How does GHCi print partially-applied values created from “pure”?
GHCi is a little bit peculiar. In particular, when you type an expression at the prompt, it tries to interpret it in two different ways, in order: As an IO action to execute. As a value to print out. Since IO is Applicative, it is interpreting pure A as an...
How to implement Future as Applicative in Scala?
scala,concurrency,functional-programming,future,applicative
None of the methods in other answers does the right thing in case of a future that fails quickly plus a future that succeeds after a long time. But such a method can be implemented manually: def smartSequence[A](futures: Seq[Future[A]]): Future[Seq[A]] = { val counter = new AtomicInteger(futures.size) val result =...
Understanding Right Apply
The *> operator is defined, by default, as xs *> ys = id <$ xs <*> ys which in turn translates, by default, to const id <$> xs <*> ys That is, it replaces each element of xs with id to get xs' and then calculates xs' <*> ys. []...
What are the applicative functor laws in terms of pure and liftA2?
Based on McBride and Paterson's laws for Monoidal(section 7) I'd suggest the following laws for liftA2 and pure. left and right identity liftA2 (\_ y -> y) (pure x) fy = fy liftA2 (\x _ -> x) fx (pure y) = fx associativity liftA2 id (liftA2 (\x y z ->...
Meaning of “closed under composition”
In general, "X is closed under Y" means that if you take some Xs and Y them, the result is an X. For example, "the set of integers is closed under addition" means that if you take two integers and add them, the result is an integer. Therefore, saying that...
Extracting nested monadic result: m (m a) -> m a
haskell,monads,functor,applicative
The easiest way would be with join: main = join $ parseArgs <$> getArgs <*> getStdGen Personally, I would prefer the form main = join $ liftM2 parseArgs getArgs getStdGen where join :: Monad m => m (m a) -> m a liftM2 :: Monad m => (a -> b...
Why is there not 'Alternative' instance for 'Control.Applicative.Const'
I believe there is a deeper reason. While it seems there is no canonical set of rules for Alternative, in order for Alternative to make sense, there definitely ought to be a relationship between Alternative and its Applicative operations (otherwise it'd be just an arbitrary monoid.) This answer to Confused...
Why should Applicative be a superclass of Monad?
haskell,monads,applicative,fam-proposal
Well, I'm not terribly satisfied with the answers given so far, but I think the comments attached to them are a bit more compelling. So I'll summarize here: I think there's only one sensible Functor instance that follows from Applicative: fmap f fa = pure f <*> fa Assuming that's...
Must I implement Applicative and Functor to implement a Monad
haskell,monads,functor,applicative,fam-proposal
With GHC 7.10 and above, you must implement Functor and Applicative. The class definitions for Monad mandate the superclass instances: class Functor f => Applicative f where ... class Applicative m => Monad m where ... Note that once you have a Monad instance, the Functor and Applicative instances can...
Applicative Functor on Lists
fs <*> xs is more-or-less [f x | f <- fs, x <- xs]. Monads have the Applicative instance fM <*> xM = do f <- fM x <- xM return (f x) which corresponds to the list comprehension pretty directly....
Struggling with Applicative parsing
I don't really have a solution for the problem, but perhaps some intuition might help you to build applicative parsers more easily. When it comes to applicative, there are two kinds of "sequencing" that needs to be taken into consideration: The sequencing of the parsing operations: this is what determines...
Why does the Alternative typeclass need to be a sub-class of Control.Applicative
I think Alt from the semigroupoids package comes closest to being a 'standard' typeclass. https://hackage.haskell.org/package/semigroupoids-5.0.0.1/docs/Data-Functor-Alt.html#t:Alt
haskell join multi-level monad
If you note that m (n _) is a monad transformer then you can define this operation. This is certainly the case when we notice that IO (Maybe a) is the same as MaybeT IO a: we then just use MaybeT's join. We can do this in no small part...
Difference between Monad and Applicative in Haskell
My favorite example is the "purely applicative Either". We'll start by analyzing the base Monad instance for Either instance Monad (Either e) where return = Right Left e >>= _ = Left e Right a >>= f = f a This instance embeds a very natural short-circuiting notion: we proceed...
How to correctly error out in JSON parsing with Data.Aeson
parsing,haskell,monads,applicative,aeson
Essentially, you need a Parser [a] -> Parser NE.NonEmpty transformer, which is relatively easy: -- toNonEmptyP :: Parser [a] -> Parser NE.NonEmtpy toNonEmptyP p = fmap NE.nonEmpty p >>= maybe mzero return We map NE.nonEmpty on our regular list parser, which gives us Parser (Maybe NE.NonEmpty). We then inspect the...
Applicative style understanding
<**> from Control.Applicative is flip <*>. Your example can work with that, slightly rearranged: >((+) <$> Just 3 <*> Just 5) <**> ((+) <$> Just 6) Just 14 ...
scalaz using reduceLeft for applicative builder
validation,scala,scalaz,applicative
This works: |@| result type is not ValidationNel (Applicative) but ApplicativeBuilder, you need yo apply it first to some function import scalaz._, Scalaz._ val x1: ValidationNel[String, Int] = 1.successNel val x2: ValidationNel[String, Int] = 2.successNel val x3: ValidationNel[String, Int] = 3.successNel println((x1 |@| x2 |@| x3)(_ + _ + _))...
Why is f <$> g <$> x equivalent to (f . g) <$> x although <$> is not right-associative?
haskell,syntax,infix-notation,applicative,infix-operator
Why is f <$> g <$> x equivalent to (f . g) <$> x ...well, this isn't so much a functor-thing as a Haskell-thing. The reason it works is that functions are functors. Both <$> operators work in different functors! f <$> g is in fact the same as...
Applicative that increments the environment in haskell
(,) a is an Applicative when a is a Monoid. We can compose (,) (Sum Int) with other applicative using Data.Functor.Compose, and get an applicative that lets us estimate the "cost" assigned to a computation before running it. To count the steps, we need a lift function from the base...
Proving Composition Law for Maybe Applicative
Applicative functor expressions are just function applications in the context of some functor. Hence: pure f <*> pure a <*> pure b <*> pure c -- is the same as: pure (f a b c) We want to prove that: pure (.) <*> u <*> v <*> w == u...
How to compose functions that return Validation?
scala,validation,scalaz,applicative
There are lots of ways to write this, but I like the following: val checkX: X => Status[X] = x => isPositive(x.x1).tuple(isEven(x.x2)).as(x) Or: val checkX: X => Status[X] = x => isPositive(x.x1) *> isEven(x.x2) *> x.point[Status] The key point is that you want to run the two validations only for...
Understanding the Haskell type system in the context of applicatives
haskell,type-systems,applicative
The first argument of <*> is supposed to be f (a -> b). So given (<*>) (pure x), this is well-typed provided that x is some kind of function. The type of 2 is Num a => a. In other words, 2 can be any possible type, so long as...
Applicative instance for ZipList in Scala
Applicatives should satisfy the law point identity <*> v == v which yours does not since point identity List(1,2,3) == List(1) pure a for a zip list should return an infinite stream of a which is why you need a lazy data structure....
Conditional looping in an Applicative Functor
The Alternative type class provides the many combinator: class Applicative f => Alternative f where empty :: f a (<|>) :: f a -> f a -> f a many :: f a -> f [a] some :: f a -> f [a] some = some' many = many' many'...
How to use Applicative for concurrency?
scala,concurrency,functional-programming,applicative,haxl
How to implement getPageData assuming Fetch[A] is an Applicative ? All we need to do is drop the monadic bind >>= in favour of the applicative <*>. So instead of val getPageData = for { recent <- getRecent popular <- getPopular topics <- getTopics } yield (recent, popular, topics)...
Why WrappedMonad and WrappedArrow?
Pretty much so for WrappedMonad. I guess it is becoming (and perhaps already was) essentially obsolete. But WrappedArrow is more difficult, because Arrow types and Applicative types have different kinds, * -> * -> * vs. * -> *. And because of the way GHC instance resolution works, adding the...
Returning NonEmptyList `sealed trait`'s w/ scalaz.Validation
You don't need to describe how to combine errors—that's taken care of by ValidationNel's applicative functor via the Semigroup instance for NonEmptyList. The argument to the applicative builder should instead indicate how to create people: def makePerson(name: String, age: Int, gender: Gender): Validation[NonEmptyList[InvalidPersonError], Person] = (validateName(name) |@| validateAge(age))(Person(_, _, gender))...
Is it possible to use a conditional statement in an Idiom Bracket in Idris?
syntactic-sugar,applicative,idris
I was able to get this working by taking care of two problems: if _ then _ else _ doesn't seem to propagate the idiom bracket to its subexpressions The default definition of if _ then _ else _ is (of course) lazy in its two branches, and Lazy' LazyEval...
ZipList with Scalaz
pure for zip lists repeats the value forever, so it's not possible to define a zippy applicative instance for Scala's List (or for anything like lists). Scalaz does provide a Zip tag for Stream and the appropriate zippy applicative instance, but as far as I know it's still pretty broken....
How to fix implicit conversion to Applicative?
The types M and A are referenced from your def so they need to be part of the thing you're returning. I can't see any way to do this completely with the structural refinement, so I think you have to define a parametrized class; how about defining an implicit class...
Why are instances matched only by their heads?
haskell,monads,typeclass,functor,applicative
There's a lot of questions rolled into one here. But let's take them one at a time. First: why doesn't the compiler look at instance contexts when choosing which instance to use? This is to keep instance search efficient. If you require the compiler to consider only instances whose instance...
Understanding the signature of <$>
a function that takes a (a -> a- > a) -> b function and an a as parameters and returns a b. This is correct. According to this reasoning , I should call this like : (<$>f) f 4 which would result in an Integer. No, because f does...
Easier way to apply multiple arguments in Haskell
haskell,functional-programming,composition,functor,applicative
The Applicative class has the <*> operator (usually pronounced "ap", and is equivalent to Control.Monad.ap for most Monads), which combined with the <$> operator (itself just an infix alias for fmap) lets you write code like -- f :: a -> b -> c -- fa :: Applicative f =>...
Lax monoidal functors with a different monoidal structure
haskell,functor,applicative,category-theory
The "neat alternative presentation" for Applicative is based on the following two equivalencies pure a = fmap (const a) unit unit = pure () ff <*> fa = fmap (\(f,a) -> f a) $ ff ** fa fa ** fb = pure (,) <*> fa <*> fb The trick to...
non-monadic error handling in Haskell?
exception,haskell,error-handling,monads,applicative
In your first example, you want to compose a function f :: a -> m a with itself. Let's pick a specific a and m for the sake of discussion: Int -> Maybe Int. Composing functions that can have errors Okay, so as you point out, you cannot just do...
How to define <*> for Option[List[_]] n Scala
Firstly, you need an instance of applicative for Option: implicit object AppOption extends Applicative[Option] { def map[A, B](f: A => B, o: Option[A]) = o.map(f) def unit[A](a: A): Option[A] = Some(a) def ap[A, B](of: Option[A => B], oa: Option[A]) = of match { case Some(f) => oa.map(f) case None =>...
Haskell - Evaluation of (+) <$> (+3) <*> (*100) $ 5
Firstly, note that both <$> and <*> associate to the left. There's nothing magical happening internally and we can see the transformation with essentially a series of eta expansions and beta reductions. Step-by-step, it looks like this: (((+) <$> (+3)) <*> (*100)) $ 5 -- Add parens ((fmap (+) (+3))...
How to define Applicative in this case?
scala,concurrency,monads,applicative
Well has soon has you have PairJob, you have what you need for applicative. With any generic type G, (here, that would be Job) if you have pairing: def pair[A,B](ga: G[A], gb: G[B]): G[(A,B)] (which for Job, is just PairJob(ga, gb)) and also map, then you can easily implement apply...
Implementing Applicative (<*>) for StateT
haskell,monad-transformers,applicative
An implementation (taken from Tony Morris' functional programming course) could be (<*>) :: (Functor f, Monad f) => StateT s f (a -> b) -> StateT s f a -> StateT s f b StateT f <*> StateT a = StateT (\s -> (\(g, t) -> (\(z, u) -> (g...