# User-defined class coercion? **URL:** \ **Category:** ruby-talk\ **Created:** [6 August 2003 03:00 UTC](https://rubytalk.org/t/user-defined-class-coercion/7239 "2003-08-06T03:00:55Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Mark\_J\_Reed](https://avatars.discourse-cdn.com/v4/letter/m/f6c823/32.png) [@Mark\_J\_Reed](https://rubytalk.org/u/Mark_J_Reed)\ **Post date:** [6 August 2003 03:00 UTC](https://rubytalk.org/t/user-defined-class-coercion/7239/1 "2003-08-06T03:00:55Z")
I would like to define my own class that works as a number when treated as a number but also has other properties. Can I do this? If so, how? -Mark ---
**Author:** ![Yukihiro\_Matsumoto2](https://avatars.discourse-cdn.com/v4/letter/y/b9e5f3/32.png) [@Yukihiro\_Matsumoto2](https://rubytalk.org/u/Yukihiro_Matsumoto2)\ **Post date:** [6 August 2003 03:20 UTC](https://rubytalk.org/t/user-defined-class-coercion/7239/2 "2003-08-06T03:20:04Z")
Hi, > I would like to define my own class that works as a number when > treated as a number but also has other properties. Can I do this? > If so, how? By defining “coerce” method: class Foo def coerce(other) … return converted\_other, self end end ``` matz. ``` > **···** > > In message “User-defined class coercion?” on 03/08/06, “Mark J. Reed” [markjreed@mail.com](mailto:markjreed@mail.com) writes: ---
**Author:** ![Dave\_Mail\_Archive](https://avatars.discourse-cdn.com/v4/letter/d/f04885/32.png) [@Dave\_Mail\_Archive](https://rubytalk.org/u/Dave_Mail_Archive)\ **Post date:** [6 August 2003 04:12 UTC](https://rubytalk.org/t/user-defined-class-coercion/7239/3 "2003-08-06T04:12:04Z")
Mark J. Reed wrote: > I would like to define my own class that works as a number when > treated as a number but also has other properties. Can I do this? > If so, how? Here’s an extract from the new reference material for the coerce method: ``` Coerce is both an instance method of Numeric and part of a type conversion protocol. When a number is asked to perform an operation, and is passed a parameter of class different to its own, it must first coerce both itself and that parameter into a common class so that the operation makes sense. For example, in the expression $1 + 2.5$, the Fixnum ``1'' must be converted to a Float to make it compatible with ``2.5''. This conversion is performed by coerce. For all numeric object, coerce is straightforward: if aNumeric is the same type as self, returns an array containing aNumeric and self. Otherwise, returns an array with both aNumeric and self represented as Float objects. If a numeric object is asked to operate on a non-numeric, it tries to invoke coerce on that other object. For example, if you write: 1 + "2" Ruby will effectively execute the code as: n1, n2 = "2".coerce(1) n2 + n1 In the more general case, this won't work, as most non-numerics don't define a coerce method. However, you can use this (if you feel so inclined) to implement part of Perl's automatic conversion of strings to numbers in expressions. class String def coerce(other) case other when Integer begin return Integer(self), other rescue return Float(self), Float(other) end when Float return Float(self), other else super end end end 1 + "2" 1 + "2.3" 1.2 + "2.3" 1.5 + "2" ``` ---
**Author:** ![Mark\_J\_Reed](https://avatars.discourse-cdn.com/v4/letter/m/f6c823/32.png) [@Mark\_J\_Reed](https://rubytalk.org/u/Mark_J_Reed)\ **Post date:** [6 August 2003 03:41 UTC](https://rubytalk.org/t/user-defined-class-coercion/7239/4 "2003-08-06T03:41:01Z")
MR = me YM = Matz > I would like to define my own class that works as a number when > treated as a number but also has other properties. Can I do this? > If so, how? > By defining “coerce” method: > > class Foo > def coerce(other) > … > return converted\_other, self > end > end That works wonderfully. Thank you, for everything! Didn’t expect my dumb newbie question to be answered by the creator of the language. 🙂 -Mark ---
**Author:** ![StephenSykes](https://avatars.discourse-cdn.com/v4/letter/s/0ea827/32.png) [@StephenSykes](https://rubytalk.org/u/StephenSykes)\ **Post date:** [6 August 2003 11:02 UTC](https://rubytalk.org/t/user-defined-class-coercion/7239/5 "2003-08-06T11:02:04Z")
> ``` > In the more general case, this won't work, as most non-numerics > don't define a coerce method. However, you can use this > (if you feel so inclined) to implement part of Perl's automatic > conversion of strings to numbers in expressions. > > class String > def coerce(other) > case other > when Integer > begin > return Integer(self), other > rescue > return Float(self), Float(other) > end > when Float > return Float(self), other > else super > end > end > end > > 1 + "2" > 1 + "2.3" > 1.2 + "2.3" > 1.5 + "2" > > ``` But… 1 - “4” =\> 3 1 - “2.4” =\> 1.4 The return values were the wrong way around, this should work better: class String def coerce(other) case other when Integer begin return other, Integer(self) rescue return Float(other), Float(self) end when Float return other, Float(self) else super end end end But even so… 8 | “16” TypeError: cannot convert String into Integer from (irb):47:in `|’ from (irb):47 from ❤:0 I feel it is an anomaly for the bitwise operators to not call coerce. Yes, it makes no sense to coerce to or from floats for bitwise, but there are other cases as the above shows. Stephen ± S.D.Sykes - [www.stephensykes.com](http://www.stephensykes.com) > **···** > > > “You are a woman, aren’t you?” – The Duke of Edinburgh checks his > > facts after receiving a gift from a woman, Kenya 1984. ---
**Author:** ![Mark\_Wilson](https://avatars.discourse-cdn.com/v4/letter/m/a3d4f5/32.png) [@Mark\_Wilson](https://rubytalk.org/u/Mark_Wilson)\ **Post date:** [6 August 2003 03:54 UTC](https://rubytalk.org/t/user-defined-class-coercion/7239/6 "2003-08-06T03:54:24Z")
And that’s only one of the many charms of Ruby. 🙂 I started out very enthusiastic about Ruby and it just keeps getting better. In some ways, encountering Ruby is like having a veil that was obscuring your vision lifted from your eyes. Regards, Mark > **···** > > On Tuesday, August 5, 2003, at 11:41 PM, Mark J. Reed wrote: > > > [snip] > > That works wonderfully. Thank you, for everything! Didn’t expect my > > dumb newbie question to be answered by the creator of the language. > > 🙂 > > > > -Mark ---
**Author:** ![Dave\_Mail\_Archive](https://avatars.discourse-cdn.com/v4/letter/d/f04885/32.png) [@Dave\_Mail\_Archive](https://rubytalk.org/u/Dave_Mail_Archive)\ **Post date:** [6 August 2003 13:23 UTC](https://rubytalk.org/t/user-defined-class-coercion/7239/7 "2003-08-06T13:23:42Z")
haldane wrote: > But… > > 1 - “4” =\> 3 > 1 - “2.4” =\> 1.4 > > The return values were the wrong way around, this should work better: Oops - and I even documented that… sigh Thanks Dave