Introduction

Martin Fowler has a good introduction to closures on his bliki.

Syntax

There are two syntaxes for closures: a block based syntax with syntactically significant whitespace and a braces based that ignores whitespace.

Block based syntax

import System.Windows.Forms
button = Button(Text: "Click me")
button.Click += def ():
    print("$button was clicked!")
    print("and yes, this is just like any other block...")

Braces based

import System.Windows.Forms
button1 = Button(Text: "Click me", Click: { print "clicked!" })
button2 = Button(Text: "Me too!")
button2.Click += { print "$button2 was clicked!";
    print "whitespace is ignored inside {}...";
    print "that's why you need to use semicolons to include multiple statements...";
    print "but please, don't write code like this just because you can :)"
}

Semantics

Boo closures have have full access (to read and write) to their enclosing lexical environment. For Instance:

a = 0 # declare a new variable
getter = { return a }
setter = { value | a = value }

assert 0 == getter()
setter(42)
assert 42 == getter()
assert 42 == a

The best source of information right now are the test cases for closures in the tests/testcases/integration directory.

Closures vs. Functions

See Functions As Objects.

Some things you can do with named functions that you cannot with closures include recursion and overloading:

This will not work because "c" is unknown from inside the closure:

c = do(x as int):
	print x
	--x
	c(x) if x > 0
c(5)

so you can use a regular named function or else create a 2nd callable to hold the name:

def c(x as int):
	print x
	--x
	c(x) if x > 0
c(5)

//or:

d as callable
c = do(x as int):
	print x
	--x
	d(x) if x > 0
d = c
c(5)

And you can use regular named functions to overload a method:

def doit(x as int):
    print x
def doit(x as string):
    print x

doit(3)

See Also

Functions, Parameters, Callable Types, Events