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POModel inheritance using functional programming style data types
 

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    <p>I've been using F# recently and tried to code in a functional way rather than doing OOP all over again in a different syntax. I've now run into a problem that I could solve with a mix of inheritance and discriminated unions but for which I'm trying to find a pure functional style representation.</p> <p>What I want to model is something like this (changed to preserve the pattern since I can't use the actual code):</p> <pre><code>type Shape = | Rectangle of Size * Size | Circle of Diameter </code></pre> <p>so far so good, but now I need to represent a collection of additional properties relevant for the different types of shapes, like:</p> <pre><code>type ShapeProperty = | Color of Shape * Color // Fine, valid for all shapes | Rotation of Shape * Angle // Wants to be Rotation of Rectangle * Angle | Details of Shape * int // Wants to be Detail of Circle * int </code></pre> <p>If instead of using a discriminated union for Shape, I'd used a base class and inheritance, I could refer to the actual types and make sure Rotation could only be applied to Rectangle and not Circle, but now I can't. Is there a way of implementing something like that while still keeping to pure functional data structures?</p> <p><strong>Edit:</strong></p> <p>My current solution is to separate the definition of a individual shapes from the fact that shapes are at all related, like this:</p> <pre><code>type Rectangle = Rectangle of Size * Size // Or using a record type type Circle = Circle of Diameter // Or using a record type type Shape = RectangleShape of Rectangle | CircleShape of Circle </code></pre> <p>which means that I then have type to refer to in ShapeProperty:</p> <pre><code>type ShapeProperty = | Color of Shape * Color | Rotation of Rectangle * Angle | Details of Circle * int </code></pre> <p>This feels a bit clumsy as now need to encapsulate each shape in the Shape type to store them in a collection, but it does give me a way of expressing the type safety I'm after. Any improvements on this would be welcome.</p>
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    1. COHow do you plan to use the ShapeProperty type? I think you need a different design here, but it is hard to say what type of design is appropriate without more details.
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    2. COI need ShapeProperty (and other classes similar to it) to add properties of various shapes without having to touch the definition of the shapes themselves as in my real case the properties are only relevant when the shapes are used in certain contexts but not all others. Basically I need a type safe representation that lets me do transformations and queries such as "find all shapes that have a rotation of 10".
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    3. COWhat does that mean? Do circles have a rotation less than 10? (Circles don't have the concept of rotation the way you described it.) I think you need more detail about precisely what it is you want.
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