Implementation of some Haskell Typeclasses in Kotlin

4 months ago 5

Do you want to write haskell-like code in kotlin? No? Well you can do it anyway!

Haskell in Kotlin
Examples
Installation

val xs = HList(1, 2, 3) val res = xs.foldr({ a, b -> a + b }, 0) assert(res == 6) { "Expected 6, but got $res" } val o = HList(1, 2, 3, 4, 5, 6) val u = o.fold(Sum).value assert(u == 21) { "Expected 21, but got $u" }

val s = Maybe.Just(1) val r = s.flatMap { a -> Maybe.Just(a + 1) } assert(r == Maybe.Just(2)) { "Expected Just(2), but got $r" } val listDo = doReturning(HList.Companion) { val c: Int = HList(0, 1, 2, 3, 4).bind() val b: Int = HList(0, 1, 2, 3, 4).bind() val bind = HList(c, b).bind() `return`(bind) }.fix() assertTrue(listDo.inner.size == 50, "Result list should contain 50 combinations of c and b")

val complexIO = doReturning(IO.Companion) { putStrLn("Enter your name:").bind() val name = getLine.bind() putStrLn("Hello, $name!").bind() `return`(name.length) }.fix() complexIO.runIO(object : IORunner { fun getChar(): Char { return System.`in`.read().toChar() } fun putChar(c: Char) { print(c) } })

Current list of monads (not with all the functions)

Maybe
Either
IO
HList (haskell's list)
State

Current list of typeclasses

Functor
Applicative
Monad
Semigroup
Monoid
Foldable

Current monoids

Sum (sum of numbers)
Endo (identity function)

This is mostly a fun project made for absolutely no reason, but it helps me learn more about Haskell's typeclasses and weird Kotlin tricks to make them work (that could break any time).

Thanks to this repository for the very hacky tricks with suspend functions, that allow do notation and also for the trick of using a star-projection as a witness for higher-kinded types:

https://github.com/h0tk3y/kotlin-monads

Also most of the code was created using either the GHC source implementation for functions (available when clicking on " #source" on hackage) or the Haskell wikibook https://en.wikibooks.org/wiki/Haskell/

Available on Jitpack (https://jitpack.io/LelouBil/KotlinMonads)

Add it in your settings.gradle.kts at the end of repositories:

dependencyResolutionManagement { repositoriesMode.set(RepositoriesMode.FAIL_ON_PROJECT_REPOS) repositories { mavenCentral() maven { url = uri("https://jitpack.io") } } }

Add the dependency in your build.gradle.kts file:

dependencies { implementation("com.github.LelouBil:KotlinMonads:0.1.0") }

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