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Franciscello
Franciscello

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From`# =>` to `#p`

Sometimes writing documentation can lead us to discover some details of a language!

I opened a PR adding documentation about Generics with a variable number of type arguments in Crystal with some code examples using # =>.

Here's a simplified example:

puts typeof("Hello!") # => String
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But then I got a suggestion that read (broadly speaking): use # => next to an expression to show what the result of that said expression would be.

In other words: If we have the expression 40 + 2 then we can add a comment using # => showing the expected result:

40 + 2 # => 42
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Let's see if that is the expected result using #p:

p 40 + 2
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the output would read:

42
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So, what was the problem with the first example?

puts typeof("Hello!") # => String
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The suggestion continued: [...] #puts returns nil.

😲 So if #puts returns nil and # => shows the result of evaluating the expression, then the correct comment would be:

puts typeof("Hello!") # => nil
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Well ... this is not what we want! 🥲

Documenting with # =>

The last part of the suggestion read: [...] Also, #p uses #inspect and returns its argument.

So, maybe we can write:

p typeof("Hello!") # => String
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Technically speaking this is correct because #p not only prints String which is the value of typeof("Hello!") but also returns that said value.

It's correct but ... we don't need to use #p. We only need # => for showing the expected value, like this:

typeof("Hello!") # => String
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And now the code is well documented! 🤓🎉

Another example

Just to reinforce what we've just learned, let's see another example. This one is taken right from the documentation:

a = 1
b = 2
"sum: #{a} + #{b} = #{a + b}" # => "sum: 1 + 2 = 3"
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In this example we are defining a string literal using interpolation (which let us embed expressions) and so with the use of # => we are showing the resulting string.


Up to this point, we have learned how to document lines of code using comments with # =>. Great!

And all this led us to see the difference between #p and #puts ... so maybe we can take a look at the implementation of these two methods? 🤔 ... yeah! Let's do that! 🤓🎉

But before doing that ...

📚 Did you know?
Methods #puts and #p also exist in Ruby.
They are implemented in module Kernel and with the same behaviour.
We can read the implementations for #puts and #p written in C in the documentation itself.

And now ... 🥁

#puts vs #p under the hood 🔬

#puts

Here is the source code for #puts:

def puts(*objects) : Nil
  STDOUT.puts *objects
end
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As we can see it forwards the responsibility to IO#puts. Let's see the implementation:

def puts(string : String) : Nil
  self << string
  puts unless string.ends_with?('\n')
  nil
end
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Great! The method writes the string and it returns nil (as we were already expecting).

#p

Now let's see the implementation of #p:

def p(object)
  object.inspect(STDOUT)
  puts
  object
end
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First, we may notice that the method returns (again, as we were expecting) the argument.

Then we may notice it forwards the "print responsibility" to Object#inspect(io : IO). Here is the source code:

def inspect(io : IO) : Nil
  to_s io
end
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Let's follow the code path and continue with the implementation of Object#to_s(io : IO):

abstract def to_s(io : IO) : Nil
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Ok, we've just found an abstract method.

Because we are trying to "print" a Class (remember p typeof("Hello!") # => String), let's see how Class#to_s(io : IO) is implemented:

def to_s(io : IO) : Nil
  io << {{ @type.name.stringify }}
end
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We can see Class#to_s outputs the Class string representation.

And we may read about the use of {{ }} in the macros docs 🤓

🤯 Did you notice?
We've just learned how the methods #puts and #p are implemented, and all the time we were reading code in Crystal.
Yes! Crystal's stdlib is written in Crystal itself, making it a lot more natural to inspect and learn how the language works under the hood. 🤩

Example

Here is another example that shows the difference between using #puts and #p:

class A
  def initialize
    @foo = "Foo"
    @bar = "Bar"
  end
end

a = A.new
puts a
p a
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The output will read:

#<A:0x7f25e8f28ea0>
#<A:0x7f25e8f28ea0 @foo="Foo", @bar="Bar">
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We can see that #p (which we now know uses #inspect) prints more information.

Farewell and see you later

Let's recap:

  • We have learned about documenting lines of code using # =>.
  • We have traveled through the implementation of #puts and #p.
  • And finally we have seen an example with the difference between using #puts and #p.

Hope you enjoyed it! 😃

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