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    <p>In F#, people don't use implementation inheritance (base classes) that often, so it would be useful to know what are you trying to do (e.g. more realistic example would be great). </p> <p>If I understand you correctly, the function <code>spread</code> should take a subtype of the type <code>Spreader</code> and return a value of the same type. You can write the signature of the function like this:</p> <pre><code>let foo&lt; 'T when 'T :&gt; Spreader &gt;(arg:'T) : 'T = // some implementation here </code></pre> <p>The type parameter <code>'T</code> will be some inherited class of <code>Spreader</code> and as the type signature shows, the function <code>foo</code> returns the same type as it takes as the argument.</p> <p>The implementation is a bit ugly, because you need to add a lot of dynamic type casts:</p> <pre><code>let foo&lt; 'T when 'T :&gt; Spreader &gt;(arg:'T) : 'T = match box arg with | :? Fire -&gt; (box (Fire())) :?&gt; 'T | :? Disease -&gt; (box (Disease())) :?&gt; 'T </code></pre> <p>Anyway, I agree with Brian that this seems like a scenario where discriminated unions would be a better choice. It would be great if you described your problem in some more details.</p> <hr> <p>Unlike the solution from Jon and <code>spread2</code> from Brian, the version above returns the same subtype as the one it gets as an argument, which means that the following should work:</p> <pre><code>let fire = new Fire() let fire2 = foo fire // 'fire2' has type 'Fire' fire2.FireSpecificThing() </code></pre>
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    1. USTomas Petricek
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    1. CO"... this seems like a scenario where discriminated unions would be a better choice. It would be great if you described your problem in some more details." Please see above, orignal post amended.
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      1. USMiloDC
    2. CO@MiloDC: Thanks for the clarification. So, you have two entities for representing fire and diseaase and both of them can have strength. Do you want to easily add other types of these things? How are you going to use them? If you had a member `Burn` in your fire entity, where would that be called from? Wouldn't you always work with these things using the base class? Is this for example for something like a game? ... These are the kind of high-level questions that you can use to decide whether to use OO (class hierarchy) or FP style (discriminated unions).
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    3. COTomas, just wanted to say that I recently bought your book _Real World Functional Programming_, and so far it seems to be very well written. I'm going to come at my problem differently, based largely on what I've read in your book. Cheers.
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