Bug #9170: Math.sqrt returns different types when mathn is included; breaks various gems - this bug can be reproduced in Ruby 1.8 as well

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-darwin12.2.1]

[kranzky (Jason Hutchens)]

Math.sqrt() usually returns a Float or throws an exception. Some gems rely on this behaviour (e.g. by calling nan? on the results of a calculation that includes a sqrt).

After requiring mathn, Math.sqrt() returns an Integer in some cases (when the results of the sqrt would be a whole number).

Here is a transcript that demonstrates the problem:

$ rvm use ree
Using /Users/jason/.rvm/gems/ree-1.8.7-2012.02
$ ruby -v
ruby 1.8.7 (2012-02-08 MBARI 8/0x6770 on patchlevel 358) [i686-darwin12.1.0], MBARI 0x6770, Ruby Enterprise Edition 2012.02
$ ruby -e "puts Math.sqrt(4).nan?"
false
$ ruby -e "require 'mathn'; puts Math.sqrt(4).nan?"
-e:1: undefined method nan?' for 2:Fixnum (NoMethodError) $ rvm use 1.9.3 Using /Users/jason/.rvm/gems/ruby-1.9.3-p448 $ ruby -v ruby 1.9.3p448 (2013-06-27 revision 41675) [x86_64-darwin12.2.0] $ ruby -e "puts Math.sqrt(4).nan?" false $ ruby -e "require 'mathn'; puts Math.sqrt(4).nan?" -e:1:in ': undefined method nan?' for 2:Fixnum (NoMethodError) $ rvm use 2.0 Using /Users/jason/.rvm/gems/ruby-2.0.0-p247 $ ruby -v ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-darwin12.2.1] $ ruby -e "puts Math.sqrt(4).nan?" false $ ruby -e "require 'mathn'; puts Math.sqrt(4).nan?" -e:1:in ': undefined method nan?' for 2:Fixnum (NoMethodError) $ rvm use jruby Using /Users/jason/.rvm/gems/jruby-1.7.4 $ ruby -v jruby 1.7.4 (1.9.3p392) 2013-05-16 2390d3b on Java HotSpot(TM) 64-Bit Server VM 1.6.0_51-b11-457-11M4509 [darwin-x86_64] $ ruby -e "puts Math.sqrt(4).nan?" false $ ruby -e "require 'mathn'; puts Math.sqrt(4).nan?" NoMethodError: undefined method nan?' for 2:Fixnum
(root) at -e:1

Here is a discussion of how the bug manifests itself: https://github.com/olbrich/ruby-units/pull/48

Inclusion of mathn causes issues in RGeo, Alchemist, ActiveShipping and pry-rails

[mrkn (Kenta Murata)]

• Category set to core
• Status changed from Open to Feedback
• Target version set to 2.6

It's not a bug.

Do you want to introduce Numeric#nan? method to resolve this problem?
If so, this issue should be in Feature tracker.

Please tell us how do you think.

[mrkn (Kenta Murata)]

I think introducing Numeric#nan? is good.

[kranzky (Jason Hutchens)]

mrkn (Kenta Murata) wrote:

It's not a bug.

I'm a middle-man here, but I have gem maintainers claiming that it is a bug, because the documentation for Math.sqrt() indicates that it returns a Float, and that it's therefore OK to assume that it can be treated as a Float by the caller. Ref: http://ruby-doc.org/core-2.1.0/Math.html#method-c-sqrt

[marcandre (Marc-Andre Lafortune)]

Isn't this more an issue with the documentation of mathn not specifying all the methods it changes and how?

It might be wise to add a warning that it is suggested not to require it from a library because it might induce unexpected changes in any code using it?

It would be a prime candidate for refinements...

[zzak (zzak _)]

I will check the docs and see if I can fix it.

On Jan 6, 2014, at 12:44 AM, "kranzky (Jason Hutchens)" [e-mail] wrote:

Issue #9170 has been updated by kranzky (Jason Hutchens).

mrkn (Kenta Murata) wrote:

It's not a bug.

I'm a middle-man here, but I have gem maintainers claiming that it is a bug, because the documentation for Math.sqrt() indicates that it returns a Float, and that it's therefore OK to assume that it can be treated as a Float by the caller. Ref: http://ruby-doc.org/core-2.1.0/Math.html#method-c-sqrt

Bug #9170: Math.sqrt returns different types when mathn is included; breaks various gems - this bug can be reproduced in Ruby 1.8 as well
https://bugs.ruby-lang.org/issues/9170#change-44105

Author: kranzky (Jason Hutchens)
Status: Feedback
Priority: Normal
Assignee:
Category: core
Target version: next minor
ruby -v: ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-darwin12.2.1]
Backport: 1.9.3: UNKNOWN, 2.0.0: UNKNOWN

Math.sqrt() usually returns a Float or throws an exception. Some gems rely on this behaviour (e.g. by calling nan? on the results of a calculation that includes a sqrt).

After requiring mathn, Math.sqrt() returns an Integer in some cases (when the results of the sqrt would be a whole number).

Here is a transcript that demonstrates the problem:

$ rvm use ree
Using /Users/jason/.rvm/gems/ree-1.8.7-2012.02
$ ruby -v
ruby 1.8.7 (2012-02-08 MBARI 8/0x6770 on patchlevel 358) [i686-darwin12.1.0], MBARI 0x6770, Ruby Enterprise Edition 2012.02
$ ruby -e "puts Math.sqrt(4).nan?"
false
$ ruby -e "require 'mathn'; puts Math.sqrt(4).nan?"
-e:1: undefined method nan?' for 2:Fixnum (NoMethodError) $ rvm use 1.9.3 Using /Users/jason/.rvm/gems/ruby-1.9.3-p448 $ ruby -v ruby 1.9.3p448 (2013-06-27 revision 41675) [x86_64-darwin12.2.0] $ ruby -e "puts Math.sqrt(4).nan?" false $ ruby -e "require 'mathn'; puts Math.sqrt(4).nan?" -e:1:in ': undefined method nan?' for 2:Fixnum (NoMethodError) $ rvm use 2.0 Using /Users/jason/.rvm/gems/ruby-2.0.0-p247 $ ruby -v ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-darwin12.2.1] $ ruby -e "puts Math.sqrt(4).nan?" false $ ruby -e "require 'mathn'; puts Math.sqrt(4).nan?" -e:1:in ': undefined method nan?' for 2:Fixnum (NoMethodError) $ rvm use jruby Using /Users/jason/.rvm/gems/jruby-1.7.4 $ ruby -v jruby 1.7.4 (1.9.3p392) 2013-05-16 2390d3b on Java HotSpot(TM) 64-Bit Server VM 1.6.0_51-b11-457-11M4509 [darwin-x86_64] $ ruby -e "puts Math.sqrt(4).nan?" false $ ruby -e "require 'mathn'; puts Math.sqrt(4).nan?" NoMethodError: undefined method nan?' for 2:Fixnum
(root) at -e:1

Here is a discussion of how the bug manifests itself: https://github.com/olbrich/ruby-units/pull/48

Inclusion of mathn causes issues in RGeo, Alchemist, ActiveShipping and pry-rails

--
http://bugs.ruby-lang.org/

[naruse (Yui NARUSE)]

• Target version deleted (2.6)

[jeremyevans0 (Jeremy Evans)]

• Status changed from Feedback to Closed
• Backport deleted (1.9.3: UNKNOWN, 2.0.0: UNKNOWN)

mathn was removed from the standard library in Ruby 2.5.