Closures

A closure is an anonymous function value. The syntax uses a backslash, an optional parameter list, an arrow, and an expression body.

Zero arguments

\-> 1

One argument

\ s -> s.length

Multiple arguments

\ x, y -> x == y

Block-body closures

The arrow can also be followed by a brace-delimited statement block instead of a single expression:

\ x -> {
  y = x + 1
  print(y)
  y
}

The block body runs its statements top-to-bottom. The value of the closure is the value of the last expression statement.

Closures as arguments

Closures can be passed to methods that expect functions:

['a', 'ab', 'abc'].map(\ s -> s.length)

Returns [1, 2, 3].

Closures in maps

Map values can be closures:

x = {foo = \-> "bar"}
print(x.foo())
print(x[:foo]())

Both print bar.

Interop with java.util.function

Notch closures convert to JVM functional interfaces:

(\ x, y -> x == y).toBiFunction()
(\ x, y -> x == y).toBiPredicate()

The returned objects implement BiFunction and BiPredicate respectively.

See also

• JVM Overview for calling JVM methods that take functional interfaces.