What does map name mean in Ruby

Question

I found this code in a RailsCast   def tag names    tag names    tags map  amp  name  join      end   What does the   amp  name  in map  amp  name  mean

User · Answer

Another cool shorthand  unknown to many  is  array each  amp method  foo     which is a shorthand for   array each    element  foo element      By calling method  foo  we took a Method object from self that represents its foo method  and used the  amp  to signify that it has a to proc method that converts it into a Proc   This is very useful when you want to do things point-free style  An example is to check if there is any string in an array that is equal to the string  foo   There is the conventional way     bar    baz    foo   any     str  str     foo      And there is the point-free way     bar    baz    foo   any   amp  foo  method         The preferred way should be the most readable one

User · Answer

tags map  amp  name    is The same as  tags map  tag  tag name     amp  name just uses the symbol as the method name to be called

User · Answer

map  amp  name  takes an enumerable object  tags in your case  and runs the name method for each element tag  outputting each returned value from the method   It is a shorthand for  array map    element  element name     which returns the array of element tag  names

User · Answer

Two things are happening here  and it s important to understand both   As described in other answers  the Symbol to proc method is being called   But the reason to proc is being called on the symbol is because it s being passed to map as a block argument  Placing  amp  in front of an argument in a method call causes it to be passed this way  This is true for any Ruby method  not just map with symbols   def some method  args   amp block    puts  args    args inspect     puts  block    block inspect   end  some method  whatever    args    whatever    block  nil  some method  amp  whatever    args       block    lt Proc 0x007fd23d010da8 gt   some method  amp  whatever     TypeError  wrong argument type String  expected Proc     String doesn t respond to  to proc    The Symbol gets converted to a Proc because it s passed in as a block  We can show this by trying to pass a proc to  map without the ampersand    arr    w apple banana  reverse upcase   proc    i  i reverse upcase   reverse upcase is a  Proc    gt  true  arr map reverse upcase    ArgumentError  wrong number of arguments  1 for 0     map expects 0 positional arguments and one block argument   arr map  amp reverse upcase    gt    ELPPA    ANANAB     Even though it doesn t need to be converted  the method won t know how to use it because it expects a block argument  Passing it with  amp  gives  map the block it expects

User · Answer

Josh Lee s answer is almost correct except that the equivalent Ruby code should have been as follows    class Symbol   def to proc     Proc new do  receiver        receiver send self     end   end end   not  class Symbol   def to proc     Proc new do  obj   args        obj send self   args     end   end end   With this code  when print   1  a    2  b    3  c    map  amp  first  is executed  Ruby splits the first input  1  a   into 1 and  a  to give obj 1 and args   a  to cause an error as Fixnum object 1 does not have the method self  which is  first       When   1  a    2  b    3  c    map  amp  first  is executed     first is a Symbol object  so when  amp  first is given to a map method as a parameter  Symbol to proc is invoked  map sends call message to  first to proc with parameter  1  a    e g    first to proc call  1  a    is executed   to proc procedure in Symbol class sends a send message to an array object   1  a    with parameter   first   e g    1  a   send  first  is executed  iterates over the rest of the elements in   1  a    2  b    3  c    object    This is the same as executing   1  a    2  b    3  c    map  e  e first  expression

User · Answer

First   amp  name is a shortcut for  amp  name to proc  where  name to proc returns a Proc  something that is similar  but not identical to a lambda  that when called with an object as  first  argument  calls the name method on that object   Second  while  amp  in def foo  amp block      end converts a block passed to foo to a Proc  it does the opposite when applied to a Proc   Thus   amp  name to proc is a block that takes an object as argument and calls the name method on it  i  e     o  o name

User · Answer

It s shorthand for tags map  amp  name to proc  join       If foo is an object with a to proc method  then you can pass it to a method as  amp foo  which will call foo to proc and use that as the method s block   The Symbol to proc method was originally added by ActiveSupport but has been integrated into Ruby 1 8 7  This is its implementation   class Symbol   def to proc     Proc new do  obj   args        obj send self   args     end   end end

User · Answer

It s equivalent to  def tag names    tag names    tags map    tag  tag name   join      end

User · Answer

While let us also note that ampersand  to proc magic can work with any class  not just Symbol  Many Rubyists choose to define  to proc on Array class   class Array   def to proc     proc    receiver  receiver send  self     end end    And then        Hello    Goodbye    map  amp         world        gt    Hello world     Goodbye world      Ampersand  amp  works by sending to proc message on its operand  which  in the above code  is of Array class  And since I defined  to proc method on Array  the line becomes      Hello    Goodbye    map    receiver  receiver send        world

User · Answer

it means  array each  amp  to sym to proc

User · Answer

It s shorthand for tags map    tag  tag name   join

User · Answer

It is same as below   def tag names   if  tag names      tag names   else     tags map   t  t name   join      end

User · Answer

Here  name is the symbol which point to the method name of tag object  When we pass  amp  name to map  it will treat name as a proc object   For short  tags map  amp  name  acts as    tags map do  tag    tag name end

User · Answer

Although we have great answers already  looking through a perspective of a beginner I d like to add the additional information      What does map  amp  name  mean in Ruby    This means  that you are passing another method as parameter to the map function    In reality you re passing a symbol that gets converted into a proc  But this isn t that important in this particular case    What is important is that you have a method named name that will be used by the map method as an argument instead of the traditional block style

User · Answer

amp  name  is short for   amp  name to proc  it is same as tags map   t  t name   join       to proc is actually implemented in C

User · Answer

It basically execute the method call tag name on each tags in the array   It is a simplified ruby shorthand

[ruby] What does map(&:name) mean in Ruby?

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