Questions about alternative functors, including Haskell's Alternative type class as well as implementations of the concept in other languages.
Questions tagged [alternative-functor]
70 questions
91
votes
1 answer
Distinction between typeclasses MonadPlus, Alternative, and Monoid?
The standard-library Haskell typeclasses MonadPlus, Alternative, and Monoid each provide two methods with essentially the same semantics:
An empty value: mzero, empty, or mempty.
An operator a -> a -> a that joins values in the typeclass together:…
00dani
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76
votes
4 answers
What does Haskell's <|> operator do?
Going through Haskell's documentation is always a bit of a pain for me, because all the information you get about a function is often nothing more than just: f a -> f [a] which could mean any number of things.
As is the case of the <|> function.
All…
Electric Coffee
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76
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5 answers
Confused by the meaning of the 'Alternative' type class and its relationship to other type classes
I've been going through the Typeclassopedia to learn the type classes. I'm stuck understanding Alternative (and MonadPlus, for that matter).
The problems I'm having:
the 'pedia says that "the Alternative type class is for Applicative functors…
Matt Fenwick
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53
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5 answers
What are Alternative's "some" and "many" useful for?
Alternative, an extension of Applicative, declares empty, <|> and these two functions:
One or more:
some :: f a -> f [a]
Zero or more:
many :: f a -> f [a]
If defined, some and many should be the least solutions of the equations:
some v = (:) <$>…
Petr
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35
votes
3 answers
Lax monoidal functors with a different monoidal structure
Applicative functors are well-known and well-loved among Haskellers, for their ability to apply functions in an effectful context.
In category-theoretic terms, it can be shown that the methods of Applicative:
pure :: a -> f a
(<*>) :: f (a -> b) ->…
Ben Millwood
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31
votes
2 answers
Haskell - What is Control.Applicative.Alternative good for?
I was looking at the Applicative class within Haskell libraries and stumbled across Alternative.
What is this class good for? A google search did not reveal anything particularly insightful. And it seems to be completely out of place, bundled as it…
Anupam Jain
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26
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1 answer
What’s an example of a Monad which is an Alternative but not a MonadPlus?
In his answer to the question “Distinction between typeclasses MonadPlus, Alternative, and Monoid?”, Edward Kmett says that
Moreover, even if Applicative was a superclass of Monad, you’d wind up needing the MonadPlus class anyways, because…
Antal Spector-Zabusky
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22
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2 answers
'some' and 'many' functions from the 'Alternative' type class
What are the functions some and many in the Alternative type class useful for? Docs provide a recursive definition which I was unable to comprehend.
missingfaktor
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18
votes
1 answer
Generate optparse-applicative parser from free alternative functor
Consider the following type signatures:
data Foo x = Foo {
name :: String
, reader :: String -> x
}
instance Functor Foo where
fmap f (Foo n r) = Foo n $ f . r
Now I show a natural transformation from Foo to optparse-applicative's Parser…
Impredicative
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18
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1 answer
Why does Parsec not use Control.Applicative operators
Using Control.Applicative is very useful with Parsec, but you need to always hide <|> and similar objects as they conflict with Parsec's own:
import Control.Applicative hiding ((<|>), many, optional)
import Text.Parsec.Combinator
import…
luispedro
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17
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2 answers
Why is there no alternative instance for Either but a semigroup that behaves similarily to alternative?
I am a Haskell newbie and I wonder why there is no alternative instance for Either but a semigroup, which behaves as I would expect it from alternative:
instance Semigroup (Either a b) where
Left _ <> b = b
a <> _ = a
This instance discards or…
user6445533
16
votes
3 answers
How to combine and then branch in MonadPlus/Alternative
I recently wrote
do
e <- (Left <$> m) <|> (Right <$> n)
more actions
case e of
Left x -> ...
Right y -> ...
This seems awkward. I know that protolude (and some other packages) define
-- Called eitherP in parser combinator…
dfeuer
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16
votes
1 answer
Why is there not 'Alternative' instance for 'Control.Applicative.Const'
There is an instance Monoid a => Monoid (Const a b) for the Const functor from Control.Applicative. There is also an instance Monoid m => Applicative (Const m).
I would therefore expect that there is also an instance Monoid m => Alternative (Const…
meiersi
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15
votes
1 answer
Structurally enforced Free Alternative, without left distributivity
There is a nice Free Alternative in the great free package, which lifts a Functor to a left-distributive Alternative.
That is, the claim is that:
runAlt :: Alternative g => (forall x. f x -> g x) -> Alt f a -> g a
is an Alternative homomorphism,…
Justin L.
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15
votes
1 answer
Why does the Alternative typeclass need to be a sub-class of Control.Applicative
Haskell provides a standard typeclass 'Alternative' that effectively provides the <|> operator for any type that is also an Applicative.
As I understand it Alternative is considered a Monoid on Applicative's, however the <|> operator seems to make…
James Davies
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