Convert a Ruby Method to a Lambda

David Eisinger, Development Director

Article Category: #Code

Posted on

Last week I tweeted:

Convert a method to a lambda in Ruby: lambda(&method(:events_path)). OR JUST USE JAVASCRIPT.

It might not be clear what I was talking about or why it would be useful, so allow me to elaborate. Say you’ve got the following bit of Javascript:

var ytmnd = function() { alert("you're the man now " + (arguments[0] || "dog")); }; 

Calling ytmnd() gets us you're the man now dog, while ytmnd("david") yields you're the man now david. Calling simply ytmnd gives us a reference to the function that we’re free to pass around and call at a later time. Consider now the following Ruby code:

def ytmnd(name = "dog") puts "you're the man now #{name}" end 

First, aren’t default argument values and string interpolation awesome? Love you, Ruby. Just as with our Javascript function, calling ytmnd() prints “you’re the man now dog”, and ytmnd("david") also works as you’d expect. But. BUT. Running ytmnd returns not a reference to the method, but rather calls it outright, leaving you with nothing but Sean Connery’s timeless words.

To duplicate Javascript’s behavior, you can convert the method to a lambda with sean = lambda(&method(:ytmnd)). Now you’ve got something you can call with or"david") and pass around with sean.

BUT WAIT. Everything in Ruby is an object, even methods. And as it turns out, a method object behaves very much like a lambda. So rather than saying sean = lambda(&method(:ytmnd)), you can simply say sean = method(:ytmnd), and then call it as if it were a lambda with .call or []. Big ups to Justin for that knowledge bomb.


All contrivances aside, there are real-life instances where you’d want to take advantage of this language feature. Imagine a Rails partial that renders a list of filtered links for a given model. How would you tell the partial where to send the links? You could pass in a string and use old-school :action and :controller params or use eval (yuck). You could create the lambda the long way with something like :base_url => lambda { |*args| articles_path(*args) }, but using method(:articles_path) accomplishes the same thing with much less line noise.

I’m not sure it would have ever occurred to me to do something like this before I got into Javascript. Just goes to show that if you want to get better as a Rubyist, a great place to start is with a different language entirely.

David Eisinger

David is Viget's managing development director. From our Durham, NC, office, he builds high-quality, forward-thinking software for PUMA, the World Wildlife Fund, NFLPA, and many others.

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