# Can you enable coercion between types? **URL:** \ **Category:** Ruby\ **Created:** [March 18, 2006, 8:50pm UTC](https://www.ruby-forum.com/t/can-you-enable-coercion-between-types/55268 "2006-03-18T20:50:40Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![larry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/47e85d/32.png) [@larry](https://www.ruby-forum.com/u/larry)\ **Post date:** [March 18, 2006, 8:50pm UTC](https://www.ruby-forum.com/t/can-you-enable-coercion-between-types/55268/1 "2006-03-18T20:50:40Z")
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:** ![larry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/47e85d/32.png) [@larry](https://www.ruby-forum.com/u/larry)\ **Post date:** [March 19, 2006, 1:35am UTC](https://www.ruby-forum.com/t/can-you-enable-coercion-between-types/55268/2 "2006-03-19T01:35:28Z")
On Sun, 19 Mar 2006, larry wrote: > larry > 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 ---
**Author:** ![larry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/47e85d/32.png) [@larry](https://www.ruby-forum.com/u/larry)\ **Post date:** [March 19, 2006, 2:20am UTC](https://www.ruby-forum.com/t/can-you-enable-coercion-between-types/55268/3 "2006-03-19T02:20:58Z")
removed\_email\_address@domain.invalid wrote: > ``` > else > [Float(@seconds), Float(other)] > end > end > end > > ``` Could you explain what this is doing? I don’t get the effect. Thanks, T. ---
**Author:** ![larry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/47e85d/32.png) [@larry](https://www.ruby-forum.com/u/larry)\ **Post date:** [March 18, 2006, 9:24pm UTC](https://www.ruby-forum.com/t/can-you-enable-coercion-between-types/55268/4 "2006-03-18T21:24:09Z")
Did you try #to\_f? T. ---
**Author:** ![larry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/47e85d/32.png) [@larry](https://www.ruby-forum.com/u/larry)\ **Post date:** [March 19, 2006, 3:27am UTC](https://www.ruby-forum.com/t/can-you-enable-coercion-between-types/55268/5 "2006-03-19T03:27:22Z")
On Sun, 19 Mar 2006, Trans wrote: > > ``` > > 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. 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 42.0 42 42.0 42.0 42.0 ``` i assume one would do something nice with the coerce method using Time, Date, and DateTime objects too… regards. -a ---
**Author:** ![larry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/47e85d/32.png) [@larry](https://www.ruby-forum.com/u/larry)\ **Post date:** [March 19, 2006, 7:29am UTC](https://www.ruby-forum.com/t/can-you-enable-coercion-between-types/55268/6 "2006-03-19T07:29:54Z")
On Sat, 2006-03-18 at 17:11, removed\_email\_address@domain.invalid wrote: > ``` > 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. –MarkusQ P.S. And I think (as Tom noted) that “to\_f” was the answer to the original question. ---
**Author:** ![larry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/47e85d/32.png) [@larry](https://www.ruby-forum.com/u/larry)\ **Post date:** [March 19, 2006, 4:20pm UTC](https://www.ruby-forum.com/t/can-you-enable-coercion-between-types/55268/7 "2006-03-19T16:20:34Z")
On Sun, 19 Mar 2006, Markus wrote: > > ``` > > if other.class == @seconds.class > > > > ``` > > 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. indeed. good catch. cheers. -a