Understanding When to Use lambda vs Proc.new in Ruby

When programming in Ruby, especially before version 1.9, many developers face a common dilemma: when should I use lambda and when should I use Proc.new? While they may seem similar, these two constructs have important differences that can impact your code’s behavior, and understanding these nuances is crucial for any Ruby programmer.

Key Differences Between lambda and Proc.new

1. Behavior of Return Statements

One of the most significant differences lies in how each construct handles return statements. Let’s break it down:

• Return in a lambda-created proc:

  • When you call return from within a lambda, it exits only from the lambda itself. After returning from the lambda, the control goes back to the calling method, and execution continues normally.

• Return in a Proc.new-created proc:

  • In comparison, calling return from a Proc.new not only exits from the proc but also from the enclosing method. This can lead to unexpected behavior in your code.

Here’s a quick illustration:

Practical Examples

Example of lambda

def whowouldwin
  mylambda = lambda { return "Freddy" }
  mylambda.call
  return "Jason"  # This line will be executed
end

puts whowouldwin  # Output: "Jason"

In the example above, the lambda executed returns “Freddy”, but control returns back to the method and executes the next line, returning “Jason” instead.

Example of Proc.new

def whowouldwin2
  myproc = Proc.new { return "Freddy" }
  myproc.call
  return "Jason"  # This line will NOT be executed
end

puts whowouldwin2  # Output: "Freddy"

In this case, when the Proc.new executes its return, control is given back from the entire method. Therefore, “Jason” is never returned.

2. Flexibility and Usage

• Use lambda when:

  • You want predictable behavior with the return statement.
  • You expect the return value to be returned only from the proc itself and continue execution of the enclosing method.

• Use Proc.new when:

  • You require a more flexible (and sometimes risky) behavior with the return statement and need the proc to exit both the proc and the method.
  • You prefer less typing and simpler syntax in certain contexts.

Conclusion

Choosing between lambda and Proc.new in Ruby can significantly affect your code’s flow and outcome. For most cases, particularly when clarity and predictability are vital, it’s generally advisable to use lambda over Proc.new. However, understanding the contexts where each applies can make your programming more effective and enjoyable.

By keeping these differences in mind and applying them judiciously in your code, you can avoid common pitfalls and write more robust Ruby programs.