Are these two essentially the same thing? They look very similar to me.

Did lambda expression borrow its 开发者_开发技巧idea from Ruby?

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Ruby actually has 4 constructs that are all extremely similar

The Block

The idea behind blocks is sort of a way to implement really light weight strategy patterns. A block will define a coroutine on the function, which the function can delegate control to with the yield keyword. We use blocks for just about everything in ruby, including pretty much all the looping constructs or anywhere you would use using in c#. Anything outside the block is in scope for the block, however the inverse is not true, with the exception that return inside the block will return the outer scope. They look like this

def foo
  yield 'called foo'
end

#usage
foo {|msg| puts msg} #idiomatic for one liners

foo do |msg| #idiomatic for multiline blocks
  puts msg
end

Proc

A proc is basically taking a block and passing it around as a parameter. One extremely interesting use of this is that you can pass a proc in as a replacement for a block in another method. Ruby has a special character for proc coercion which is &, and a special rule that if the last param in a method signature starts with an &, it will be a proc representation of the block for the method call. Finally, there is a builtin method called block_given?, which will return true if the current method has a block defined. It looks like this

def foo(&block)
  return block
end

b = foo {puts 'hi'}
b.call # hi

To go a little deeper with this, there is a really neat trick that rails added to Symbol (and got merged into core ruby in 1.9). Basically, that & coercion does its magic by calling to_proc on whatever it is next to. So the rails guys added a Symbol#to_proc that would call itself on whatever is passed in. That lets you write some really terse code for any aggregation style function that is just calling a method on every object in a list

class Foo
  def bar
    'this is from bar'
  end
end

list = [Foo.new, Foo.new, Foo.new]

list.map {|foo| foo.bar} # returns ['this is from bar', 'this is from bar', 'this is from bar']
list.map &:bar # returns _exactly_ the same thing

More advanced stuff, but imo that really illustrates the sort of magic you can do with procs

Lambdas

The purpose of a lambda is pretty much the same in ruby as it is in c#, a way to create an inline function to either pass around, or use internally. Like blocks and procs, lambdas are closures, but unlike the first two it enforces arity, and return from a lambda exits the lambda, not the containing scope. You create one by passing a block to the lambda method, or to -> in ruby 1.9

l = lambda {|msg| puts msg} #ruby 1.8
l = -> {|msg| puts msg} #ruby 1.9

l.call('foo') # => foo

Methods

Only serious ruby geeks really understand this one :) A method is a way to turn an existing function into something you can put in a variable. You get a method by calling the method function, and passing in a symbol as the method name. You can re bind a method, or you can coerce it into a proc if you want to show off. A way to re-write the previous method would be

l = lambda &method(:puts)
l.call('foo')

What is happening here is that you are creating a method for puts, coercing it into a proc, passing that in as a replacement for a block for the lambda method, which in turn returns you the lambda

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Feel free to ask about anything that isn't clear (writing this really late on a weeknight without an irb, hopefully it isn't pure gibberish)

EDIT: To address questions in the comments

list.map &:bar Can I use this syntax with a code block that takes more than one argument? Say I have hash = { 0 => "hello", 1 => "world" }, and I want to select the elements that has 0 as the key. Maybe not a good example. – Bryan Shen

Gonna go kind of deep here, but to really understand how it works you need to understand how ruby method calls work.

Basically, ruby doesn't have a concept of invoking a method, what happens is that objects pass messages to each other. The obj.method arg syntax you use is really just sugar around the more explicit form, which is obj.send :method, arg, and is functionally equivalent to the first syntax. This is a fundamental concept in the language, and is why things like method_missing and respond_to? make sense, in the first case you are just handling an unrecognized message, the second you are checking to see if it is listening for that message.

The other thing to know is the rather esoteric "splat" operator, *. Depending on where its used, it actually does very different things.

def foo(bar, *baz)

In a method call, if it is the last parameter, splat will make that parameter glob up all additional parameters passed in to the function (sort of like params in C#)

obj.foo(bar, *[biz, baz])

When in a method call (or anything else that takes argument lists), it will turn an array into a bare argument list. The snippet below is equivilent to the snippet above.

obj.foo(bar, biz, baz)

Now, with send and * in mind, Symbol#to_proc is basically implemented like this

class Symbol
  def to_proc
    Proc.new { |obj, *args| obj.send(self, *args) }
  end
end

So, &:sym is going to make a new proc, that calls .send :sym on the first argument passed to it. If any additional args are passed, they are globbed up into an array called args, and then splatted into the send method call.

I notice that & is used in three places: def foo(&block), list.map &:bar, and l = lambda &method(:puts). Do they share the same meaning? – Bryan Shen

Yes, they do. An & will call to_proc on what ever it is beside. In the case of the method definition it has a special meaning when on the last parameter, where you are pulling in the co-routine defined as a block, and turning that into a proc. Method definitions are actually one of the most complex parts of the language, there are a huge amount of tricks and special meanings that can be in the parameters, and the placement of the parameters.

b = {0 => "df", 1 => "kl"} p b.select {|key, value| key.zero? } I tried to transform this to p b.select &:zero?, but it failed. I guess that's because the number of parameters for the code block is two, but &:zero? can only take one param. Is there any way I can do that? – Bryan Shen

This should be addressed earlier, unfortunately you can't do it with this trick.

"A method is a way to turn an existing function into something you can put in a variable." why is l = method(:puts) not sufficient? What does lambda & mean in this context? – Bryan Shen

That example was exceptionally contrived, I just wanted to show equivalent code to the example before it, where I was passing a proc to the lambda method. I will take some time later and re-write that bit, but you are correct, method(:puts) is totally sufficient. What I was trying to show is that you can use &method(:puts) anywhere that would take a block. A better example would be this

['hello', 'world'].each &method(:puts) # => hello\nworld

l = -> {|msg| puts msg} #ruby 1.9: this doesn't work for me. After I checked Jörg's answer, I think it should be l = -> (msg) {puts msg}. Or maybe i'm using an incorrect version of Ruby? Mine is ruby 1.9.1p738 – Bryan Shen

Like I said in the post, I didn't have an irb available when I was writing the answer, and you are right, I goofed that (spend the vast majority of my time in 1.8.7, so I am not used to the new syntax yet)

There is no space between the stabby bit and the parens. Try l = ->(msg) {puts msg}. There was actually a lot of resistance to this syntax, since it is so different from everything else in the language.

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C# vs. Ruby

Are these two essentially the same thing? They look very similar to me.

They are very different.

First off, lambdas in C# do two very different things, only one of which has an equivalent in Ruby. (And that equivalent is, surprise, lambdas, not blocks.)

In C#, lambda expression literals are overloaded. (Interestingly, they are the only overloaded literals, as far as I know.) And they are overloaded on their result type. (Again, they are the only thing in C# that can be overloaded on its result type, methods can only be overloaded on their argument types.)

C# lambda expression literals can either be an anonymous piece of executable code or an abstract representation of an anonymous piece of executable code, depending on whether their result type is Func / Action or Expression.

Ruby doesn't have any equivalent for the latter functionality (well, there are interpreter-specific non-portable non-standardized extensions). And the equivalent for the former functionality is a lambda, not a block.

The Ruby syntax for a lambda is very similar to C#:

->(x, y) { x + y }           # Ruby
(x, y) => { return x + y; } // C#

In C#, you can drop the return, the semicolon and the curly braces if you only have a single expression as the body:

->(x, y) { x + y }  # Ruby
(x, y) => x + y    // C#

You can leave off the parentheses if you have only one parameter:

-> x { x }  # Ruby
x => x     // C#

In Ruby, you can leave off the parameter list if it is empty:

-> { 42 }  # Ruby
() => 42  // C#

An alternative to using the literal lambda syntax in Ruby is to pass a block argument to the Kernel#lambda method:

->(x, y) { x + y }
lambda {|x, y| x + y } # same thing

The main difference between those two is that you don't know what lambda does, since it could be overridden, overwritten, wrapped or otherwise modified, whereas the behavior of literals cannot be modified in Ruby.

In Ruby 1.8, you can also use Kernel#proc although you should probably avoid that since that method does something different in 1.9.

Another difference between Ruby and C# is the syntax for calling a lambda:

l.()  # Ruby
l()  // C#

I.e. in C#, you use the same syntax for calling a lambda that you would use for calling anything else, whereas in Ruby, the syntax for calling a method is different from the syntax for calling any other kind of callable object.

Another difference is that in C#, () is built into the language and is only available for certain builtin types like methods, delegates, Actions and Funcs, whereas in Ruby, .() is simply syntactic sugar for .call() and can thus be made to work with any object by just implementing a call method.

procs vs. lambdas

So, what are lambdas exactly? Well, they are instances of the Proc class. Except there's a slight complication: there are actually two different kinds of instances of the Proc class which are subtly different. (IMHO, the Proc class should be split into two classes for the two different kinds of objects.)

In particular, not all Procs are lambdas. You can check whether a Proc is a lambda by calling the Proc#lambda? method. (The usual convention is to call lambda Procs "lambdas" and non-lambda Procs just "procs".)

Non-lambda procs are created by passing a block to Proc.new or to Kernel#proc. However, note that before Ruby 1.9, Kernel#proc creates a lambda, not a proc.

What's the difference? Basically, lambdas behave more like methods, procs behave more like blocks.

If you have followed some of the discussions on the Project Lambda for Java 8 mailinglists, you might have encountered the problem that it is not at all clear how non-local control-flow should behave with lambdas. In particular, there are three possible sensible behaviors for return (well, three possible but only two are really sensible) in a lambda:

• return from the lambda
• return from the method the lambda was called from
• return from the method the lambda was created in

That last one is a bit iffy, since in general the method will have already returned, but the other two both make perfect sense, and neither is more right or more obvious than the other. The current state of Project Lambda for Java 8 is that they use two different keywords (return and yield). Ruby uses the two different kinds of Procs:

• procs return from the calling method (just like blocks)
• lambdas return from the lambda (just like methods)

They also differ in how they handle argument binding. Again, lambdas behave more like methods and procs behave more like blocks:

• you can pass more arguments to a proc than there are parameters, in which case the excess arguments will be ignored
• you can pass less arguments to a proc than there are parameters, in which case the excess parameters will be bound to nil
• if you pass a single argument which is an Array (or responds to to_ary) and the proc has multiple parameters, the array will be unpacked and the elements bound to the parameters (exactly like they would in the case of destructuring assignment)

Blocks: lightweight procs

A block is essentially a lightweight proc. Every method in Ruby has exactly one block parameter, which does not actually appear in its parameter list (more on that later), i.e. is implicit. This means that on every method call you can pass a block argument, whether the method expects it or not.

Since the block doesn't appear in the parameter list, there is no name you can use to refer to it. So, how do you use it? Well, the only two things you can do (not really, but more on that later) is call it implicitly via the yield keyword and check whether a block was passed via block_given?. (Since there is no name, you cannot use the call or nil? methods. What would you call them on?)

Most Ruby implementations implement blocks in a very lightweight manner. In particular, they don't actually implement them as objects. However, since they have no name, you cannot refer to them, so it's actually impossible to tell whether they are objects or not. You can just think of them as procs, which makes it easier since there is one less different concept to keep in mind. Just treat the fact that they aren't actually implemented as blocks as a compiler optimization.

to_proc and &

There is actually a way to refer to a block: the & sigil / modifier / unary prefix operator. It can only appear in parameter lists and argument lists.

In a parameter list, it means "wrap up the implicit block into a proc and bind it to this name". In an argument list, it means "unwrap this Proc into a block".

def foo(&bar)
end

Inside the method, bar is now bound to a proc object that represents the block. This means for example that you can store it in an instance variable for later use.

baz(&quux)

In this case, baz is actually a method which takes zero arguments. But of course it takes the implicit block argument which all Ruby methods take. We are passing the contents of the variable quux, but unroll it into a block first.

This "unrolling" actually works not just for Procs. & calls to_proc on the object first, to convert it to a proc. That way, any object can be converted into a block.

The most widely used example is Symbol#to_proc, which first appeared sometime during the late 90s, I believe. It became popular when it was added to ActiveSupport from where it spread to Facets and other extension libraries. Finally, it was added to the Ruby 1.9 core library and backported to 1.8.7. It's pretty simple:

class Symbol
  def to_proc
    ->(recv, *args) { recv.send self, *args }
  end
end

%w[Hello StackOverflow].map(&:length) # => [5, 13]

Or, if you interpret classes as functions for creating objects, you can do something like this:

class Class
  def to_proc
    -> *args { new *args }
  end
end

[1, 2, 3].map(&Array) # => [[nil], [nil, nil], [nil, nil, nil]]

Methods and UnboundMethods

Another class to represent a piece of executable code, is the Method class. Method objects are reified proxies for methods. You can create a Method object by calling Object#method on any object and passing the name of the method you want to reify:

m = 'Hello'.method(:length)
m.() #=> 5

or using the method reference operator .::

m = 'Hello'.:length
m.() #=> 5

Methods respond to to_proc, so you can pass them anywhere you could pass a block:

[1, 2, 3].each(&method(:puts))
# 1
# 2
# 3

An UnboundMethod is a proxy for a method that hasn't been bound to a receiver yet, i.e. a method for which self hasn't been defined yet. You cannot call an UnboundMethod, but you can bind it to an object (which must be an instance of the module you got the method from), which will convert it to a Method.

UnboundMethod objects are created by calling one of the methods from the Module#instance_method family, passing the name of the method as an argument.

u = String.instance_method(:length)

u.()
# NoMethodError: undefined method `call' for #<UnboundMethod: String#length>

u.bind(42)
# TypeError: bind argument must be an instance of String

u.bind('Hello').() # => 5

Generalized callable objects

Like I already hinted at above: there's not much special about Procs and Methods. Any object that responds to call can be called and any object that responds to to_proc can be converted to a Proc and thus unwrapped into a block and passed to a method which expects a block.

History

Did lambda expression borrow its idea from Ruby?

Probably not. Most modern programming languages have some form of anonymous literal block of code: Lisp (1958), Scheme, Smalltalk (1974), Perl, Python, ECMAScript, Ruby, Scala, Haskell, C++, D, Objective-C, even PHP(!). And of course, the whole idea goes back to Alonzo Church's λ-calculus (1935 and even earlier).

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Not exactly. But they're very similar. The most obvious difference is that in C# a lambda expression can go anywhere where you might have a value that happens to be a function; in Ruby you only have one code block per method call.

They both borrowed the idea from Lisp (a programming language dating back to the late 1950s) which in turn borrowed the lambda concept from Church's Lambda Calculus, invented in the 1930s.