Blocks and yields in Ruby

Question

I am trying to understand blocks and yield and how they work in Ruby   How is yield used  Many of the Rails applications I ve looked at use yield in a weird way   Can someone explain to me or show me where to go to understand them

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In Ruby  methods can check to see if they were called in such a way that a block was provided in addition to the normal arguments   Typically this is done using the block given  method but you can also refer to the block as an explicit Proc by prefixing an ampersand   amp   before the final argument name   If a method is invoked with a block then the method can yield control to the block  call the block  with some arguments  if needed   Consider this example method that demonstrates   def foo x    puts  OK  called as foo   x inspect      yield  A gift from foo    if block given  end  foo 10    OK  called as foo 10  foo 123    y  puts  BLOCK    y  How nice        OK  called as foo 123    BLOCK  A gift from foo  How nice      Or  using the special block argument syntax   def bar x   amp block    puts  OK  called as bar   x inspect      block call  A gift from bar    if block end  bar 10    OK  called as bar 10  bar 123    y  puts  BLOCK    y  How nice        OK  called as bar 123    BLOCK  A gift from bar  How nice

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Yield can be used as nameless block to return a value in the method  Consider the following code   Def Up anarg    yield anarg  end   You can create a method  Up  which is assigned one argument  You can now assign this argument to yield which will call and execute an associated block  You can assign the block after the parameter list   Up  Here is a string    x  x reverse   puts x     When the Up method calls yield  with an argument  it is passed to the block variable to process the request

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It s quite possible that someone will provide a truly detailed answer here  but I ve always found this post from Robert Sosinski to be a great explanation of the subtleties between blocks  procs  amp  lambdas   I should add that I believe the post I m linking to is specific to ruby 1 8  Some things have changed in ruby 1 9  such as block variables being local to the block  In 1 8  you d get something like the following    gt  gt  a    Hello    gt   Hello   gt  gt  1 times    a  a    Goodbye      gt  1  gt  gt  a   gt   Goodbye    Whereas 1 9 would give you    gt  gt  a    Hello    gt   Hello   gt  gt  1 times    a  a    Goodbye      gt  1  gt  gt  a   gt   Hello    I don t have 1 9 on this machine so the above might have an error in it

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In Ruby  a block is basically a chunk of code that can be passed to and executed by any method  Blocks are always used with methods  which usually feed data to them  as arguments     Blocks are widely used in Ruby gems  including Rails  and in well-written Ruby code  They are not objects  hence cannot be assigned to variables    Basic Syntax  A block is a piece of code enclosed by     or do  end  By convention  the curly brace syntax should be used for single-line blocks and the do  end syntax should be used for multi-line blocks        This is a single line block    do     This is a multi-line block end    Any method can receive a block as an implicit argument  A block is executed by the yield statement within a method  The basic syntax is    def meditate   print  Today we will practice zazen    yield   This indicates the method is expecting a block end     We are passing a block as an argument to the meditate method meditate   print   for 40 minutes      Output  Today we will practice zazen for 40 minutes    When the yield statement is reached  the meditate method yields control to the block  the code within the block is executed and control is returned to the method  which resumes execution immediately following the yield statement    When a method contains a yield statement  it is expecting to receive a block at calling time  If a block is not provided  an exception will be thrown once the yield statement is reached  We can make the block optional and avoid an exception from being raised    def meditate   puts  Today we will practice zazen     yield if block given   end meditate  Output  Today we will practice zazen     It is not possible to pass multiple blocks to a method  Each method can receive only one block    See more at  http   www zenruby info 2016 04 introduction-to-blocks-in-ruby html

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There are two points I want to make about yield here  First  while a lot of answers here talk about different ways to pass a block to a method which uses yield  let s also talk about the control flow  This is especially relevant since you can yield MULTIPLE times to a block  Let s take a look at an example   class Fruit   attr accessor  kinds    def initialize       kinds    w orange apple pear banana    end    def each      puts  inside each      3 times   yield   kinds tap   kinds  puts  selecting from   kinds      sample     end   end  f   Fruit new f each do  kind    puts  inside block  end        gt  inside each   gt  selecting from   orange    apple    pear    banana     gt  inside block   gt  selecting from   orange    apple    pear    banana     gt  inside block   gt  selecting from   orange    apple    pear    banana     gt  inside block   When the each method is invoked  it executes line by line  Now when we get to the 3 times block  this block will be invoked 3 times  Each time it invokes yield  That yield is linked to the block associated with the method that called the each method  It is important to notice that each time yield is invoked  it returns control back to the block of the each method in client code  Once the block is finished executing  it returns back to the 3 times block  And this happens 3 times  So that block in client code is invoked on 3 separate occasions since yield is explicitly called 3 separate times    My second point is about enum for and yield  enum for instantiates the Enumerator class and this Enumerator object also responds to yield    class Fruit   def initialize      kinds    w orange apple    end    def kinds     yield  kinds shift     yield  kinds shift   end end  f   Fruit new enum   f to enum  kinds  enum next    gt   orange   enum next    gt   apple     So notice every time we invoke kinds with the external iterator  it will invoke yield only once  The next time we call it  it will invoke the next yield and so on    There s an interesting tidbit with regards to enum for  The documentation online states the following   enum for method    each   args    enum Creates a new Enumerator which will enumerate by calling method on obj  passing args if any   str    xyz  enum   str enum for  each byte  enum each    b  puts b           gt  120     gt  121     gt  122   If you do not specify a symbol as an argument to enum for  ruby will hook the enumerator to the receiver s each method  Some classes do not have an each method  like the String class    str    I like fruit  enum   str to enum enum next   gt  NoMethodError  undefined method  each  for  I like fruit  String   Thus  in the case of some objects invoked with enum for  you must be explicit as to what your enumerating method will be

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Yields  to put it simply  allow the method you create to take and call blocks   The yield keyword specifically is the spot where the  stuff  in the block will be performed

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I sometimes use  yield  like this   def add to http     http     yield   end  puts add to http    www example com    puts add to http    www victim com

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I found this article to be very useful  In particular  the following example      usr bin ruby  def test   yield 5   puts  You are in the method test    yield 100 end  test   i  puts  You are in the block   i     test do  i      puts  You are in the block   i   end   which should give the following output   You are in the block 5 You are in the method test You are in the block 100 You are in the block 5 You are in the method test You are in the block 100   So essentially each time a call is made to yield ruby will run the code in the do block or inside     If a parameter is provided to yield then this will be provided as a parameter to the do block   For me  this was the first time that I understood really what the do blocks were doing  It is basically a way for the function to give access to internal data structures  be that for iteration or for configuration of the function   So when in rails you write the following   respond to do  format    format html   render template   my view   layout   my layout    end   This will run the respond to function which yields the do block with the  internal  format parameter  You then call the  html function on this internal variable which in turn yields the code block to run the render command  Note that  html will only yield if it is the file format requested   technicality  these functions actually use block call not yield as you can see from the source but the functionality is essentially the same  see this question for a discussion   This provides a way for the function to perform some initialisation then take input from the calling code and then carry on processing if required   Or put another way  it s similar to a function taking an anonymous function as an argument and then calling it in javascript

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Yes  it is a bit puzzling at first    In Ruby  methods may receive a code block in order to perform arbitrary segments of code    When a method expects a block  it invokes it by calling the yield function    This is very handy  for instance  to iterate over a list or to provide a custom algorithm    Take the following example   I m going to define a Person class initialized with a name  and provide a do with name method that when invoked  would just pass the name attribute   to the block received   class Person      def initialize  name              name   name     end      def do with name          yield   name        end end   This would allow us to call that method and pass an arbitrary code block   For instance  to print the name we would do   person   Person new  Oscar     invoking the method passing a block person do with name do  name      puts  Hey  his name is   name   end   Would print   Hey  his name is Oscar   Notice  the block receives  as a parameter  a variable called name  N B  you can call this variable anything you like  but it makes sense to call it name   When the code invokes yield it fills this parameter with the value of  name    yield   name     We could provide another block to perform a different action  For example  reverse the name    variable to hold the name reversed reversed name        invoke the method passing a different block person do with name do  name       reversed name   name reverse end  puts reversed name    gt   racsO    We used exactly the same method  do with name  - it is just a different block   This example is trivial  More interesting usages are to filter all the elements in an array    days     monday    tuesday    wednesday    thursday    friday         select those which start with  t    days select do   item        item match   t   end    gt    tuesday    thursday     Or  we can also provide a custom sort algorithm  for instance based on the string size    days sort do  x y      x size  lt   gt  y size  end    gt    monday    friday    tuesday    thursday    wednesday     I hope this helps you to understand it better   BTW  if the block is optional you should call it like   yield value  if block given    If is not optional  just invoke it    EDIT   hmak created a repl it for these examples  https   repl it  makstaks blocksandyieldsrubyexample

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I wanted to sort of add why you would do things that way to the already great answers   No idea what language you are coming from  but assuming it is a static language  this sort of thing will look familiar  This is how you read a file in java  public class FileInput      public static void main String   args         File file   new File  C   MyFile txt        FileInputStream fis   null      BufferedInputStream bis   null      DataInputStream dis   null       try         fis   new FileInputStream file             Here BufferedInputStream is added for fast reading        bis   new BufferedInputStream fis         dis   new DataInputStream bis             dis available   returns 0 if the file does not have more lines        while  dis available      0              this statement reads the line from the file and print it to            the console          System out println dis readLine                       dispose all the resources after using them        fis close          bis close          dis close           catch  FileNotFoundException e          e printStackTrace          catch  IOException e          e printStackTrace                  Ignoring the whole stream chaining thing  The idea is this   Initialize resource that needs to be cleaned up use resource make sure to clean it up   This is how you do it in ruby  File open  readfile rb    r   do  infile      while  line   infile gets          puts    counter     line           counter   counter   1     end end   Wildly different  Breaking this one down   tell the File class how to initialize the resource tell the file class what to do with it laugh at the java guys who are still typing  -    Here  instead of handling step one and two  you basically delegate that off into another class  As you can see  that dramatically brings down the amount of code you have to write  which makes things easier to read  and reduces the chances of things like memory leaks  or file locks not getting cleared    Now  its not like you can t do something similar in java  in fact  people have been doing it for decades now  It s called the Strategy pattern  The difference is that without blocks  for something simple like the file example  strategy becomes overkill due to the amount of classes and methods you need to write  With blocks  it is such a simple and elegant way of doing it  that it doesn t make any sense NOT to structure your code that way   This isn t the only way blocks are used  but the others  like the Builder pattern  which you can see in the form for api in rails  are similar enough that it should be obvious whats going on once you wrap your head around this  When you see blocks  its usually safe to assume that the method call is what you want to do  and the block is describing how you want to do it

[ruby] Blocks and yields in Ruby

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