prelude Text.Regex.Posix Text.Regex.Base.RegexLike Text.Regex.Posix.String> makeRegex ".*" (makeRegex is a shortcut for makeRegexOpts with predefined…

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