Feature #14609: Let `Kernel#p` without an argument print the receiver

• Status: Open
• Priority: Normal
• Assignee: matz (Yukihiro Matsumoto)
• Target version: -

[ko1 (Koichi Sasada)]

Abstract

Kernel#p(obj) prints obj as inspected.
How about printing the receiver if an argument is not given?

Background

We recently introduced yield_self, which encourages block chain.

https://zverok.github.io/blog/2018-01-24-yield_self.html
Quoting from this article, we can write method chain with blocks:

construct_url
  .yield_self { |url| Faraday.get(url) }.body
  .yield_self { |response| JSON.parse(response) }
  .dig('object', 'id')
  .yield_self { |id| id || '<undefined>' }
  .yield_self { |id| "server:#{id}" }

There is a small problem concerning debugging.
If we want to see the intermediate values in the method/block chain, we need to insert tap{|e| p e}.

With the above example,

construct_url
  .yield_self { |url| Faraday.get(url) }.body
  .yield_self { |response| JSON.parse(response) }.tap{|e| p e} # debug print
  .dig('object', 'id')
  .yield_self { |id| id || '<undefined>' }.tap{|e| p e} # debug print
  .yield_self { |id| "server:#{id}" }

Proposal

Let obj.p work the same as p(obj).

We can replace
block{...}.tap{|e| p e}
with
block{...}.p

For the above example, we can simply add .p at the end of a line:

construct_url
  .yield_self { |url| Faraday.get(url) }.body
  .yield_self { |response| JSON.parse(response) }.p # debug print
  .dig('object', 'id')
  .yield_self { |id| id || '<undefined>' }.p # debug print
  .yield_self { |id| "server:#{id}" }

Compatibility issues

(1) Shorthand for nil

This spec change can introduce compatibility issues because p returns nil and does not output anything.
That is to say, p is a shorthand for nil. Some code-golfers use it.

Maybe we can ignore them :p

(2) make it a public method

Kernel#p a is private method, so if we mistype obj.x as obj.p (not sure how it is feasible), it will raise a NoMethodError because of visibility.
We need to change this behavior.

Note

Past proposal and discussion

Endoh-san proposed the same idea 10+ years ago [ruby-dev:29736] in Japanese.
I think we should revisit this idea because of yield_self introduction.

In this thread, Matz said "simple p shows p(self), it is not clear".

[ruby-dev:30903]

  p

はどう動くのかとか(p selfと同じ、は変な気が)

  self.p(obj)

はどうなのかとか。その辺が解決(納得)できたら、ということで。

English translation:

What would the behavior of:

  p

be? (I feel strange for it to be equivalent to `p(self)`.) What would happen to

  self.p(obj)

pp

If this proposal is accepted, we also need to change the behavior of pp.

gems

tapp method is provided by a gem.
https://github.com/esminc/tapp

I thought about proposing this method in core. But I found that p is shorter than tapp.
A disadvantage is that p is too short and difficult to grep.

[mame (Yusuke Endoh)]

+1

Kernel#p is one of the greatest feature in Ruby. It would be further great to make it useful.

[zverok (Victor Shepelev)]

Small notice: If #13581 would be once acted upon, attaching p in the middle of the chain could be as simple as (using one of the proposed syntaxes)

  .yield_self { |response| JSON.parse(response) }.tap(&:.p)
  .dig('object', 'id')
  .yield_self { |id| id || '<undefined>' }.tap(&:.p)

Just .p is still nicer, though.

[Hanmac (Hans Mackowiak)]

hm i have a slightly problem with this

check out the different return types there:

a = []
p *a #=> nil

a = [1]
p *a #=> 1

a = [1,2]
p *a #=> [1,2]

[ko1 (Koichi Sasada)]

Hanmac: do you rely on this behavior?

[Hanmac (Hans Mackowiak)]

ko1 (Koichi Sasada) wrote:

Hanmac: do you rely on this behavior?

Me? not so much, in general i don't trust the return value of print commands (print returns null)

it was months ago, i probably wanted to point out that the return value of p might be differ depending on the parameters
It might be inconsistency?

There is Object.display too but i don't use it much because it doesn't have new lines

[osyo (manga osyo)]

Good proposal.
This can solve the following problem.

# NG: syntax error, unexpected ':', expecting '}'
p { name: "homu", age: 14 }

# OK:
{ name: "homu", age: 14 }.p
# => {:name=>"homu", :age=>14}

[nobu (Nobuyoshi Nakada)]

osyo (manga osyo) wrote:

Good proposal.
This can solve the following problem.

# NG: syntax error, unexpected ':', expecting '}'
p { name: "homu", age: 14 }

You can use parentheses.

p(name: "home", age: 14)

[shevegen (Robert A. Heiler)]

I don't mind, personally. To me, the biggest improvement was
that we could omit doing:

require 'pp'

;)

Since I did that a lot in my code. (I love pp; I think I use it
more than just p)

I personally have not been using (or needing) yield_self or tap so
far, so the change would probably not be of immediate benefit to
me; but probably also not require of me to change anything either.

To the name "tapp" - that name is a bit weird. To me it reads as
if we combine "tap" and then add a "p" to it. Reminds me of a
joke proposal to condense "end" into "enddd" and such. :D

To be fair, I consider the name yield_self to be also weird :D -
but matz added an alias called "then" to it if I understand it
correctly (though the semantic confuses me a bit as well ... but
I don't really want to distract here since I don't really feel
too strongly either way; picking good names is not always easy).

On a side note, perhaps in the long run we could have something
to "experiment" with - like new or changed features in ruby that
have not been 100% approved in the sense of a name AND the associated
functionality, so we can try them out for some time, which may help
build a stronger opinion either way. (I mean this in general, not
just in regards to #p here).

It may still be best to ask matz again though. Syntax shortcuts
(syntactic sugar) has always been an area in ruby where code changes
has happened (e. g. yield_self to then, or omitting the end value
for infinite ranges and so forth).

[nobu (Nobuyoshi Nakada)]

How about:

self.P

:P

[matz (Yukihiro Matsumoto)]

I vote for #tapp.

Matz.

[mrkn (Kenta Murata)]

• Related to Feature #15112: Introducing the short form of `STDERR.puts expr.inspect`. added

[docx (Lukas Dolezal)]

Allowing debugging within yield_self is great! If yield_self was inspired by Elixir, we can look at Elixir for inspiration here as well:

https://hexdocs.pm/elixir/IO.html#inspect/2

One of the examples that is very interesting is that they have optional parameter to describe the printout:

[1, 2, 3]
|> IO.inspect(label: "before")
|> Enum.map(&(&1 * 2))
|> IO.inspect(label: "after")
|> Enum.sum

Prints:

before: [1, 2, 3]
after: [2, 4, 6]

I'm wondering, would that be something we would like in Ruby too?

The problem with the proposal of making obj.p is that the same method already is defined with parameter in the form of p(obj).

Hence it would be impossible to implement obj.p("after") - as that would conflict with the current version of p with parameter.

My feeling is that given that both yield_self and tap are defined on Object, it does not feel to me right that .p should be defined on Kernel. I can see the advantage of already existing method there, but I feel like it would be wrong overloading.

Continuing on the argument that yield_self is defined on Object, should we define this "print inspect and return self" method be defined on Object too?

If so, we could make new method p_self (terrible name yes) and implement it with new interface when called as instance method and avoid making Kernel.p public:

Object.p_self() - p self and return self
Object.p_self(label:) - puts "#{label}: #{self.inspect}" and return self

What do you think?

[zverok (Victor Shepelev)]

docx (Lukas Dolezal) That are pretty interesting comparison with Elixir!
In fact, what the examples show (to me) is a need for more powerful partial application of procs. E.g. what would be a parity for Elixir is something like...

def p_with_label(label, obj)
  puts "#{label}#{obj.inspect}"
end

[1, 2, 3]
  .tap(&method(:p_with_label).curry['before: '])
  .map { |x| x * 2 }
  .tap(&method(:p_with_label).curry['after: '])
  .sum

This example already works, yet doesn't look VERY expressive, to be honest. I see two pretty different solutions to the problem:

First is core. Considering the shorten of &method(:name) is discussed for ages (and almost agreed to be &:.name), it could be discussed on addition of more powerful partial application than current (mostly unusable) Proc#curry to language core. The result may look somewhat like (fantasizing)...

[1, 2, 3]
  .tap(&:.p.with('before: '))
  .map { |x| x * 2 }
  .tap(&:.p.with('after: '))
  .sum

The second one is rise of proc-producing libraries:

module Functional
  module_function
  def inspect(label:)
    ->(obj) { puts "#{label}#{obj.inspect}" }
  end
end

[1, 2, 3]
  .tap(&Functional.inspect(label: 'before: '))
  .map { |x| x * 2 }
  .tap(&Functional.inspect(label: 'after: '))
  .sum

[nobu (Nobuyoshi Nakada)]

zverok (Victor Shepelev) wrote:

First is core. Considering the shorten of &method(:name) is discussed for ages (and almost agreed to be &:.name),

You may want to say &self.:name at https://bugs.ruby-lang.org/issues/13581#change-72072?
It wouldn't be able to omit the receiver, syntactically.

[zverok (Victor Shepelev)]

It wouldn't be able to omit the receiver, syntactically.

Hmm, I missed this point is the discussion. This limitation makes :. syntax sugar substantially less useful :(

[ko1 (Koichi Sasada)]

How about obj.p! or obj.pp! ?

[naruse (Yui NARUSE)]

• Target version deleted (2.6)

[akr (Akira Tanaka)]

matz (Yukihiro Matsumoto) wrote:

I vote for #tapp.

I don't like #tappp (for pp self ).

[nobu (Nobuyoshi Nakada)]

obj.?

[ko1 (Koichi Sasada)]

Discussion:

• obj.p has compatibility issue (some code expect p returns nil) -> reject
• p! is not danger. -> reject
• tapp seems long. tappp is weird
• tap_p, tap_pp are too long.
• introduce obj.? new method name -> don't modify syntax! (by matz). it is not predicate.

No conclusion.

[larskanis (Lars Kanis)]

+1 for obj.p, but I don't like the alternative names. IMHO obj.p is the natural extension of Kernel#p. Any other name doesn't feel right. Does the current p() => nil really justify a reject of such a useful feature?

[jeremyevans0 (Jeremy Evans)]

larskanis (Lars Kanis) wrote in #note-21:

+1 for obj.p, but I don't like the alternative names. IMHO obj.p is the natural extension of Kernel#p. Any other name doesn't feel right. Does the current p() => nil really justify a reject of such a useful feature?

I don't code like this is a major issue:

def foo
  p
end

That seems unlikely to occur in real world code as it doesn't do anything useful. Where I think you get into compatibility issues is code like:

def foo
  debug = []
  debug << 1 if something
  debug << 2 if something_else
  p(*debug)
end

Here, you would expect no output, not outputting of the receiver.

[sawa (Tsuyoshi Sawada)]

• Description updated (diff)
• Subject changed from `Kernel#p` without args shows the receiver to Let `Kernel#p` without an argument print the receiver

[sawa (Tsuyoshi Sawada)]

What about obj.tap? Currently, tap without a block raises a LocalJumpError, so such extended feature of tap without a block would not have compatibility issues. Furthermore, the meaning of the word "tap", as well as the current usage of the method tap with a block, matches the use case we have here.