Differences between Proc and Lambda

Question

Ruby has differences between Procs created via Proc.new and lambda (or the ->() operator in 1.9). It appears that non-lambda Procs will splat an array passed in across the block arguments; Procs created via lambda do not.

p = Proc.new { |a,b| a + b}
p[[1,2]] # => 3

l = lambda { |a,b| a + b }
l[[1,2]] # => ArgumentError: wrong number of arguments (1 for 2)

Does anyone have any insight into the motivations behind this behavior?

Answer 1

There are two main differences between lambdas and non-lambda Procs:

1. Just like methods, lambdas return from themselves, whereas non-lambda Procs return from the enclosing method, just like blocks.
2. Just like methods, lambdas have strict argument checking, whereas non-lambda Procs have loose argument checking, just like blocks.

Or, in short: lambdas behave like methods, non-lambda Procs behave like blocks.

What you are seeing there is an instance of #2. Try it with a block and a method in addition to a non-lambda Proc and a lambda, and you'll see. (Without this behavior, Hash#each would be a real PITA to use, since it does yield an array with two-elements, but you pretty much always want to treat it as two arguments.)