[LRUG] The advantage of immutability?

[Chris Patuzzo]

It's probably worth throwing up a reference to functional purity:

https://en.wikipedia.org/wiki/Pure_function

On 9 September 2015 at 11:43, Roland Swingler <roland.swingler at gmail.com>
wrote:

> > why an immutable object is an advantage
>
> In general immutability is nice because a lot, lot less can go wrong with
> immutable objects. A lot of bugs are caused by editing state incorrectly -
> if you can't change the state of an object then a whole class of bugs
> vanish by design.
>
> In terms of your specific problem a way to approach it might be - you have
> a Request of some form - instead of changing state model what is going on -
> so create a Response model (almost certainly a better name - I don't know
> your problem beyond what you've told us) with a state field of denied. To
> deny a request, create a Response object set to denied.
>
> This gives you some nice properties - you can start doing things like
> recording like when the request was denied, who denied it etc. if you like;
> also if a request is denied by mistake for then you can just create a new
> Response and you have a history of all the state changes.
>
> Now, none of this may be practical in your application (for example if
> lots of other things are already coupled to a mutable state field on your
> Request). In which case do whatever is simplest.
>
> Thanks,
> Roland
>
> On Wed, Sep 9, 2015 at 11:25 AM, Jonathon Horsman <jonathon at arctickiwi.com
> > wrote:
>
>> Hey smart people
>>
>> Can you shed some light on why an immutable object is an advantage for a
>> web-based Ruby app?
>>
>> For example this app I'm working on has these Request things and a user
>> has the ability to deny a Request.
>>
>> So the user would click a button which performs a post to the Request
>> controller's deny action.
>> If I were using Rails or some non-immutable based system I would fetch
>> the object from the database, set it's status to denied, and save it.
>>
>> However since this Request is an immutable object, I have to either:
>>
>> 1. Write an update_status method which sets that value in the database,
>> which becomes tedious when there's lots of possible attribute update
>> combinations
>> or
>> 2. Read the object from the database, copy all the values to a new object
>> with the denied status, and stick that back into the database. Seems like
>> pointless overhead and could be dangerous if later that object gets another
>> attribute which I forget to copy.
>>
>> My knowledge of immutable objects originates with Java Strings which I
>> understand makes sense for performance and memory management reasons.
>> I don't think this applies here though?
>>
>> Thanks!
>>
>> Jonno
>>
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