There was a very simple code to implemented. It brings me a lot of fun when I started the research. I had an array of different objects and I wanted to join them into one string in a special way. An important question here was: How objects will look like after casting into a string? The answer was short - good ;) But this is not the clue of this article. The most important question is: Why objects are cast into string in this way? Here is what I discover during my research.
Casting objects into a string
Let’s start with a very simple case. Array with string and symbol:
[:symbol, 'string'].join(' ')
=> "symbol string"
Everything looks fine. This is exactly what I expected. OK, what about an array in the array?
[[:symbol], 'string'].join(' ')
=> "symbol string"
Looks nice! I don’t need to worry about array and I get exactly what I wanted to. I also noticed one more thing here. When I use the puts method, it behaves similarly to the join method. We see what is inside of the array, but we don’t see brackets:
puts [:one, :two, :three]
one
two
three
=> nil
This is the moment when the most important question pops up. Why join and puts behave like this, when the conversion of the array into a string looks different?
[:symbol].to_s
=> "[:symbol]"
So, my logical part of the brain says: After method join I should get "[:symbol] string" and this is not what I have. Hmm… why?. I started my research. In the beginning, I focused on a difference between the to_s method and the join method.
Join method under the hood
First, what I discovered was that the join method doesn’t use to_s under the hood. At least this was my first understanding of this problem. It uses the to_str method. Let me show this in the example. I will declare a new object and I will check how it behaves in join.
class RubyStringTest
def to_s
'calls to_s'
end
def to_str
'calls to_str'
end
end
string_test = RubyStringTest.new
string_test.to_s
=> "calls to_s"
[:it, string_test, :method].join(' ')
=> "it calls to_str method"
But why we have these two methods? On the string, they work the same, but we see that we can provide different implementations for them. Do I need them both?
"String".to_s
=> "String"
"String".to_str
=> "String"
Difference between to_s and to_str
We need to understand what is a difference between method to_s and to_str. Method to_s is defined in Object class. So, all object in Ruby has method to_s. In case of to_str method not all objects implement it. When we try to call it on array we will get an exception:
[:symbol].to_str
Traceback (most recent call last):
4: from /home/agnieszka/.rvm/rubies/ruby-2.6.0/bin/irb:23:in `<main>'
3: from /home/agnieszka/.rvm/rubies/ruby-2.6.0/bin/irb:23:in `load'
2: from /home/agnieszka/.rvm/rubies/ruby-2.6.0/lib/ruby/gems/2.6.0/gems/irb-1.0.0/exe/irb:11:in `<top (required)>'
1: from (irb):7
NoMethodError (undefined method `to_str' for [:symbol]:Array)
Did you mean? to_set
to_s
This is because of the meaning of to_str method. to_s method allows us to cast an object into a string and to_str allow us to behave like a string. If we go back to our RubyStringTest example we can see:
string_test = RubyStringTest.new
"This test #{string_test}"
=> "This test calls to_s"
"This test " + string_test
=> "This test calls to_str"
Do you see the difference? In the first case, we cast an object into a string, so we call the to_s method. In the second example, we don’t cast object into a string. We want to object behave like a string. We want to object allow to use the :+ method the same as strings do. When we look at the array example we will see that Array is not behaving like a String:
"This is an array #{[:symbol]}"
=> "This is an array [:symbol]"
'This is an array' + [:symbol]
Traceback (most recent call last):
6: from /home/agnieszka/.rvm/rubies/ruby-2.6.0/bin/irb:23:in `<main>'
5: from /home/agnieszka/.rvm/rubies/ruby-2.6.0/bin/irb:23:in `load'
4: from /home/agnieszka/.rvm/rubies/ruby-2.6.0/lib/ruby/gems/2.6.0/gems/irb-1.0.0/exe/irb:11:in `<top (required)>'
3: from (irb):8
2: from (irb):8:in `rescue in irb_binding'
1: from (irb):8:in `+'
TypeError (no implicit conversion of Array into String)
We can of course force array to behave like a string, but by default, it is not like that.
class Array
def to_str
to_s
end
end
'This is an array ' + [:symbol]
=> "This is an array [:symbol]"
OK, but this is not the answer to my main question. We see that in the previous example by default array don’t behave like a string so it doesn’t implement method to_str. As we saw, when we try to call that method on the array, we will get an exception.
[:symbol].to_s
=> "[:symbol]"
[:symbol].to_str
Traceback (most recent call last):
4: from /home/agnieszka/.rvm/rubies/ruby-2.6.0/bin/irb:23:in `<main>'
3: from /home/agnieszka/.rvm/rubies/ruby-2.6.0/bin/irb:23:in `load'
2: from /home/agnieszka/.rvm/rubies/ruby-2.6.0/lib/ruby/gems/2.6.0/gems/irb-1.0.0/exe/irb:11:in `<top (required)>'
1: from (irb):2
NoMethodError (undefined method `to_str' for [:symbol]:Array)
Did you mean? to_set
to_s
How join method works on the array?
To understand how the join method works we need to go to the source. I mean, we need to go to the Ruby source code.
/*
* call-seq:
* ary.join(separator=$,) -> str
*
* Returns a string created by converting each element of the array to
* a string, separated by the given +separator+.
* If the +separator+ is +nil+, it uses current <code>$,</code>.
* If both the +separator+ and <code>$,</code> are +nil+,
* it uses an empty string.
*
* [ "a", "b", "c" ].join #=> "abc"
* [ "a", "b", "c" ].join("-") #=> "a-b-c"
*
* For nested arrays, join is applied recursively:
*
* [ "a", [1, 2, [:x, :y]], "b" ].join("-") #=> "a-1-2-x-y-b"
*/
This is a description of how the join method works. Two sentences give us the answer: Returns a string created by converting each element of the array to a string, separated by the given separator. For nested arrays, join is applied recursively. So, the to_str method isn’t called on an array, but on each element of this array. This match to what we get from our examples:
[[:symbol], 'string'].join(' ')
=> "symbol string"
:symbol.to_str
Traceback (most recent call last):
4: from /home/agnieszka/.rvm/rubies/ruby-2.6.0/bin/irb:23:in `<main>'
3: from /home/agnieszka/.rvm/rubies/ruby-2.6.0/bin/irb:23:in `load'
2: from /home/agnieszka/.rvm/rubies/ruby-2.6.0/lib/ruby/gems/2.6.0/gems/irb-1.0.0/exe/irb:11:in `<top (required)>'
1: from (irb):17
NoMethodError (undefined method `to_str' for :symbol:Symbol)
Did you mean? to_s
to_sym
What? This doesn’t match. Symbol class doesn’t implement the to_str method! Let’s go back to our example with RubyStringTest, run a new console, and do that example one more time step by step.
class RubyStringTest
end
string_test = RubyStringTest.new
string_test.to_s
=> "#<RubyStringTest:0x000055567edb0bd0>"
[:it, string_test, :method].join(' ')
=> "it #<RubyStringTest:0x000055567edb0bd0> method"
We see that in this case, the join method uses the to_s method under the hood. When we override the to_s method, we will see our implementation:
class RubyStringTest
def to_s
'calls to_s'
end
end
string_test = RubyStringTest.new
string_test.to_s
=> "calls to_s"
[:it, string_test, :method].join(' ')
=> "it calls to_s method"
When we override both methods to_s and to_str we will see, that join start using to_str method:
class RubyStringTest
def to_s
'calls to_s'
end
def to_str
'calls to_str'
end
end
string_test = RubyStringTest.new
string_test.to_s
=> "calls to_s"
[:it, string_test, :method].join(' ')
=> "it calls to_str method"
So, based on this example we see that join try to use to_str method and when it is not there join use to_s method. This matches, what we see for Symbol:
[[:symbol], 'string'].join(' ')
=> "symbol string"
:symbol.to_s
=> "symbol"
How puts method works on the array?
Similar behavior has a puts method. When we call puts on an array we will see elements of the array. Each element in the new line. puts doesn’t work on a whole array:
[1, 2, 3].to_s
=> "[1, 2, 3]"
puts [1, 2, 3]
1
2
3
=> nil
So, both methods join and puts don’t call to_str and to_s methods on whole array but an each element in the array. You can also see this directly in Ruby code for puts method:
/*
* call-seq:
* ios.puts(obj, ...) -> nil
*
* Writes the given object(s) to <em>ios</em>.
* Writes a newline after any that do not already end
* with a newline sequence. Returns +nil+.
*
* The stream must be opened for writing.
* If called with an array argument, writes each element on a new line.
* Each given object that isn't a string or array will be converted
* by calling its +to_s+ method.
* If called without arguments, outputs a single newline.
*
* $stdout.puts("this", "is", ["a", "test"])
*
* <em>produces:</em>
*
* this
* is
* a
* test
*
* Note that +puts+ always uses newlines and is not affected
* by the output record separator (<code>$\\</code>).
*/
Summary
- Two methods are allowing us to be like a string:
to_smethod - allow casting object into a string. Every object has that method.to_strmethod - allow the object to behave like a string. Not all object has this method. Not all objects behave like a string ;)
joinmethod uses under the hoodto_strmethod if it is implemented. If no, then thejoinmethod uses theto_smethod.putsmethod always uses under the hoodto_smethod.joinandputsmethods callsto_strandto_smethods on each element of the array not on the array it’s self.SymbolandArrayclass don’t implement theto_strmethod by default.
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