How does defining [square bracket] method in Ruby work?

Question

I am going through Programming Ruby - a pragmatic programmers guide and have stumbled on this piece of code:

class SongList   def [](key)     if key.kind_of?(Integer)       return @songs[key]     else       for i in [email protected]         return @songs[i] if key == @songs[i].name       end     end     return nil   end end 

I do not understand how defining [ ] method works?

Why is the key outside the [ ], but when the method is called, it is inside [ ]?

Can key be without parenthesis?

I realize there are far better ways to write this, and know how to write my own method that works, but this [ ] method just baffles me... Any help is greatly appreciated, thanks

Answer 1

It's just syntactic sugar. There are certain syntax patterns that get translated into message sends. In particular

a + b 

is the same as

a.+(b) 

and the same applies to ==, !=, <, >, <=, >=, <=>, ===, &, |, *, /, -, %, **, >>, <<, !==, =~ and !~ as well.

Also,

!a 

is the same as

a.! 

and the same applies to ~.

Then,

+a 

is the same as

a.+@ 

and the same applies to -.

Plus,

a.(b) 

is the same as

a.call(b) 

There is also special syntax for setters:

a.foo = b 

is the same as

a.foo=(b) 

And last but not least, there is special syntax for indexing:

a[b] 

is the same as

a.[](b) 

and

a[b] = c 

is the same as

a.[]=(b, c)