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Arity difference between Lambda and Proc in Ruby

Many devs think Procs & Lambas in Ruby are interchangable… and it a lot of cases they can be.

However I did come across a difference to be aware of.

Procs do not enforce arity where a Lambda will.

Let’s take a look at a Proc’s behavior:

# Argument provided
p = Proc.new { |arg| puts arg }
p.call("TEST")
TEST
=> nil
# No argument provided
p = Proc.new { |arg| puts arg }
p.call

=> nil

Now let’s look at lambdas:

# Argument provided
l = ->(arg) { puts arg }
l.call("TEST")
TEST
=> nil
# No argument provided
l = ->(arg) { puts arg }
l.call

=> wrong number of arguments (given 0, expected 1) (ArgumentError)

See how there is strict arity on a lambda where the proc will not complain.

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