meet it

A is a proc with strict rules.

Write a lambda with an arrow: ->(weight, zone) { weight * zone }, or with lambda do |weight, zone| for longer ones. Like a proc, you keep it in a variable and run it with .call. Unlike a proc, a lambda is strict about its , the number of arguments it expects.

Give a lambda the wrong number and it raises . A proc shrugs instead: a missing argument becomes nil and extras are dropped. That silence hides bugs, so professionals reach for lambdas when they store a rule or a calculation in a variable.

A lambda also treats return like a method does: it leaves the lambda only, and the code around it carries on.

shipping = ->(weight, zone) { weight * zone }
loose = proc { |weight, zone| [weight, zone] }
puts shipping.call(2, 3)
puts shipping.arity
p loose.call(2)

A proc's return tries to leave the whole method around it, which surprises people, so lambdas are the safer choice.

guess first

A `return` inside a lambda. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

def shop_status
  status = lambda do |count|
    return "sold out" if count == 0
    "in stock"
  end
  result = status.call(0)
  "Shop says: #{result}"
end
puts shop_status

your turn

Your turn. Store a password rule in a lambda.

Pick a minimum password length from 4 to 11 and type it in the blank. The first password should pass, the second should fail.

Your code

press run to see what happens

fix it

Each room's floor area should print. Fix the call.

Broken code to fix

press run to see what happens

Read the error: how many arguments did area get, and how many does it want?
Each room is one array. A lambda will not split it into two numbers for you.
Pass both numbers on line 4: area.call(room[0], room[1]).

nailed it

you made lambdas keep to the rules. :D

->(x) { } makes a lambda, and .call runs it.
The wrong arity raises ArgumentError instead of guessing quietly.
return inside a lambda leaves only the lambda.

new words in your collection

lambda
arity
ArgumentError

This lesson isn’t saved. A free account remembers every lesson you finish.

A lambda is a proc with strict rules.

A return inside a lambda. What will this show?

Your turn. Store a password rule in a lambda.

Each room's floor area should print. Fix the call.

you made lambdas keep to the rules. :D

A is a proc with strict rules.

A `return` inside a lambda. What will this show?