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    <p>Self types allow you to define cyclical dependencies. For example, you can achieve this:</p> <pre><code>trait A { self: B =&gt; } trait B { self: A =&gt; } </code></pre> <p>Inheritance using <code>extends</code> does not allow that. Try:</p> <pre><code>trait A extends B trait B extends A error: illegal cyclic reference involving trait A </code></pre> <p>In the Odersky book, look at section 33.5 (Creating spreadsheet UI chapter) where it mentions:</p> <blockquote> <p>In the spreadsheet example, class Model inherits from Evaluator and thus gains access to its evaluation method. To go the other way, class Evaluator defines its self type to be Model, like this:</p> </blockquote> <pre><code>package org.stairwaybook.scells trait Evaluator { this: Model =&gt; ... </code></pre> <p>Hope this helps.</p>
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    1. POWhat is the difference between self-types and trait subclasses?
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    1. USJacek Laskowski
    1. USMushtaq Ahmed
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    1. COI hadn't considered this scenario. Its the first example of something that I've seen that isn't the same as a self-type as it is with a subclass. However, it seems kind of edge-casey and, more important, it seems like a bad idea (I usually go far out of my way NOT to define cyclic dependencies!). Do you find this to be the most important distinction?
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    2. COI think so. I do not see any other reason why I would prefer self-types to extends clause. Self-types are verbose, they do not get inherited (so you have to add self-types to all subtypes as a ritual) and you can only see member but can't override them. I am well aware of Cake pattern and many posts mentioning self-types for DI. But somehow I am not convinced. I had created a sample app here long back (http://bitbucket.org/mushtaq/scala-di/). Look specifically at /src/configs folder. I achieved DI to replace complex Spring configurations without self-types.
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      1. USMushtaq Ahmed
    3. COMushtaq, we're in agreement. I think Daniel's statement about not exposing unintentional functionality is an important one but, as you put it, there is a mirror view of this 'feature'... that you cannot override the functionality or use it in future subclasses. This pretty clearly tells me when design will call for one over the other. I'll be avoiding self-types until I find a genuine need -- ie if I start using objects as modules as Daniel points out. I'm autowiring dependencies with implicit parameters and a straightforward bootstrapper object. I like the simplicity.
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