Mastering Ruby Closures

Lambdas—Same, But Different

Procs have a lot of similarities with lambdas. In fact, you might be surprised to learn that a lambda is a Proc:

>> lambda {}.class
=> Proc

A Proc, however, is not a lambda:

>> proc {}.class
=> Proc

Fortunately, Ruby has a helpful predicate method that lets you disambiguate procs and lambdas via the Proc#lambda? method:

>> lambda {}.lambda?
=> true

>> proc {}.lambda?
=> false

You might be wondering why there isn’t a corresponding proc? method. Some thinking leads you to realize that this method would be pretty useless. Let’s assume the method existed:

# NOTE: This method doesn't exist!
>> lambda {}.proc?
=> true

# NOTE: This method doesn't exist!
>> proc {}.proc? ...