This adds pretty printing to Crystal objects. And now when you do p obj that actually pretty prints obj instead of using inspect. The reason for this is that I always find Ruby's pp better whenever I want to quickly "inspect" an object, so I think pretty printing should be preferred. There's also Elixir, which only has IO.inspect which does pretty printing, and that never bothered me.

Example:

class Point
  def initialize
    @x = 1
    @y = 2
    @z = 3
  end
end

class Foo
  @points : Array(Point)

  def initialize
    @points = [Point.new] * 3
  end
end

class Bar
  def initialize
    @foo = Foo.new
  end
end

bar = Bar.new
p bar

Output before:

#<Bar:0x103173f00 @foo=#<Foo:0x103173ee0 @points=[#<Point:0x103173ea0 @x=1, @y=2, @z=3>, #<Point:0x103173ea0 @x=1, @y=2, @z=3>, #<Point:0x103173ea0 @x=1, @y=2, @z=3>]>>

Output after:

#<Bar:0x109f16f00
 @foo=
  #<Foo:0x109f16ee0
   @points=
    [#<Point:0x109f16ea0 @x=1, @y=2, @z=3>,
     #<Point:0x109f16ea0 @x=1, @y=2, @z=3>,
     #<Point:0x109f16ea0 @x=1, @y=2, @z=3>]>>

I think I prefer the later to be outputted, always (one can do puts obj.inspect to get the old behaviour)

I'll be honest: I read the papers about pretty printing and did an implementation myself, but then I checked how it was done in Ruby and it was much better. The difference is that Ruby streams directly to the given output whenever possible, while my implementation constructed the whole tree and then outputted. In the end, my implementation consumed a lot of memory... so I decided to first port Ruby's code to Crystal, check if it works, and then try to understand it. Not surprisingly, porting the code was extremely easy! And it worked out of the box. I think I understand now how it works, though the algorithm is pretty complex (if somebody would like to add more comments afterwards, please do so!). As always, I don't know if I should do something about the copyright, the algorithms seem to be public though I'm not so sure about Ruby's code.

Now we'll have to_s, inspect and pretty_print... maybe we can remove inspect? Or maybe we can declare it such that it creates a pretty printer that will output everything into a single line and then invoke pretty_print with it? In that way users would only need to override to_s and/or pretty_print, but never inspect. In any case, maybe that can be decided after we merge this PR.