Catless Category Theory in Scala 3, meow ~

libraryDependencies += "us.oyanglul" %% "meow" % "0.4.+"

The most frustrating thing about using Cats is always such complaints from my teammates:

Q: What's missing? What should I import?

A: Oh, if you are trying to use <+>, you should import cats.syntax.semigroupk._

A: Or, if you don't mind it slowing down you IDE you can just import cats.syntax.all._

Q: But it doesn't work, the compiler is not happy and I can't make sense of it.

A: Oh, you are trying to combineK a List, you need import cats.instances.list._ as well, since blah blah blah.

A: Or, if you don't mind you IDE get even slower, just import cats.implicits._.

This is extremely not user friendly, even though it makes perfect sense for lib maintainer to know that <+> should be in SemigroupK. But as a user, why should I care where <+> is in the package? I should be able to guess that this Option[_] and other Option[_] can be glued together with <+>, I don't need to remember if it is Semigroup, Semigroupal, SemigroupK or Alternative.

Why can't it be just like Haskell, all I need to know is that I can <|> two Maybe, I can >>= or <$> Maybe without worrying anything about what to import.

Meow is a completely new design to be user friendly, not cat friendly, by using the latest Scala 3 features.

Meow provides idiomatic haskell-like typeclasses by using functions over methods, contextual binding over hierarchy.

All you have to do is import prelude.{given, _} same as importing Prelude in Haskell.

Don't worry, it won't slow down the compiler I promise, no weird class hierarchy and unnecessary syntax lookup.

import meow.prelude.{given, *}

val fa = pure[Option](1)
val fb: Option[Int] = pure(2)
val f = (x: Int) => (y: Int) => x + y

assertEquals(f `<$>` fa <*> fb, Option(3))

With the super powerful of Scala 3 inline and Tuple, we no longer need shapeless to auto derive typeclasses instance for data type.

  case class Inner[A](a: A)
  case class Outer[A](a: A, b: Inner[A], c: List[A], d: Option[String]) derives Functor

  test("derive functor") {
    assertEquals(
      map[Outer]((a: Int) => a + 1)(Outer(1, Inner(2), List(3,4), Option("hehe"))),
      Outer(2, Inner(3), List(4,5), Option("hehe")))
  }

Simply just derives Functor and your data type will be mappable over A.

More Details...

You may want to refresh your Scala 3 knowledge before jump into the source code.

• Rank-N Types
• FunctionK
• GADT
• Phantom Types
• Dependent Types

sbt test