Why does explicit return make a difference in a Proc?

Ruby has three constructs:

1. A block is not an object and is created by { ... } or do ... end.
2. A proc is a Proc object created by Proc.new or proc.
3. A lambda is a Proc created by lambda (or proc in Ruby 1.8).

Ruby has three keywords that return from something:

1. return terminates the method or lambda it is in.
2. next terminates the block, proc, or lambda it is in.
3. break terminates the method that yielded to the block or invoked the proc or lambda it is in.

In lambdas, return behaves like next, for whatever reason. next and break are named the way they are because they are most commonly used with methods like each, where terminating the block will cause the iteration to resume with the next element of the collection, and terminating each will cause you to break out of the loop.

def foo  f = Proc.new { next "return from foo from inside proc" }  f.call # control leaves foo here  return "return from foo" endputs foo # prints "return from foo"

This is the semantics for Procs; it is not necessarily the semantics for all blocks. I agree this is a bit confusing. It is there for added flexibility (and perhaps partially cause Ruby has no spec except for its implementation).

The behavior is defined in the Proc implementation. Lambdas behave differently, so if you would like your returns not to exit out of the enclosing method, use lambdas. Or, omit the return keyword from your Proc.

A deep investigation of Rubys closures is here. It is a fantastic exposé.

So:

def foo     f = Proc.new {    p2 = Proc.new { return "inner proc"};    p2.call    return "proc"  }  f.call  return "foo"enddef foo2  result = Proc.new{"proc"}.call  "foo2 (proc result is: #{result})"enddef bar  l = lambda { return "lambda" }  result = l.call  return "bar (lambda result is: #{result})"endputs foo# inner procputs foo2# foo (proc result is: proc) puts bar# bar (lambda result is: lambda) 

Think of it this way: Proc.new just create a block of code that is part of the calling function. proc/lambda create an anonymous function that has special bindings. A little code examples will help:

def foo  f = Proc.new { return "return from foo from inside Proc.new" }  f.call # control leaves foo here  return "return from foo" end

is equivalent to

def foo  begin    return "return from foo from inside begin/end" }  end  return "return from foo" end

so it is clear that the return will just return from the function 'foo'

in contrast:

def foo  f = proc { return "return from foo from inside proc" }  f.call # control stasy in foo here  return "return from foo" end

is equivalent to (ignoring the bindings since not used in this example):

def unonymous_proc  return "return from foo from inside proc"enddef foo  unonymous_proc()  return "return from foo" end

Which is as clearly will not return from foo and continue to the next statement instead.