Koka language preview

1. Effects and handlers

an effect declaration, an effect type, and a handler

horizon-dark

// prints a greeting with an effect
effect ctl ask() : int

fun greet() : <ask,console> ()
  val name = ask()
  println("Hello, " ++ name.show)

fun main()
  handle(greet)
    ctl ask() -> resume(42)

atom-one-dark

// prints a greeting with an effect
effect ctl ask() : int

fun greet() : <ask,console> ()
  val name = ask()
  println("Hello, " ++ name.show)

fun main()
  handle(greet)
    ctl ask() -> resume(42)

github-dark

// prints a greeting with an effect
effect ctl ask() : int

fun greet() : <ask,console> ()
  val name = ask()
  println("Hello, " ++ name.show)

fun main()
  handle(greet)
    ctl ask() -> resume(42)

dracula

// prints a greeting with an effect
effect ctl ask() : int

fun greet() : <ask,console> ()
  val name = ask()
  println("Hello, " ++ name.show)

fun main()
  handle(greet)
    ctl ask() -> resume(42)

nord

// prints a greeting with an effect
effect ctl ask() : int

fun greet() : <ask,console> ()
  val name = ask()
  println("Hello, " ++ name.show)

fun main()
  handle(greet)
    ctl ask() -> resume(42)

github

// prints a greeting with an effect
effect ctl ask() : int

fun greet() : <ask,console> ()
  val name = ask()
  println("Hello, " ++ name.show)

fun main()
  handle(greet)
    ctl ask() -> resume(42)

2. Pattern matching

match with the Just/Nothing literals

horizon-dark

fun describe(x : maybe<int>) : string
  match x
    Just(n) -> "got " ++ n.show
    Nothing -> "nothing"

atom-one-dark

fun describe(x : maybe<int>) : string
  match x
    Just(n) -> "got " ++ n.show
    Nothing -> "nothing"

github-dark

fun describe(x : maybe<int>) : string
  match x
    Just(n) -> "got " ++ n.show
    Nothing -> "nothing"

dracula

fun describe(x : maybe<int>) : string
  match x
    Just(n) -> "got " ++ n.show
    Nothing -> "nothing"

nord

fun describe(x : maybe<int>) : string
  match x
    Just(n) -> "got " ++ n.show
    Nothing -> "nothing"

github

fun describe(x : maybe<int>) : string
  match x
    Just(n) -> "got " ++ n.show
    Nothing -> "nothing"

3. Trailing lambdas

a function passed as a trailing block

horizon-dark

fun main()
  [1, 2, 3].map fn(x)
    x * 2

atom-one-dark

fun main()
  [1, 2, 3].map fn(x)
    x * 2

github-dark

fun main()
  [1, 2, 3].map fn(x)
    x * 2

dracula

fun main()
  [1, 2, 3].map fn(x)
    x * 2

nord

fun main()
  [1, 2, 3].map fn(x)
    x * 2

github

fun main()
  [1, 2, 3].map fn(x)
    x * 2