When to use RSpec let()?

I tend to use before blocks to set instance variables. I then use those variables across my examples. I recently came upon let(). According to RSpec docs, it is used to

... to define a memoized helper method. The value will be cached across multiple calls in the same example but not across examples.

How is this different from using instance variables in before blocks? And also when should you use let() vs before()?

I always prefer let to an instance variable for a couple of reasons:

• Instance variables spring into existence when referenced. This means that if you fat finger the spelling of the instance variable, a new one will be created and initialized to nil, which can lead to subtle bugs and false positives. Since let creates a method, you'll get a NameError when you misspell it, which I find preferable. It makes it easier to refactor specs, too.
• A before(:each) hook will run before each example, even if the example doesn't use any of the instance variables defined in the hook. This isn't usually a big deal, but if the setup of the instance variable takes a long time, then you're wasting cycles. For the method defined by let, the initialization code only runs if the example calls it.
• You can refactor from a local variable in an example directly into a let without changing the referencing syntax in the example. If you refactor to an instance variable, you have to change how you reference the object in the example (e.g. add an @).
• This is a bit subjective, but as Mike Lewis pointed out, I think it makes the spec easier to read. I like the organization of defining all my dependent objects with let and keeping my it block nice and short.

A related link can be found here: http://www.betterspecs.org/#let

The difference between using instances variables and let() is that let() is lazy-evaluated. This means that let() is not evaluated until the method that it defines is run for the first time.

The difference between before and let is that let() gives you a nice way of defining a group of variables in a 'cascading' style. By doing this, the spec looks a little better by simplifying the code.

I have completely replaced all uses of instance variables in my rspec tests to use let(). I've written a quickie example for a friend who used it to teach a small Rspec class: http://ruby-lambda.blogspot.com/2011/02/agile-rspec-with-let.html

As some of the other answers here says, let() is lazy evaluated so it will only load the ones that require loading. It DRYs up the spec and make it more readable. I've in fact ported the Rspec let() code to use in my controllers, in the style of inherited_resource gem. http://ruby-lambda.blogspot.com/2010/06/stealing-let-from-rspec.html

Along with lazy evaluation, the other advantage is that, combined with ActiveSupport::Concern, and the load-everything-in spec/support/ behavior, you can create your very own spec mini-DSL specific to your application. I've written ones for testing against Rack and RESTful resources.

The strategy I use is Factory-everything (via Machinist+Forgery/Faker). However, it is possible to use it in combination with before(:each) blocks to preload factories for an entire set of example groups, allowing the specs to run faster: http://makandra.com/notes/770-taking-advantage-of-rspec-s-let-in-before-blocks

It is important to keep in mind that let is lazy evaluated and not putting side-effect methods in it otherwise you would not be able to change from let to before(:each) easily. You can use let! instead of let so that it is evaluated before each scenario.