Ruby tips: Procs vs lambdas
14 May 2013
I recently ran into a Ruby subtlety that I had heard of but never encountered before, regarding the use of the return keyword within blocks. Let’s jump right into an example. Let’s say you have an array of numbers and you want to select all the even ones. What will the following output?
def evensWithReturn(items)
items.select do |item|
if item.even?
return true
else
return false
end
end
end
> evensWithReturn([1,2,3,4,5,6])
# => false
Wait, what? We’re expecting to see [2, 4, 6], not false. What’s going on here? Let’s try it again, this time removing the return keyword from the select block.
def evensWithoutReturn(items)
items.select do |item|
if item.even?
true
else
false
end
end
end
> evensWithoutReturn([1,2,3,4,5,6])
# => [2, 4, 6]
Much better. Although why was the return keyword messing things up? It turns out that this is a result of the subtle differences between Procs and lambdas in Ruby, both of which represent chunks of reusable code. For the most part, the two are identical, although one of the differences is that a Proc returns from its enclosing block/method, while a lamba simply returns from the lambda. Let’s see some more examples. First, using Procs:
def procWithReturn(items)
myProc = Proc.new do |item|
if item.even?
return true
else
return false
end
end
items.select(&myProc)
end
> procWithReturn([1,2,3,4,5,6])
# => false
This is the same result as we saw with the block. Still not the result we wanted, but at least we know why. As soon as the proc gets the first item (1), it will call .even? on it and return false, and consequently return false from the entire procWithReturn method. However, it we try the same example using a lambda instead of a proc:
def lambdaWithReturn(items)
myLambda = lambda do |item|
if item.even?
return true
else
return false
end
end
items.select(&myLambda)
end
> lambdaWithReturn([1,2,3,4,5,6])
# => [2, 4, 6]
Success! Here, the lambda returns values to the select block, but doesn’t exit the enclosing method. For completeness, the other difference between Procs and lambdas is that lambdas enforce arity, meaning it will throw an ArgumentError if you don’t pass the number of arguments it specifies.
For the record, this example is fairly contrived for illustrative purposes - before you jump down my throat, I know that you can do
[1,2,3,4,5,6].select(&:even?)
I hope this clarifies some of the subtleties between procs and lambdas. For interested readers, I recommend this excellent blog post which goes into further detail.
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