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POHow to organize Haskell modules with instances: stick to data type vs type class?
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  1. POHow to organize Haskell modules with instances: stick to data type vs type class?
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    copied!<p>The general question is which module structure is more convenient when adding instances for existing objects? Which pros and cons there are?</p> <p>Let's say I want to add NFData instance for Seq type. I can place it in:</p> <ul> <li><code>Data.Sequence.Extra</code> (as the same thing is done in the <a href="https://github.com/coreyoconnor/vty/blob/master/src/Data/Sequence/Extra.hs">vty</a> package)</li> <li><code>Data.Sequence.Instances.NFData</code> (more precise)</li> <li><code>Control.DeepSeq.Instances</code></li> <li><code>Control.DeepSeq.Instances.Sequence</code></li> </ul> <p>It's the case when I don't own neither the type class nor the data type. The other common situation is when I own a type type class and want to add instances for data type(s) from some "heavy" package from Hackage, like OpenGL. Let's say the type class I designed is very light and has no direct relation to OpenGL. I don't want my type class depend on "heavy" package, so I want to place OpenGL instances in a separate module (it's my intuitive feeling, if you have other opinion, let's discuss it). So, what this module should be:</p> <ul> <li><code>MyClass.Instances.OpenGL</code></li> <li><code>Graphics.Rendering.OpenGL.Extra</code> (together with instances for other classes)</li> <li><code>Graphics.Rendering.OpenGL.Instances.MyClass</code></li> </ul> <p>What is more flexible solution? At some point OpenGL can be replaced with other library, or MyClass can be replaced too. Are there any subtle nuances?</p> <p>Also, which scheme is better if choose <code>MyClass.Instances</code> variant:</p> <ul> <li><code>MyClass.Class</code> module for the class itself and basic instances and <code>MyClass</code> module reexports it (and maybe <code>MyClass.Instances</code>)</li> <li><code>MyClass</code> module for the class and basic instances, and <code>MyClass.All</code> reexports everything</li> <li><code>MyClass</code> module for the class and basic instances and no module for reexporting.</li> </ul>
 

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