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POHow to generalize f# option?
 

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  1. POHow to generalize f# option?
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    <p>I'm getting result of some calculations in form of <code>'a option when 'a :&gt; IBaseType</code>. There is tree of types derived from <code>IBaseType</code>, and I don't really know option of what particular type this one is, but the important thing is that it's option of specific derived, not base type. So I want to upcast it to <code>IBaseType option</code> to process it further. As option is generic type, it's impossible to do cast directly (in F#), and I have to do cast inside of Option.map. Nothing complicated, type inference works as expected... <img src="https://content.screencast.com/users/AndriyKvasnytsya/folders/Jing/media/2e93e2ab-2871-4b22-8747-284374f166d2/2012-11-13_2029.png" alt="inference ok"> intermediate casted option is also resolved as expected... <img src="https://content.screencast.com/users/AndriyKvasnytsya/folders/Jing/media/fd77a7cb-60db-41d8-828a-f77e15f27258/2012-11-13_2026.png" alt="intermediate inference ok"> until function is complete. At this point for some reason type inference decided that original option must be already of type <code>IBaseType option</code>: <img src="https://content.screencast.com/users/AndriyKvasnytsya/folders/Jing/media/50208452-1ef7-4886-b20d-5be206416a96/2012-11-13_2025.png" alt="inference not ok"></p> <p>Intermediate type was already resolved earlier, why did it decided to reassign infered type of the <code>op</code>? Sure this leads to runtime exceptions. Looks like compiler error, but the main rule is that there are no errors in compiler. </p> <p>So in the end it sounds really stupid: I'm out of ideas how to simply upcast simple option. Just to make picture clearer: <code>processResult</code> takes <code>IBaseType option</code> as argument. And here is source of troublesome function:</p> <pre><code>(fun (x: obj) -&gt; let op = x :?&gt; _ option let upcastOp = op |&gt; Option.map (fun y -&gt; y :&gt; IBaseType) upcastOp |&gt; processResult) </code></pre> <p>Any ideas how to deal with this?</p>
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    1. USAndriy K
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    1. COWhich editor is this? Looks terribly familiar, but I cannot find it.
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      1. USRamon Snir
    2. COAre you able to change `processResult` to accept `option<#IBaseType>`?
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      1. USDaniel
    3. CO@RamonSnir: That's Visual Studio, just with few tweaks: [Sons of Obsidian theme](http://studiostyl.es/schemes/son-of-obsidian-modified-for-resharper), [Envy Code R font](http://damieng.com/blog/2008/05/26/envy-code-r-preview-7-coding-font-released) and [View Whitespace on](http://blogs.msdn.com/b/zainnab/archive/2012/06/05/9990405.aspx).
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