ruby - Wann sollte Lambda verwendet werden, wann sollte verwendet werden?

original title: "ruby - When to use lambda, when to use"


In Ruby 1.8, there are subtle differences between proc/lambda on the one hand, and on the other.

  • What are those differences?
  • Can you give guidelines on how to decide which one to choose?
  • In Ruby 1.9, proc and lambda are different. What's the deal?

In Ruby 1.8 gibt es subtile Unterschiede zwischen proc / lambda einerseits und andererseits. Was sind diese Unterschiede? Können Sie Richtlinien geben, wie Sie entscheiden können, welche Sie wählen möchten ...

Dies ist die Zusammenfassung nach der Übersetzung. Wenn Sie die vollständige Übersetzung anzeigen möchten, klicken Sie auf das Symbol "Übersetzen"

Alle Antworten
  • Translate

    Another important but subtle difference between procs created with lambda and procs created with is how they handle the return statement:

    • In a lambda-created proc, the return statement returns only from the proc itself
    • In a proc, the return statement is a little more surprising: it returns control not just from the proc, but also from the method enclosing the proc!

    Here's lambda-created proc's return in action. It behaves in a way that you probably expect:

    def whowouldwin
      mylambda = lambda {return "Freddy"}
      # mylambda gets called and returns "Freddy", and execution
      # continues on the next line
      return "Jason"
    #=> "Jason"

    Now here's a proc's return doing the same thing. You're about to see one of those cases where Ruby breaks the much-vaunted Principle of Least Surprise:

    def whowouldwin2
      myproc = {return "Freddy"}
      # myproc gets called and returns "Freddy", 
      # but also returns control from whowhouldwin2!
      # The line below *never* gets executed.
      return "Jason"
    #=> "Freddy"

    Thanks to this surprising behavior (as well as less typing), I tend to favor using lambda over when making procs.

  • Translate

    To provide further clarification:

    Joey says that the return behavior of is surprising. However when you consider that behaves like a block this is not surprising as that is exactly how blocks behave. lambas on the other hand behave more like methods.

    This actually explains why Procs are flexible when it comes to arity (number of arguments) whereas lambdas are not. Blocks don't require all their arguments to be provided but methods do (unless a default is provided). While providing lambda argument default is not an option in Ruby 1.8, it is now supported in Ruby 1.9 with the alternative lambda syntax (as noted by webmat):

    concat = ->(a, b=2){ "#{a}#{b}" },5) # => "45"   # => "12"

    And Michiel de Mare (the OP) is incorrect about the Procs and lambda behaving the same with arity in Ruby 1.9. I have verified that they still maintain the behavior from 1.8 as specified above.

    break statements don't actually make much sense in either Procs or lambdas. In Procs, the break would return you from which has already been completed. And it doesn't make any sense to break from a lambda since it's essentially a method, and you would never break from the top level of a method.

    next, redo, and raise behave the same in both Procs and lambdas. Whereas retry is not allowed in either and will raise an exception.

    And finally, the proc method should never be used as it is inconsistent and has unexpected behavior. In Ruby 1.8 it actually returns a lambda! In Ruby 1.9 this has been fixed and it returns a Proc. If you want to create a Proc, stick with

    For more information, I highly recommend O'Reilly's The Ruby Programming Language which is my source for most of this information.

  • Translate

    I found this page which shows what the difference between and lambda are. According to the page, the only difference is that a lambda is strict about the number of arguments it accepts, whereas converts missing arguments to nil. Here is an example IRB session illustrating the difference:

    irb(main):001:0> l = lambda { |x, y| x + y }
    => #<Proc:0x00007fc605ec0748@(irb):1>
    irb(main):002:0> p = { |x, y| x + y }
    => #<Proc:0x00007fc605ea8698@(irb):2>
    irb(main):003:0> "hello", "world"
    => "helloworld"
    irb(main):004:0> "hello", "world"
    => "helloworld"
    irb(main):005:0> "hello"
    ArgumentError: wrong number of arguments (1 for 2)
        from (irb):1
        from (irb):5:in `call'
        from (irb):5
        from :0
    irb(main):006:0> "hello"
    TypeError: can't convert nil into String
        from (irb):2:in `+'
        from (irb):2
        from (irb):6:in `call'
        from (irb):6
        from :0

    The page also recommends using lambda unless you specifically want the error tolerant behavior. I agree with this sentiment. Using a lambda seems a tad more concise, and with such an insignificant difference, it seems the better choice in the average situation.

    As for Ruby 1.9, sorry, I haven't looked into 1.9 yet, but I don't imagine they would change it all that much (don't take my word for it though, it seems you have heard of some changes, so I am probably wrong there).

  • Louise Lee

    Proc is older, but the semantics of return are highly counterintuitive to me (at least when I was learning the language) because:

    1. If you are using proc, you are most likely using some kind of functional paradigm.
    2. Proc can return out of the enclosing scope (see previous responses), which is a goto basically, and highly non-functional in nature.

    Lambda is functionally safer and easier to reason about - I always use it instead of proc.

  • Translate

    I can't say much about the subtle differences. However, I can point out that Ruby 1.9 now allows optional parameters for lambdas and blocks.

    Here's the new syntax for the stabby lambdas under 1.9:

    stabby = ->(msg='inside the stabby lambda') { puts msg }

    Ruby 1.8 didn't have that syntax. Neither did the conventional way of declaring blocks/lambdas support optional args:

    # under 1.8
    l = lambda { |msg = 'inside the stabby lambda'|  puts msg }
    SyntaxError: compile error
    (irb):1: syntax error, unexpected '=', expecting tCOLON2 or '[' or '.'
    l = lambda { |msg = 'inside the stabby lambda'|  puts msg }

    Ruby 1.9, however, supports optional arguments even with the old syntax:

    l = lambda { |msg = 'inside the regular lambda'|  puts msg }
    #=> #<Proc:0x0e5dbc@(irb):1 (lambda)>
    #=> inside the regular lambda'jeez')
    #=> jeez

    If you wanna build Ruby1.9 for Leopard or Linux, check out this article (shameless self promotion).

  • Translate

    Short answer: What matters is what return does: lambda returns out of itself, and proc returns out of itself AND the function that called it.

    What is less clear is why you want to use each. lambda is what we expect things should do in a functional programming sense. It is basically an anonymous method with the current scope automatically bound. Of the two, lambda is the one you should probably be using.

    Proc, on the other hand, is really useful for implementing the language itself. For example you can implement "if" statements or "for" loops with them. Any return found in the proc will return out of the method that called it, not the just the "if" statement. This is how languages work, how "if" statements work, so my guess is Ruby uses this under the covers and they just exposed it because it seemed powerful.

    You would only really need this if you are creating new language constructs like loops, if-else constructs, etc.

  • Translate

    A good way to see it is that lambdas are executed in their own scope (as if it was a method call), while Procs may be viewed as executed inline with the calling method, at least that's a good way of deciding wich one to use in each case.

  • Translate

    I didn't notice any comments on the third method in the queston, "proc" which is deprecated, but handled differently in 1.8 and 1.9.

    Here's a fairly verbose example that makes it easy to see the differences between the three similar calls:

    def meth1
      puts "method start"
      pr = lambda { return }
      puts "method end"  
    def meth2
      puts "method start"
      pr = { return }
      puts "method end"  
    def meth3
      puts "method start"
      pr = proc { return }
      puts "method end"  
    puts "Using lambda"
    puts "--------"
    puts "using"
    puts "--------"
    puts "using proc"

  • Geoffrey Lee

    Closures in Ruby is a good overview for how blocks, lambda and proc work in Ruby, with Ruby.

  • Translate

    lambda works as expected, like in other languages.

    The wired is surprising and confusing.

    The return statement in proc created by will not only return control just from itself, but also from the method enclosing it.

    def some_method
      myproc = {return "End."}
      # Any code below will not get executed!
      # ...

    You can argue that inserts code into the enclosing method, just like block. But creates an object, while block are part of an object.

    And there is another difference between lambda and, which is their handling of (wrong) arguments. lambda complains about it, while ignores extra arguments or considers the absence of arguments as nil.

    irb(main):021:0> l = -> (x) { x.to_s }
    => #<Proc:0x8b63750@(irb):21 (lambda)>
    irb(main):022:0> p = { |x| x.to_s}
    => #<Proc:0x8b59494@(irb):22>
    ArgumentError: wrong number of arguments (0 for 1)
            from (irb):21:in `block in irb_binding'
            from (irb):25:in `call'
            from (irb):25
            from /usr/bin/irb:11:in `<main>'
    => ""
    irb(main):049:0> 1, 2
    ArgumentError: wrong number of arguments (2 for 1)
            from (irb):47:in `block in irb_binding'
            from (irb):49:in `call'
            from (irb):49
            from /usr/bin/irb:11:in `<main>'
    irb(main):050:0> 1, 2
    => "1"

    BTW, proc in Ruby 1.8 creates a lambda, while in Ruby 1.9+ behaves like, which is really confusing.

  • Translate

    To elaborate on Accordion Guy's response:

    Notice that creates a proc out by being passed a block. I believe that lambda {...} is parsed as a sort of literal, rather than a method call which passes a block. returning from inside a block attached to a method call will return from the method, not the block, and the case is an example of this at play.

    (This is 1.8. I don't know how this translates to 1.9.)

  • Translate

    I am a bit late on this, but there is one great but little known thing about not mentioned in comments at all. As by documentation:

    Proc::new may be called without a block only within a method with an attached block, in which case that block is converted to the Proc object.

    That said, lets to chain yielding methods:

    def m1
      yield 'Finally!' if block_given?
    def m2
      m1 &
    m2 { |e| puts e } 
    #⇒ Finally!

  • Translate

    The difference in behaviour with return is IMHO the most important difference between the 2. I also prefer lambda because it's less typing than :-)

  • Translate

    It's worth emphasizing that return in a proc returns from the lexically enclosing method, i.e. the method where the proc was created, not the method that called the proc. This is a consequence of the closure property of procs. So the following code outputs nothing:

    def foo
      proc ={return}
      puts 'foo'
    def foobar(proc)
      puts 'foobar'

    Although the proc executes in foobar, it was created in foo and so the return exits foo, not just foobar. As Charles Caldwell wrote above, it has a GOTO feel to it. In my opinion, return is fine in a block that is executed in its lexical context, but is much less intuitive when used in a proc that is executed in a different context.