Ruby method definition

ruby-talk
1

i'm somewhat new to ruby, and as it seems this language is redefining
programming for me. there is a piece of code i'm trying to understand:

class SongList
  def [](key)
    if key.kind_of?(Integer)
      @songs[key]
    else
      # ...
    end
  end
end

list = SongList.new
list.append(Song.new('title1', 'artist1', 1)).
list.append(Song.new('title2', 'artist2', 2)).
list.append(Song.new('title3', 'artist3', 3)).
list.append(Song.new('title4', 'artist4', 4))

list[0] » Song: title1--artist1 (1)
list[2] » Song: title3--artist3 (3)
list[9] » nil

i can't understand why i'm allowed to call a [] method in this manner,
i.e. list[index], shouldn't i call it like list.[](index)

any help and explanation is appreciated

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2

Hi Rail, welcome to ruby,

you are right. is a special function among a list of other (like
= / + - @-) that don't work regularily, so as to allow syntactic
sugar. This makes a great fit when you want to build Hash-like or
Array-like objects, just make sure not to over-use it.

···

2011/1/6 Rail Shafigulin <rail.shafigulin@gmail.com>:

i can't understand why i'm allowed to call a method in this manner,
i.e. list[index], shouldn't i call it like list.(index)

3

you are right. is a special function among a list of other (like
= / + - @-) that don't work regularily, so as to allow syntactic
sugar. This makes a great fit when you want to build Hash-like or
Array-like objects, just make sure not to over-use it.

where can i read more about this syntactic sugar? is there some sort of
tutorial?

···

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4

where can i read more about this syntactic sugar? is there some sort of
tutorial?

Flanagan, and Matz' book is an excellent book to learn Ruby from. I
would suggest getting a copy for reference.

···

--
Anurag Priyam
http://about.me/yeban/

5

I'm not sure if you are asking about 'syntactic sugar' in general or
specific examples of such in Ruby.

In a general sense, syntactic sugar is a textual shortcut that a
language parser/interpreter supports to provide alternate (and hopefully
more useful) syntax for a standard feature.

The general Ruby syntax for method calls:

  receiver.method(arg1, ar2)

is somewhat ugly when the method name is '':

  receiver.(3)

But the syntactic sugar provided by Ruby's parser lets it accept

  receiver[3]

while interpreting it as just a standard method call to the
method named '' with an argument of 3, just as if you
had used the standard method calling syntax:

  receiver.(3)

Another example of this is Ruby's attribute writer methods
('setter methods'):

  customer.name = "Joe Smith"

is syntactic sugar for:

  customer.name=("Joe Smith")

which is just the standard method call syntax when the method
name is 'name='.

Operators are another example of this in Ruby.

  a = 1 + 2

is syntactic sugar for

  a = 1.+(2)

where 1 is the receiver, '+' is the method name, and 2 is the
first and only argument to the method. A slightly more
complicated example

  a += 1

is sugar for

  a = a + 1

which is sugar for

  a = a.+(1)

I don't know of a definitive list of these 'sugars' but I'm sure there are all mentioned somewhere in "The Ruby Programming Language", which is my favorite Ruby book if you are interested in a reference style exposition rather than a tutorial style exposition.

Gary Wright

···

On Jan 6, 2011, at 2:11 PM, Rail Shafigulin wrote:

you are right. is a special function among a list of other (like
= / + - @-) that don't work regularily, so as to allow syntactic
sugar. This makes a great fit when you want to build Hash-like or
Array-like objects, just make sure not to over-use it.

where can i read more about this syntactic sugar? is there some sort of
tutorial?

6

thanks for the explanation. i think i got it.

···

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