# Can you enable coercion between types? **URL:** \ **Category:** ruby-talk\ **Created:** [18 March 2006 19:48 UTC](https://rubytalk.org/t/can-you-enable-coercion-between-types/25895 "2006-03-18T19:48:49Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Larry\_White](https://avatars.discourse-cdn.com/v4/letter/l/ebca7d/32.png) [@Larry\_White](https://rubytalk.org/u/Larry_White)\ **Post date:** [18 March 2006 19:48 UTC](https://rubytalk.org/t/can-you-enable-coercion-between-types/25895/1 "2006-03-18T19:48:49Z")
I have a ruby class called Duration that represents a unit of time. It's main attribute is a number that represents the duration in seconds. If I try to save it to a DB using Rails, I get a "Duration can not be coerced into Float" error. Is there a way to modify the definition of Duration to enable automatic coercion into a float? (I'm thinking of something like adding a "to\_float" method to the class.) thanks in advance larry ---
**Author:** ![7rans](https://avatars.discourse-cdn.com/v4/letter/7/a8b319/32.png) [@7rans](https://rubytalk.org/u/7rans)\ **Post date:** [18 March 2006 20:23 UTC](https://rubytalk.org/t/can-you-enable-coercion-between-types/25895/2 "2006-03-18T20:23:49Z")
Did you try #to\_f? T. ---
**Author:** ![Ara.T.Howard6](https://avatars.discourse-cdn.com/v4/letter/a/958977/32.png) [@Ara.T.Howard6](https://rubytalk.org/u/Ara.T.Howard6)\ **Post date:** [18 March 2006 23:11 UTC](https://rubytalk.org/t/can-you-enable-coercion-between-types/25895/3 "2006-03-18T23:11:59Z")
try this:      harp:~ \> cat a.rb      class Duration        def initialize seconds          @seconds = seconds        end        def coerce other          if other.class == @seconds.class            [@seconds, other]          else            [Float(@seconds), Float(other)]          end        end      end      p(40.0 + Duration::new(2.0))      p(40 + Duration::new(2))      harp:~ \> ruby a.rb      42.0      42 hth. -a > **···** > > On Sun, 19 Mar 2006, larry wrote: > > > I have a ruby class called Duration that represents a unit of time. > > It's main attribute is a number that represents the duration in > > seconds. > > > > If I try to save it to a DB using Rails, I get a "Duration can not be > > coerced into Float" error. Is there a way to modify the definition of > > Duration to enable automatic coercion into a float? (I'm thinking of > > something like adding a "to\_float" method to the class.) > > > > thanks in advance > > larry > > -- > share your knowledge. it's a way to achieve immortality. > - h.h. the 14th dali lama ---
**Author:** ![7rans](https://avatars.discourse-cdn.com/v4/letter/7/a8b319/32.png) [@7rans](https://rubytalk.org/u/7rans)\ **Post date:** [19 March 2006 01:18 UTC](https://rubytalk.org/t/can-you-enable-coercion-between-types/25895/4 "2006-03-19T01:18:48Z")
Could you explain what this is doing? I don't get the effect. Thanks, T. > **···** > > ara.t.howard@noaa.gov wrote: > > > try this: > > > >      harp:~ \> cat a.rb > >      class Duration > >        def initialize seconds > >          @seconds = seconds > >        end > >        def coerce other > >          if other.class == @seconds.class > >            [@seconds, other] > >          else > >            [Float(@seconds), Float(other)] > >          end > >        end > >      end ---
**Author:** ![Markus](https://avatars.discourse-cdn.com/v4/letter/m/b782af/32.png) [@Markus](https://rubytalk.org/u/Markus)\ **Post date:** [19 March 2006 05:20 UTC](https://rubytalk.org/t/can-you-enable-coercion-between-types/25895/5 "2006-03-19T05:20:28Z")
There's a gottcha here. Coerce is used to implement double dispatch and so needs to return the coerced values in the opposite order (see Pick Axe)--(other,self), not (self,other). The way you wrote it works fine for addition & multiplication, but you'll pull your hair out the first time you try to subtract or divide. --MarkusQ P.S. And I think (as Tom noted) that "to\_f" was the answer to the original question. > **···** > > On Sat, 2006-03-18 at 17:11, ara.t.howard@noaa.gov wrote: > > > try this: > > > >      harp:~ \> cat a.rb > >      class Duration > >        def initialize seconds > >          @seconds = seconds > >        end > >        def coerce other > >          if other.class == @seconds.class > >            [@seconds, other] > >          else > >            [Float(@seconds), Float(other)] > >          end > >        end ---
**Author:** ![Ara.T.Howard6](https://avatars.discourse-cdn.com/v4/letter/a/958977/32.png) [@Ara.T.Howard6](https://rubytalk.org/u/Ara.T.Howard6)\ **Post date:** [19 March 2006 02:25 UTC](https://rubytalk.org/t/can-you-enable-coercion-between-types/25895/6 "2006-03-19T02:25:04Z")
my understanding of coerce is that it's used to produce a pair of operands - myself and other - which can be used to perform operations. so, in the case of Fixnum#+ we might see    def + other      x, y = other.coerce self      x + y    end which is to say Fixnum need not know about how to add every possible class - rather it leaves that knowledge up to those classes to export via thier coerce method. as i understand it that method should return two new objects which interoperate so, in the case above (which is made up) we might have    duration = Duration::new 42.0    42 + duration here would call duration(42) and expect back an x and y which could be added together. so, if @seconds happens to be a Fixnum the result will be another Fixnum. if @seconds happens to a Float the result would be a Float. i admit that i'm not crystal clear on this, but it sure seems to work:      harp:~ \> cat a.rb      class Duration        def initialize seconds          @seconds = seconds        end        def coerce other          if other.class == @seconds.class            [@seconds, other]          else            [Float(@seconds), Float(other)]          end        end        def + other          x, y = coerce(other)          x + y        end      end      def Duration(\*a, &b); Duration::new(\*a, &b); end      p( 40 + Duration(2.0) ) # Float 42.0      p( 40 + Duration(2) ) # Fixnum 42      p( 40.0 + Duration(2.0) ) # Float 42.0      p( 40.0 + Duration(2) ) # Float 42.0      p( Duration(40) + 2 ) # Fixnum 42      p( Duration(40) + 2.0 ) # Float 42.0      p( Duration(40.0) + 2) # Float 42.0      p( Duration(40.0) + 2.0 ) # Float 42.0      harp:~ \> ruby a.rb      42.0      42      42.0      42      42.0 i assume one would do something nice with the coerce method using Time, Date, and DateTime objects too... regards. -a > **···** > > On Sun, 19 Mar 2006, Trans wrote: > > > ara.t.howard@noaa.gov wrote: > > > > > try this: > > > > > >      harp:~ \> cat a.rb > > >      class Duration > > >        def initialize seconds > > >          @seconds = seconds > > >        end > > >        def coerce other > > >          if other.class == @seconds.class > > >            [@seconds, other] > > >          else > > >            [Float(@seconds), Float(other)] > > >          end > > >        end > > >      end > > > > Could you explain what this is doing? I don't get the effect. > > -- > share your knowledge. it's a way to achieve immortality. > - h.h. the 14th dali lama ---
**Author:** ![Ara.T.Howard6](https://avatars.discourse-cdn.com/v4/letter/a/958977/32.png) [@Ara.T.Howard6](https://rubytalk.org/u/Ara.T.Howard6)\ **Post date:** [19 March 2006 15:20 UTC](https://rubytalk.org/t/can-you-enable-coercion-between-types/25895/7 "2006-03-19T15:20:15Z")
indeed. good catch. cheers. -a > **···** > > On Sun, 19 Mar 2006, Markus wrote: > > > On Sat, 2006-03-18 at 17:11, ara.t.howard@noaa.gov wrote: > > > > > try this: > > > > > >      harp:~ \> cat a.rb > > >      class Duration > > >        def initialize seconds > > >          @seconds = seconds > > >        end > > >        def coerce other > > >          if other.class == @seconds.class > > >            [@seconds, other] > > >          else > > >            [Float(@seconds), Float(other)] > > >          end > > >        end > > > > There's a gottcha here. Coerce is used to implement double dispatch and > > so needs to return the coerced values in the opposite order (see Pick > > Axe)--(other,self), not (self,other). > > > > The way you wrote it works fine for addition & multiplication, but > > you'll pull your hair out the first time you try to subtract or divide. > > -- > share your knowledge. it's a way to achieve immortality. > - h.h. the 14th dali lama