Lamdas and Procs

2012.09.17

One of my favorite little details from Ruby is its use of Proc and lambda, two cool syntactic features which create anonymous functions for simple tasks. For instance, to create a basic function that calculates square roots (as if Math.sqrt() weren’t easy enough), you first create a new Proc while handing it a block. Then, calling that Proc is simply a matter of sending the call method with however many arguments are required.

pr = Proc.new { |x| x**x }
pr.call(2) # => 4

Incidentally, a Proc is not picky about the number of arguments. If you pass pr above more than one argument, it will happily ignore everything after the first argument.

Meanwhile, Ruby offers a similar feature with lambda:

la = lambda { |x| x**x }
la.call(2) # => 4

The lambda feature also allows for a slightly different syntax as well, using -> in place of lambda and requiring arguments within parentheses before the block.

lam = ->(x) { x**x }
lam.call(2) # => 4

One key difference with Proc, though, is that lambda will complain about extra arguments, yielding an ArgumentError. Yet another difference is the way lambda handles return. Whereas Proc will return immediately from a function, lambda will only stop its execution of a block. The example from The Well Grounded Rubyist is as follows:

def return_test
  lam = lambda { return }
  lam.call
  puts "Still here!"
  pro = Proc.new { return }
  pro.call
  puts "You won't see this message!"
end

Interestingly, calling a Proc which simply returns when not within a function will create a LocalJumpError.

As a side note, there is a lambda function in Python. When doing something simple like in the examples above, the functionality seems nearly identical:

al = lambda x: x**x
al(2) # 4

However, as many comparisons between Ruby and Python point out, only Ruby can have multi-line blocks (using do and end). In Python, lambda is always one line.