What does print call internally?

ruby-talk
1

irb(main):001:0> puts nil
nil
=> nil
irb(main):002:0> print nil
nil=> nil
irb(main):003:0> class NilClass
irb(main):004:1> def inspect
irb(main):005:2> "foo"
irb(main):006:2> end
irb(main):007:1> end
=> foo
irb(main):008:0> puts nil
nil
=> foo
irb(main):009:0> nil.to_s
=> ""
irb(main):010:0> $_
=> foo

$ ruby -v
ruby 1.8.5 (2006-12-25 patchlevel 12) [x86_64-linux]

martin

2

it calls #to_s.

···

On Oct 10, 2007, at 15:31 , Martin DeMello wrote:

irb(main):001:0> puts nil
nil
=> nil
irb(main):002:0> print nil
nil=> nil
irb(main):003:0> class NilClass
irb(main):004:1> def inspect
irb(main):005:2> "foo"
irb(main):006:2> end
irb(main):007:1> end
=> foo

--
Poor workers blame their tools. Good workers build better tools. The
best workers get their tools to do the work for them. -- Syndicate Wars

3

Eric Hodel wrote:

···

On Oct 10, 2007, at 15:31 , Martin DeMello wrote:

irb(main):001:0> puts nil
nil
=> nil
irb(main):002:0> print nil
nil=> nil
irb(main):003:0> class NilClass
irb(main):004:1> def inspect
irb(main):005:2> "foo"
irb(main):006:2> end
irb(main):007:1> end
=> foo

it calls #to_s.

Hm. What's going on here then:

$ ruby -e 'p nil.to_s'
""
$ ruby -e 'print nil'
nil

--
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407

4

Eric Hodel wrote:

=> foo

it calls #to_s.

class Dog
  def to_s
    print "dog"
  end
end

d = Dog.new

print d
puts

puts d

--output:--
dog#<Dog:0x25634>
dog#<Dog:0x25634>

···

On Oct 10, 2007, at 15:31 , Martin DeMello wrote:

--
Posted via http://www.ruby-forum.com/\.

5

How does print nil print "nil" then?

martin

···

On 10/10/07, Eric Hodel <drbrain@segment7.net> wrote:

On Oct 10, 2007, at 15:31 , Martin DeMello wrote:

> irb(main):001:0> puts nil
> nil
> => nil
> irb(main):002:0> print nil
> nil=> nil
> irb(main):003:0> class NilClass
> irb(main):004:1> def inspect
> irb(main):005:2> "foo"
> irb(main):006:2> end
> irb(main):007:1> end
> => foo

it calls #to_s.

6

7stud -- wrote:

class Dog
  def to_s
    print "dog"
  end
end

d = Dog.new

print d
puts

puts d

--output:--
dog#<Dog:0x25634>
dog#<Dog:0x25634>

I guess that should be:

class Dog
  def to_s
    "dog"
  end
end

d = Dog.new

print d
puts

puts d

--output:--
dog
dog

I don't understand the output of my first example: print returns nil, so
print 'dog' should return nil, which means to_s returns nil, and so
print d should be equivalent to print nil.

···

--
Posted via http://www.ruby-forum.com/\.

7

Quoth Joel VanderWerf:

Eric Hodel wrote:
>
>> irb(main):001:0> puts nil
>> nil
>> => nil
>> irb(main):002:0> print nil
>> nil=> nil
>> irb(main):003:0> class NilClass
>> irb(main):004:1> def inspect
>> irb(main):005:2> "foo"
>> irb(main):006:2> end
>> irb(main):007:1> end
>> => foo
>
> it calls #to_s.

Hm. What's going on here then:

$ ruby -e 'p nil.to_s'
""
$ ruby -e 'print nil'
nil

--
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407

Irb calls NilClass#inspect, as the OP demonstrated.

  irb(main):001:0> class NilClass; def inspect() "foo" end; end # => foo
  irb(main):002:0> nil # => foo
  irb(main):003:0> puts nil # => foo
  nil

I'm guessing puts/print treat nil as a special case, but I may be wrong.

···

> On Oct 10, 2007, at 15:31 , Martin DeMello wrote:

--
Konrad Meyer <konrad@tylerc.org> http://konrad.sobertillnoon.com/

8

switch (TYPE(argv[i])) {
           case T_NIL:
             rb_io_write(out, rb_str_new2("nil"));
             break;
           default:
             rb_io_write(out, argv[i]);
             break;
         }

···

On Oct 10, 2007, at 19:52 , Joel VanderWerf wrote:

Eric Hodel wrote:

On Oct 10, 2007, at 15:31 , Martin DeMello wrote:

irb(main):001:0> puts nil
nil
=> nil
irb(main):002:0> print nil
nil=> nil
irb(main):003:0> class NilClass
irb(main):004:1> def inspect
irb(main):005:2> "foo"
irb(main):006:2> end
irb(main):007:1> end
=> foo

it calls #to_s.

Hm. What's going on here then:

$ ruby -e 'p nil.to_s'
""
$ ruby -e 'print nil'
nil

--
Poor workers blame their tools. Good workers build better tools. The
best workers get their tools to do the work for them. -- Syndicate Wars

9

In IO#write, if #to_s returns nil, rb_any_to_s() gets called, which returns "#<#{self.class}:0x#{object_id.to_s 16}>", which gets printed instead of your broken #to_s.

···

On Oct 10, 2007, at 19:58 , 7stud -- wrote:

Eric Hodel wrote:

On Oct 10, 2007, at 15:31 , Martin DeMello wrote:

=> foo

it calls #to_s.

class Dog
  def to_s
    print "dog"
  end
end

d = Dog.new

print d
puts

puts d

--output:--
dog#<Dog:0x25634>

--
Poor workers blame their tools. Good workers build better tools. The
best workers get their tools to do the work for them. -- Syndicate Wars

10

If to_s returns something other than a string, Ruby shakes a naughty
finger at you and ignores you, and falls back on the result of
Kernel#inspect instead.

irb(main):001:0> class Foo; def to_s; 42; end; end
=> nil
irb(main):002:0> puts Foo.new
#<Foo:0x359224>
=> nil
irb(main):003:0> class Foo; def to_s; "42"; end; end
=> nil
irb(main):004:0> puts Foo.new
42

···

On Oct 10, 9:25 pm, 7stud -- <dol...@excite.com> wrote:

I don't understand the output of my first example: print returns nil, so
print 'dog' should return nil, which means to_s returns nil, and so
print d should be equivalent to print nil.

11

That's quite counterintuitive, though. If true, the question becomes
"why?" - I can't see any useful purpose it'd serve, save maybe
debugging.

martin

···

On 10/10/07, Konrad Meyer <konrad@tylerc.org> wrote:

I'm guessing puts/print treat nil as a special case, but I may be wrong.

12

Martin DeMello wrote:

I'm guessing puts/print treat nil as a special case, but I may be wrong.

That's quite counterintuitive, though. If true, the question becomes
"why?" - I can't see any useful purpose it'd serve, save maybe
debugging.

martin

pickaxe2, p. 131:

"With a couple of exceptions, every object you pass to puts and print is
converted to a string by calling that object's to_s method. If for some
reason the to_s method doesn't return a valid string, a string is
created containing the object's class name and ID....

The exceptions are simple, too. The nil object will print as the string
'nil', and an array passed to puts will be written as if each of its
elements in turn were passed separately to puts."

Eric Hodel wrote:

···

On 10/10/07, Konrad Meyer <konrad@tylerc.org> wrote:
On Oct 10, 2007, at 19:58 , 7stud -- wrote:

end
dog#<Dog:0x25634>

In IO#write, if #to_s returns nil, rb_any_to_s() gets called, which
returns "#<#{self.class}:0x#{object_id.to_s 16}>", which gets printed
instead of your broken #to_s.

How do you get from puts/print to IO.write?
--
Posted via http://www.ruby-forum.com/\.

13

Hi,

···

On Thu, 2007-10-11 at 15:26 +0900, Martin DeMello wrote:

That's quite counterintuitive, though. If true, the question becomes
"why?" - I can't see any useful purpose it'd serve, save maybe
debugging.

Debugging is right. Otherwise, you print nothing (or "" -- who knows?).
It's probably better to see 'nil' and have to toss up between print nil
and print "nil" (or print ["nil"], or whatever!), rather than never
seeing a thing and constantly guessing if you're perhaps seeing print
nil, "", [""], etc....

- Arlen

14

Follow the calls in rb_f_print() (Kernel#print).

···

On Oct 11, 2007, at 05:03 , 7stud -- wrote:

Eric Hodel wrote:

On Oct 10, 2007, at 19:58 , 7stud -- wrote:

end
dog#<Dog:0x25634>

In IO#write, if #to_s returns nil, rb_any_to_s() gets called, which
returns "#<#{self.class}:0x#{object_id.to_s 16}>", which gets printed
instead of your broken #to_s.

How do you get from puts/print to IO.write?

--
Poor workers blame their tools. Good workers build better tools. The
best workers get their tools to do the work for them. -- Syndicate Wars