Blocks, procs, lambdas

Methods that yield

A method runs the attached block wherever it calls yield, pausing itself until the block finishes. A method may yield several times or not at all. The predicate block_given? reports whether a caller attached a block, so yielding stays safe.

def twice yield yield end twice do puts "hi" end

hi
hi

Block params with &

Prefix the last parameter with &block to capture the block as an ordinary object. Calling its call method runs the stored code, possibly much later. The same object can also travel onward into another method.

def run(&block) puts block.class block.call end run do puts "hello" end

Proc
hello

Procs

The method proc builds a block object that can sit in a variable and run later through call. Procs are lenient about arguments: missing ones arrive as nil, and extra ones are quietly ignored.

hi = proc { |name| puts "hi #{name}" } puts hi.class hi.call("Ada")

Proc
hi Ada

Lambdas vs procs

The method lambda builds a stricter cousin of a proc: a wrong argument count raises an ArgumentError at once. The predicate lambda? tells the two apart, returning true only for lambdas. Reach for lambdas when the stored code should behave like a strict function.

add = lambda { |a, b| puts a + b } add.call(2, 3) say = proc { |name| puts "hi #{name}" } puts add.lambda? puts say.lambda?

5
true
false

Next: Arguments: splats and kwargs

Article author: Arthur Isaev