meow

A cute little library for Cats.

What meow has (currently)

• Structures: Arrow, Semigroup, Monoid, Functor, Applicative, Monad, Alternative ...
• Currying helpers
• Peano number and a dependently typed list
• Natural transformations
• Set isomorphism and monad isomorphism
• Monads: State, Parser, Free
• Monad Transformers: StateT, ReaderT, OptionT

Importing

import meow._
import Meow._

Functor

See Examples.

val m1: Option[Int] = Some(1)
m1 <#> (_ + 1)
// => Some(2)

val m2: List[Int] = (0 until 3).toList
m2 <#> (_ + 1)
// => List(1, 2, 3)

Do it in Haskell style:

val m1: Option[Int] = Some(1)
((x: Int) => x + 1) <%> m1
// => Some(2)

val m2: List[Int] = (0 until 3).toList
((x: Int) => x + 1) <%> m2
// => List(1, 2, 3)

Use <%> instead of <$>.

Applicative

See Examples.

Monad

See Examples.

Currying

Currying a function:

def add(x: Int, y: Int): Int = x + y
val f: Int => Int => Int = curry2(add)

Currying a 2-ary function implicitly when mapping:

def add(x: Int, y: Int): Int = x + y
add _ <%%> List(1, 2) <*> List(3, 4)

State Monad

See Examples.

val mult: (Int, Int) => Int = (x, y) => x * y
val app: State[Int, Unit] =
  mult <%%> get[Int] <*> get >>= set
app.exec(-5)

Parser Monad (Parser Combinator)

See Examples.

sealed trait UrlScheme
case object Http extends UrlScheme
case object Https extends UrlScheme
case object Ftp extends UrlScheme
case object File extends UrlScheme

case class Url(scheme: UrlScheme, term: List[String])
val pScheme: Parser[UrlScheme] = choice(
  string("https") <*| Https,
  string("http") <*| Http,
  string("ftp") <*| Ftp,
  string("file") <*| File,
) <* string("://")
val pTerm: Parser[String] = some(satisfy(_.isLetter)) <%| (_ mkString "")
val pUrl: Parser[Url] = Url <%%> pScheme <*> pTerm.sepBy1(char('.'))
println(pUrl parseOnly "https://example.com.cn")
// => Some(Url(Https,List(example, com, cn)))
// or ...
println("https://example.com.cn" ~~> pUrl)

SParser Monad (Parser Combinator)

SParser is also a Monad for parser combinators. The difference between it and Parser is that SParser is based on StateT and Option, while Parser is based on List. Generally, SParser will have better performance than Parser, since Parser uses a List instead of a LazyList.

See Examples.

References

• cats
• scalaz
• Lazy Functional State Threads