# Can't change the value of self **URL:** \ **Category:** Ruby\ **Created:** [January 2, 2006, 1:39am UTC](https://www.ruby-forum.com/t/cant-change-the-value-of-self/51509 "2006-01-02T01:39:36Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![johanatan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/8491ac/32.png) [@johanatan](https://www.ruby-forum.com/u/johanatan)\ **Post date:** [January 2, 2006, 1:39am UTC](https://www.ruby-forum.com/t/cant-change-the-value-of-self/51509/1 "2006-01-02T01:39:36Z")
This code: * * * class Time # Rounds a time sensibly. # Currently just hour is supported (as that’s the only one needed) def round\_to(unit = :hour) unless min == 0 new\_hour = hour new\_hour += 1 if min \> 30 time = Time.local(year, month, day, new\_hour) end time end def round\_to!(\*args) self = round\_to \*args end ## end Throws this error: Can’t change the value of self (SyntaxError) self = round\_to \*args Why doesn’t it work? What can I do instead? Thanks ---
**Author:** ![johanatan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/8491ac/32.png) [@johanatan](https://www.ruby-forum.com/u/johanatan)\ **Post date:** [January 2, 2006, 1:42am UTC](https://www.ruby-forum.com/t/cant-change-the-value-of-self/51509/2 "2006-01-02T01:42:38Z")
Jonathan L. wrote: > Can’t change the value of self (SyntaxError) > self = round\_to \*args > > Why doesn’t it work? What can I do instead? “Destructive” methods must alter self using that self’s existing destructive methods. James – [http://www.ruby-doc.org](http://www.ruby-doc.org) - Ruby Help & Documentation [Ruby Code & Style](http://www.artima.com/rubycs/) - Ruby Code & Style: Writers wanted [http://www.rubystuff.com](http://www.rubystuff.com) - The Ruby Store for Ruby Stuff [http://www.jamesbritt.com](http://www.jamesbritt.com) - Playing with Better Toys [http://www.30secondrule.com](http://www.30secondrule.com) - Building Better Tools ---
**Author:** ![johanatan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/8491ac/32.png) [@johanatan](https://www.ruby-forum.com/u/johanatan)\ **Post date:** [January 2, 2006, 1:48am UTC](https://www.ruby-forum.com/t/cant-change-the-value-of-self/51509/3 "2006-01-02T01:48:39Z")
James B. wrote: > Jonathan L. wrote: > > > Can’t change the value of self (SyntaxError) > > self = round\_to \*args > > > > Why doesn’t it work? What can I do instead? > > “Destructive” methods must alter self using that self’s existing > destructive methods. To which we should add: Time has no destructive methods, since Time objects are immutable. But someone once created a MutableTime class – anyone recall who/when/where and whether it’s complete? Hal ---
**Author:** ![johanatan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/8491ac/32.png) [@johanatan](https://www.ruby-forum.com/u/johanatan)\ **Post date:** [January 2, 2006, 2:12am UTC](https://www.ruby-forum.com/t/cant-change-the-value-of-self/51509/4 "2006-01-02T02:12:58Z")
On Mon, 2006-01-02 at 09:47 +0900, Hal F. wrote: > > destructive methods. > > To which we should add: Time has no destructive methods, since > Time objects are immutable. > > But someone once created a MutableTime class – anyone recall > who/when/where and whether it’s complete? Thanks for the replies. Why is Time immutable (presumably it’s deliberately designed like that)? The MutableTime class sounds interesting but it’s probably overkill for what I wanted to do – I’ll just modify variables at a higher level. Thanks ---
**Author:** ![johanatan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/8491ac/32.png) [@johanatan](https://www.ruby-forum.com/u/johanatan)\ **Post date:** [January 2, 2006, 4:22am UTC](https://www.ruby-forum.com/t/cant-change-the-value-of-self/51509/5 "2006-01-02T04:22:12Z")
On 1/1/06, Hal F. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > destructive methods. > > To which we should add: Time has no destructive methods, since > Time objects are immutable. > > But someone once created a MutableTime class – anyone recall > who/when/where and whether it’s complete? This is why I can’t do: class Fixnum def double! self \*= 2 end end …right? Too bad. Speaking of methods… is there a way to do: class Something def \<\<(a, b) # do something with a and b end end s = Something.new s \<\< ‘a’, ‘b’ results in a syntax error. Is that another of those YACC limitations? ---
**Author:** ![johanatan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/8491ac/32.png) [@johanatan](https://www.ruby-forum.com/u/johanatan)\ **Post date:** [January 2, 2006, 2:19am UTC](https://www.ruby-forum.com/t/cant-change-the-value-of-self/51509/6 "2006-01-02T02:19:01Z")
On Jan 1, 2006, at 7:37 PM, Jonathan L. wrote: > Can’t change the value of self (SyntaxError) > self = round\_to \*args > > Why doesn’t it work? What can I do instead? Instead of ``` a.round_to!(:hour) ``` just write ``` a = a.round_to(:hour) ``` Or maybe a different name would help: ``` a = a.closest(:hour) ``` Gary W. ---
**Author:** ![johanatan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/8491ac/32.png) [@johanatan](https://www.ruby-forum.com/u/johanatan)\ **Post date:** [January 2, 2006, 5:16am UTC](https://www.ruby-forum.com/t/cant-change-the-value-of-self/51509/7 "2006-01-02T05:16:18Z")
Wilson B. wrote: > …right? Too bad. Well, that, and what would “2.double!” do? > My guess is you’re out of luck, there. But you could change it to take s \<\< [‘a’,‘b’] or s \<\< ‘a’ \<\< ‘b’. Devin ---
**Author:** ![johanatan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/8491ac/32.png) [@johanatan](https://www.ruby-forum.com/u/johanatan)\ **Post date:** [January 2, 2006, 8:23am UTC](https://www.ruby-forum.com/t/cant-change-the-value-of-self/51509/8 "2006-01-02T08:23:37Z")
Wilson B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes: > You could do s.\<\<(‘a’, ‘b’), but it seems unrubyish. Perhaps you could settle for: s \<\< [‘a’, ‘b’] > Is that another of those YACC limitations? Hmmm, I don’t know if it’s a limitation, but it could get confusing. Consider: four, three = 1 \<\< 2, 3 arr = [x \<\< 1, x \<\< 2] And if you do it for `<<', then what about `+‘, `-', `%’? If you do do it for those, then you start to lose really basic stuff like: arr = [a + 1, b + 1] b, a = a % b, b until b == 0 ---
**Author:** ![johanatan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/8491ac/32.png) [@johanatan](https://www.ruby-forum.com/u/johanatan)\ **Post date:** [January 2, 2006, 1:06pm UTC](https://www.ruby-forum.com/t/cant-change-the-value-of-self/51509/9 "2006-01-02T13:06:59Z")
Thanks very much for this explanation, the restriction makes a lot more sense to me now. Jon ---
**Author:** ![johanatan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/8491ac/32.png) [@johanatan](https://www.ruby-forum.com/u/johanatan)\ **Post date:** [January 2, 2006, 11:33am UTC](https://www.ruby-forum.com/t/cant-change-the-value-of-self/51509/10 "2006-01-02T11:33:27Z")
On Mon, 2 Jan 2006 09:37:45 +0900, Jonathan L. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Can’t change the value of self (SyntaxError) > self = round\_to \*args > > Why doesn’t it work? What can I do instead? One way of thinking about reassigning self is if you imagine in a numeric class saying def double self \*= 2 end That would amount to saying, if we did 1.double, 1 becomes 2 which would mean that 1+1 would thereafter be 4, and that would be bad. What we need is not the value 1 to become 2, but to get the value 2 back when we say 1.double. So we say def double return self\*2 end or, since “return” is implied def double self\*2 end In general in OO, an object can change its contents, but it cannot change its “self”. What should happen in this program? one = 1 another\_one = 1 assert\_equal(2, one+one) assert\_equal(2, another\_one+another\_one) one.double assert\_equal(4, one+one) assert\_equal(???,another\_one+another\_one) assert\_equal(???, 1+1) When we double one, if the self\*=2 thing worked, the ??? asserts would also find 4. We might be able to understand another\_one being 2, but to have all occurrences of 1+1 in the program turn into 4 … that would be bad. Certain objects are called “value objects”, and the idea is that like numbers, they never change value. For many objects it is a design choice whether or not to make them value objects, even if they are quite complex. For example, in the articles I’m currently writing on extended set theory on my site, I’m moving in the direction of making my sets value objects. They are essentially arbitrarily large collections of records (like relational database relations), and most DB apps think of relations as mutable. I’m going to make a different decision, namely that any set, once created, is a constant. It’ll be interesting to see what happens. ---
**Author:** ![johanatan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/8491ac/32.png) [@johanatan](https://www.ruby-forum.com/u/johanatan)\ **Post date:** [January 2, 2006, 8:45pm UTC](https://www.ruby-forum.com/t/cant-change-the-value-of-self/51509/11 "2006-01-02T20:45:37Z")
Jonathan L. wrote: > Thanks very much for this explanation, the restriction makes a lot more > sense to me now. You should also know that more complex objects (as you know) can often change their state. In particular, Array and String both implement a replace method, which totally replaces their contents. But even then, the object id stays the same. Hal ---
**Author:** ![johanatan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/8491ac/32.png) [@johanatan](https://www.ruby-forum.com/u/johanatan)\ **Post date:** [January 5, 2006, 2:33am UTC](https://www.ruby-forum.com/t/cant-change-the-value-of-self/51509/12 "2006-01-05T02:33:40Z")
Immutable classes are less prone to bugs than mutable classes. This is because they have a single state. They are also thread safe. It is good practise to favor immutability when there are no compelling reasons for making a class mutable. A Time object represents an instance in time. Making it mutable could easily get confusing. Consider: time = MutableTime.at(946702800) rocket.launch\_time = time … time.change(946701565) # Changes the launch time of the rocket The launch time of the rocket can be changed without setting the launch\_time property explicitly. This is a potential source of bugs. /Henrik ---
**Author:** ![johanatan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/8491ac/32.png) [@johanatan](https://www.ruby-forum.com/u/johanatan)\ **Post date:** [January 5, 2006, 4:31am UTC](https://www.ruby-forum.com/t/cant-change-the-value-of-self/51509/13 "2006-01-05T04:31:09Z")
Henrik M. wrote: > rocket.launch\_time = time > … > time.change(946701565) # Changes the launch time of the rocket > > The launch time of the rocket can be changed without setting the > launch\_time property explicitly. This is a potential source of bugs. Thanks, Henrik. That’s a great discussion and a great example. Hal ---
**Author:** ![johanatan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/8491ac/32.png) [@johanatan](https://www.ruby-forum.com/u/johanatan)\ **Post date:** [January 5, 2006, 7:29am UTC](https://www.ruby-forum.com/t/cant-change-the-value-of-self/51509/14 "2006-01-05T07:29:12Z")
On Jan 1, 2006, at 5:47 PM, Hal F. wrote: > Time objects are immutable. > > But someone once created a MutableTime class – anyone recall > who/when/where and whether it’s complete? That’d be me, and it’s documented at [RubyLibs on Phrogz](http://phrogz.net/RubyLibs/rdoc/) classes/MutableTime.html and available for download at http:// [phrogz.net/RubyLibs/MutableTime.rb](http://phrogz.net/RubyLibs/MutableTime.rb) As to whether it’s ‘complete’…I dunno. I nominally labeled it v1.0.5, but who knows what that means 🙂