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Wojciech Sznapka:
Immutable value objects in PHP
May 16, 2014 @ 09:04:27

Wojciech Sznapka has a quick post to his site today looking at a possible implementation of Value Objects (immutable objects) in PHP applications.

Value objects are one of building blocks in Domain Driven Design. They represents a value and does not have an identity. That said, two value objects are equal if their values are equal. Other important feature is that Value Objects are immutable, i.e. they can not be modified after creation. [...] This post isn't about obvious advantages of representing domain logic with support of Value Object. As well, we wouldn't elaborate here about pros and cons of immutable objects. I'd rather would like to show an attempt to change Value Object.

His change method isn't so much a "change" as a "duplicate with new values" process. In his example he creates a EmailValueObject with "host" and "mailbox" properties. This object has a "changemailbox" method that seems to update the "mailbox" property, but in actuality clones the current object with a new "mailbox" value in the constructor.

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value object immutable clone change property

Link: http://blog.sznapka.pl/immutable-value-objects-in-php/

Nicolas Bérard-Nault's Blog:
The unknown value of Value Objects
March 31, 2011 @ 12:12:46

Nicolas Bérard-Nault has put together a new post looking at the role that Value Objects play in application development and, more specifically, how they fit in with domain driven design (DDD). He looks to explain the unknown value of Value Objects to developers that might not know how helpful they really can be.

One of the main rules of DDD is that value objects are immutable. This is often not as self-evident as it seems, as many programmers are not even aware of the state they leak and create. Sadly, state is often positively correlated with entropy. Hence, taking steps to contain and to limit state is one of the keys to taming complexity in an application.

He notes that even outside of DDD Value Objects can be quite useful. He gives an example of a "RationalNumber" class with methods for basic things like addition and subtraction. He shows the more traditional mutable version that most developers would start with and uses it in several examples to show its flaws. He finishes up the post with a look at the "more correct" immutable version of the class and a sample call that would result in the correct output of a simple matematical operation.

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valueobject domaindrivendesign mutable immutable


Bradley Holt's Blog:
Immutable Value Objects in PHP
October 01, 2010 @ 11:15:40

Bradley Holt has a new post to his blog about a subject he's recently been learning about, Domain-Driven Design, and how immutable value objects could be useful in PHP.

Yesterday I tweeted: Modern object-oriented programming languages need support for immutable Value Objects. #DDD The "DDD" in that tweet stands for Domain-Driven Design. There were several interesting responses to this tweet.

Responses to his tweet included comments from Matthew Weier O'Phinney, Ralph Schindler, Nicolas Berard-Nault and Giorgio Sironi. He notes that, while all of the suggestions are good, they're only half of the issue. They make it immutable when defined but not during execution. Currently PHP lacks this functionality, but something like this could be worked in with the concept of a "final" class.

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immutable object domaindrivendesign ddd valueobject



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