what does print call internally?

[Martin DeMello]

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

 

[Eric Hodel]

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.

 

[Joel VanderWerf]

it calls #to_s.

Hm. What's going on here then:

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

 

[7stud --]

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>

 

[7stud --]

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.

 

[Konrad Meyer]

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Quoth Joel VanderWerf:

=20
Hm. What's going on here then:
=20
$ ruby -e 'p nil.to_s'
""
$ ruby -e 'print nil'
nil
=20
--=20
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 # =3D> foo
irb(main):002:0> nil # =3D> foo
irb(main):003:0> puts nil # =3D> foo
nil

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

=2D-=20
Konrad Meyer <[email protected]> http://konrad.sobertillnoon.com/

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[Phrogz]

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.

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

 

[Martin DeMello]

it calls #to_s.

How does print nil print "nil" then?

martin

 

[Martin DeMello]

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

 

[Eric Hodel]

Hm. What's going on here then:

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

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

 

[Eric Hodel]

class Dog
def to_s
print "dog"
end
end

d = Dog.new

print d
puts

puts d

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

 

[7stud --]

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."

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?

 

[Eric Hodel]

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

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