Ruby IO - File input/output indirectly - ruby

Question

Recently I've been learning Ruby.I hit a problem while I'm writing a subclass of File.
class MyFile < File
end
file_path = "text_file"
file = MyFile.open(file_path) do | file |
file.each_line do | line |
puts line
end
file.close
end
result:
line 1
line 2
line 3
If I want output by calling a method:
class MyFile < File
def foo
self.each_line do | line |
puts line
end
end
end
file_path = "text_file"
my_file = MyFile.open(file_path) do | file |
file.foo
file.close
end
Result:
/Users/veightz/Developer/RubyCode/io_error.rb:4:in `write': not opened for writing (IOError)
from /Users/veightz/Developer/RubyCode/io_error.rb:4:in `puts'
from /Users/veightz/Developer/RubyCode/io_error.rb:4:in `block in foo'
from /Users/veightz/Developer/RubyCode/io_error.rb:3:in `each_line'
from /Users/veightz/Developer/RubyCode/io_error.rb:3:in `foo'
from /Users/veightz/Developer/RubyCode/io_error.rb:20:in `block in <main>'
from /Users/veightz/Developer/RubyCode/io_error.rb:19:in `open'
from /Users/veightz/Developer/RubyCode/io_error.rb:19:in `<main>'
Then I add new method bar
class MyFile < File
def foo
self.each_line do | line |
puts line
end
end
def bar
self.each_line do | line |
p line
end
end
end
my_file = MyFile.open(file_path) do | file |
file.bar
file.close
end
Result:
"line 1\n"
"line 2\n"
"line 3\n"
So, I'm so confused about IO in Ruby.Why puts line in foo can't work well.

Answer 1

There is a puts method in IO and IO is a superclass of File. That means that this:
puts line
is actually self.puts rather than Kernel#puts as it is pretty much everywhere else that you use puts. Hence the "not opened for writing" error message.
You need an explicit receiver to get the same effect as Kernel#puts; Kernel#puts is equivalent to $stdout.puts so you want:
file.each_line do | line |
$stdout.puts line
end
Your p line version works fine because there is no IO#p or File#p method, p is Kernel#p just like it is everywhere else.
Keep in mind that we don't have functions in Ruby the way other languages have global functions. Methods that you use like a function are almost always methods in Kernel.

Answer 2

You can also use $> "the default output stream" to redirect output of methods such as Kernel#puts... Which is the sole job of $> .
def foo
self.each_line do | line |
$>.puts line
end
end
I added one more method showing how you can also borrow "require" other gems (e.g pretty-printed gem) and the use of class << slef....Hence , you said "Recently I've been learning Ruby" :|
The below works as intended.
#!/usr/bin/env ruby
require 'pp'
class MyFile < File
class << self
def foo
each_line do | line |
$>.puts line
end
end
def bar
each_line do | line |
p line
end
end
def bam
each_line do | line |
pp line
end
end
end
file_path = "/Users/path/ofdirectory_to/somefile.txt"
my_file = MyFile.open(file_path) do | file |
file.bam
file.foo
file.bar
File.close
end
Note: the use of NameOfClass << self ... wana read more SO question
What seems btter :
non-pretty-printed output by p is:
#<PP:0x81fedf0 #genspace=#<Proc:0x81feda0>, #group_queue=#<PrettyPrint::GroupQueue:0x81fed3c #queue=[[#<PrettyPrint::Group:0x81fed78 #breakables=[], #depth=0, #break=false>], []]>, #buffer=[], #newline="\n", #group_stack=[#<PrettyPrint::Group:0x81fed78 #breakables=[], #depth=0, #break=false>], #buffer_width=0, #indent=0, #maxwidth=79, #output_width=2, #output=#<IO:0x8114ee4>>
pretty-printed output by pp is:
#<PP:0x81fedf0
#buffer=[],
#buffer_width=0,
#genspace=#<Proc:0x81feda0>,
#group_queue=
#<PrettyPrint::GroupQueue:0x81fed3c
#queue=
[[#<PrettyPrint::Group:0x81fed78 #break=false, #breakables=[], #depth=0>],
[]]>,
#group_stack=
[#<PrettyPrint::Group:0x81fed78 #break=false, #breakables=[], #depth=0>],
#indent=0,
#maxwidth=79,
#newline="\n",
#output=#<IO:0x8114ee4>,
#output_width=2>
For more about pp go here Ruby-Doc