Bug #15596: Kernel.warn without arguments should do the same as Kernel.warn(nil)

• Status: Rejected
• Assignee: -
• Target version: -
• ruby -v: ruby 2.6.1p33 (2019-01-30 revision 66950) [x86_64-darwin18]
• Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN

[kke (Kimmo Lehto)]

Kernel.warn without arguments does not print an empty line to $stderr.

This is inconsistent with Kernel.puts and it feels weird, because it does not act like a regular Ruby method would (if it was written in Ruby instead of C, it would probably be defined like def warn(msg = nil) and calling it with or without nil as argument would make no difference)

Expected behavior:

irb(main):001:0> warn nil

=> nil
irb(main):002:0> warn

=> nil

Actual behavior:

irb(main):001:0> warn nil

=> nil
irb(main):002:0> warn
=> nil

[shevegen (Robert A. Heiler)]

This is indeed a (to me) somewhat surprising behaviour; not that I guess
many have encountered it (and it probably is quite minor, which may
explain why that has not been reported before? Unless that was a more
recent change perhaps).

The current documentation is at:

https://ruby-doc.org/core-2.6.1/Kernel.html#method-i-warn

If the behaviour is not a bug then I think it should be mentioned
in the documentation at the least briefly to explain why it behaves
that way.

If the behaviour is a bug, though, then I think I concur with Kimmo,
but I really do not know any specifics. It did seem strange though,
since I was not able to determine as to why warn nil would be
treated differently than warn without arguments.

[nobu (Nobuyoshi Nakada)]

It is a natural behavior.

This method prints multiple arguments to STDERR, per line.
warn nil passes one argument and prints one line, whereas warn passes no argument and prints zero line.

[jeremyevans0 (Jeremy Evans)]

• Status changed from Open to Rejected

As nobu stated, the current behavior is expected in regards to not printing a newline if no arguments are given, and the documentation is accurate for the current behavior. It states converts each of the messages to strings, appends a newline character to the string if the string does not end in a newline, and calls Warning.warn with the string.. To me, that implies if there are no messages/arguments, it does not do anything.

It is true that warn is different than puts in how a zero argument call is handled. warn it is similar to p in this regard. puts is specifically documented as adding a newline for no arguments (If called without arguments, outputs a single newline), and warn is not.