# Commutativity of '\*' **URL:** \ **Category:** ruby-talk\ **Created:** [2 May 2004 11:23 UTC](https://rubytalk.org/t/commutativity-of/11321 "2004-05-02T11:23:59Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![Stephan\_Kamper3](https://avatars.discourse-cdn.com/v4/letter/s/d9b06d/32.png) [@Stephan\_Kamper3](https://rubytalk.org/u/Stephan_Kamper3)\ **Post date:** [2 May 2004 11:23 UTC](https://rubytalk.org/t/commutativity-of/11321/1 "2004-05-02T11:23:59Z")
Hi all, say I have a new class Foo which provides \* as a multiplicative method. Now, given the following lines: def some\_operation\_using( lft, rgt ) return "#{lft} times #{rgt}" end class Foo def \*( other ) # do something res = some\_operation\_using( other, self ) return res end def to\_s return "a Foo thingy." end end # That given, the following lines will work f = Foo.new puts f \* 42.0 # But this won’t. Instead I get: # …/lm.rb:25:in `\*’: Foo can’t be coerced into Float (TypeError) # from …/lm.rb:25 puts 42.0 \* f What would be a (or even the) Ruby Way to get the last line to work? Some method to coerce a Foo into a Float? Or some hacking of Float itself? Like adding that, for example: class Float alias old\_mult \* def \*( other ) if other.class == Foo return other \* self else return self.old\_mult( other ) end end end That doesn’t seem to be terribly elegant, I think. Any other way? Happy rubying Stephan ---
**Author:** ![ts1](https://avatars.discourse-cdn.com/v4/letter/t/a8b319/32.png) [@ts1](https://rubytalk.org/u/ts1)\ **Post date:** [2 May 2004 11:43 UTC](https://rubytalk.org/t/commutativity-of/11321/2 "2004-05-02T11:43:20Z")
> # But this won't. Instead I get: > # .../lm.rb:25:in `\*': Foo can't be coerced into Float (TypeError) Well, ruby is saying that it can coerce, learn it how to do it One way to do it svg% cat b.rb #!/usr/bin/ruby def some\_operation\_using( lft, rgt )    "#{lft} times #{rgt}" end class Foo    def \*(other)       # do something       res = some\_operation\_using( other, self )       res    end    def coerce(other)       case other       when Numeric          [self, other]       else          raise TypeError       end    end    def to\_s       "a Foo thingy."    end end f = Foo.new puts f \* 42.0 puts 42.0 \* f svg% svg% b.rb 42.0 times a Foo thingy. 42.0 times a Foo thingy. svg% Guy Decoux ---
**Author:** ![Robert](https://avatars.discourse-cdn.com/v4/letter/r/e95f7d/32.png) [@Robert](https://rubytalk.org/u/Robert)\ **Post date:** [2 May 2004 11:53 UTC](https://rubytalk.org/t/commutativity-of/11321/3 "2004-05-02T11:53:59Z")
“Stephan Kämper” [Stephan.Kaemper@Schleswig-Holstein.de](mailto:Stephan.Kaemper@Schleswig-Holstein.de) schrieb im Newsbeitrag [news:c72lo9$hi3jp$1@ID-96582.news.uni-berlin.de](mailto:news:c72lo9$hi3jp$1@ID-96582.news.uni-berlin.de)… > Hi all, > > say I have a new class Foo which provides \* as a multiplicative method. > Now, given the following lines: > > def some\_operation\_using( lft, rgt ) > return “#{lft} times #{rgt}” > end > > class Foo > > def \*( other ) > # do something > res = some\_operation\_using( other, self ) > return res > end > > def to\_s > return “a Foo thingy.” > end > > end > > # That given, the following lines will work > > f = Foo.new > puts f \* 42.0 > > # But this won’t. Instead I get: > > # …/lm.rb:25:in `\*': Foo can’t be coerced into Float (TypeError) > > # from …/lm.rb:25 > > puts 42.0 \* f > > What would be a (or even the) Ruby Way to get the last line to work? > Some method to coerce a Foo into a Float? Or some hacking of Float > itself? Like adding that, for example: > > class Float > alias old\_mult \* > def \*( other ) > if other.class == Foo > return other \* self > else > return self.old\_mult( other ) > end > end > end > > That doesn’t seem to be terribly elegant, I think. > > Any other way? You want coerce. See my posting in thread “coerce(), what protocol to implement it” from 30 Apr. [http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/98791](http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/98791) Kind regards ``` robert ``` ---
**Author:** ![Kristof\_Bastiaensen1](https://avatars.discourse-cdn.com/v4/letter/k/9fc29f/32.png) [@Kristof\_Bastiaensen1](https://rubytalk.org/u/Kristof_Bastiaensen1)\ **Post date:** [2 May 2004 19:33 UTC](https://rubytalk.org/t/commutativity-of/11321/4 "2004-05-02T19:33:58Z")
It is maybe not a good idea to reverse the order in coerce. It may work with \* and +, but you will get a problem when order matters, like in / (division): #— including the above code — class Foo def /(other) “divide #{self} by #{other}” end end puts (f / 2) =\> “divide a Foo thingy. by 2” puts (2 / f) =\> “divide a Foo thingy. by 2” #— end code — Note that some classes which use \*, and don’t represent a number just give an error: “string” \* 2 =\> “stringstring” 2 \* “string” =\> TypeError: String can’t be coerced into Fixnum That behaviour is probably the best if you are not representing a number. Kristof > **···** > > On Sun, 02 May 2004 20:43:20 +0900, ts wrote: > > > #!/usr/bin/ruby > > def some\_operation\_using( lft, rgt ) > > “#{lft} times #{rgt}” > > end > > > > class Foo > > > > def \*(other) > > # do something > > res = some\_operation\_using( other, self ) > > res > > end > > > > def coerce(other) > > case other > > when Numeric > > [self, other] > > else > > raise TypeError > > end > > end > > > > def to\_s > > “a Foo thingy.” > > end > > > > end > > > > f = Foo.new > > puts f \* 42.0 > > puts 42.0 \* f > > svg% > > > > svg% b.rb > > 42.0 times a Foo thingy. > > 42.0 times a Foo thingy. > > svg% > > > > Guy Decoux