Rails - Oct 2012 - struct square brackets method used to instantiate?

I know that Struct class defines the [] instance method:

1.9.3p0 :014 > Struct.instance_methods(false)
 =>
[:==, :eql?, :hash, :inspect, :to_s, :to_a, :values, :size, :length, :each,
:each_pair, :
[], :
[]=, :select, :values_at, :members, :pretty_print, :pretty_print_cycle,
:as_json]

This allows you to mimic hash behavior via Struct:

1.9.3p0 :016 > HashLike = Struct.new(:x,:y)
 => HashLike
1.9.3p0 :017 > h = HashLike.new(1,2)
 => #<struct HashLike x=1, y=2>
1.9.3p0 :018 > h[:x]
 => 1
1.9.3p0 :019 > h[:y] = 100
 => 100

But I come across a use case where the square brackets method was used
as instantiation:

1.9.3p0 :001 > Point = Struct.new :x, :y do
1.9.3p0 :002 >     def distance(point)
1.9.3p0 :003?>     Math.sqrt((point.x - self.x) ** 2 +
1.9.3p0 :004 >         (point.y - self.y) ** 2)
1.9.3p0 :005?>     end
1.9.3p0 :006?>   end
 => Point
1.9.3p0 :007 > Point[3,4].distance Point[0,0]
 => 5.0

That confuses me. How can [] be used both as both hash key accessors
and also instantiation (e.g. new)?

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2012/10/1 John Merlino <stoicism1-YDxpq3io04c@public.gmane.org>
> I know that Struct class defines the [] instance method:
>
> 1.9.3p0 :014 > Struct.instance_methods(false)
>  =>
> [:==, :eql?, :hash, :inspect, :to_s, :to_a, :values, :size, :length,
> :each, :each_pair, :
> [], :
> []=, :select, :values_at, :members, :pretty_print, :pretty_print_cycle,
> :as_json]
>
> This allows you to mimic hash behavior via Struct:
>
> 1.9.3p0 :016 > HashLike = Struct.new(:x,:y)
>  => HashLike
> 1.9.3p0 :017 > h = HashLike.new(1,2)
>  => #<struct HashLike x=1, y=2>
> 1.9.3p0 :018 > h[:x]
>  => 1
> 1.9.3p0 :019 > h[:y] = 100
>  => 100
>
> But I come across a use case where the square brackets method was used
> as instantiation:
>
> 1.9.3p0 :001 > Point = Struct.new :x, :y do
> 1.9.3p0 :002 >     def distance(point)
> 1.9.3p0 :003?>     Math.sqrt((point.x - self.x) ** 2 +
> 1.9.3p0 :004 >         (point.y - self.y) ** 2)
> 1.9.3p0 :005?>     end
> 1.9.3p0 :006?>   end
>  => Point
> 1.9.3p0 :007 > Point[3,4].distance Point[0,0]
>  => 5.0
>
> That confuses me. How can [] be used both as both hash key accessors
> and also instantiation (e.g. new)?
>
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>
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>
>
>
That''s because the :[] method is both defined as an instance method and
as
a class method of the Point class.
>Point.singleton_class.instance_methods false
=> [:new, :[], :members]

-- 
Piantanida, Ignacio Julián

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