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    <p>Let me start by saying that, personally, an OOP principle that does not work on both dynamically and statically typed languages isn't a principle.</p> <p>That said, here is an example: </p> <p>The Interface Segregation Principle (<a href="http://objectmentor.com/resources/articles/isp.pdf" rel="nofollow noreferrer">http://objectmentor.com/resources/articles/isp.pdf</a>) states that clients should depend on the most specific interface that meets their needs. If client code needs to use two methods of class C then C should implement interface, I, containing only these two methods and the client will use I rather than C. This principle is irrelevant in dynamically typed languages where interfaces are not needed (since interfaces defined types, and types are not needed in a language where variables are type-less)</p> <p>[edit]</p> <p>Second example - The Dependency Inversion Principle (<a href="http://objectmentor.com/resources/articles/dip.pdf" rel="nofollow noreferrer">http://objectmentor.com/resources/articles/dip.pdf</a>). This principle argues is "the strategy of depending upon interfaces or abstract functions and classes, rather than upon concrete functions and classes". Again, in dynamically typed language client code does not depend on anything - it just specifies method signatures - thereby obviating this principle.</p> <p>Third example - Liskov Substitution Principle (<a href="http://objectmentor.com/resources/articles/lsp.pdf" rel="nofollow noreferrer">http://objectmentor.com/resources/articles/lsp.pdf</a>). The text book example for this principle is a Square class that subclasses a Rectangle class. And then client code that invokes a setWidth() method on a Rectangle variable is surprised when the height is also changed since the actual object is a Square. Again, in a dynamically typed language the variables are type-less, the Rectangle class will not be mentioned in the client code and hence no such surprises will arise. </p>
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    1. USItay Maman
    1. USItay Maman
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    1. COType-less? A dynamically typed language has types. In Common Lisp, for example, any data object is of a type (or, more likely, a hierarchy of types), and at any time in the execution of the program it's possible to determine that type precisely. Further, conversions between types follow strict rules, and therefore CL is a strongly typed language. You just can't look at a symbol in the source code and know what it represents.
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      1. USDavid Thornley
    2. CODavid: What you mean is that object have types, with which I fully agree. I said that the variables are type-less: there is no type associated with a variable and it can point to object of any type.
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      1. USItay Maman
    3. CO@Itay RE: Dependency Inversion -- but even in a dynamic environment, is it not better to inject an instance on a constructor or setter rather than instantiate an instance? The former keeps dependency at the method level you describe, the latter creates a tight coupling to the entire class. Also, I use factories all the time in Ruby to hide dependence on concrete classes.
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      1. USDave Sims
 

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