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    <p>Interfaces might be a closer fit to what you have described in the updated question:</p> <pre><code>type Small = abstract v1 : int type Large = inherit Small abstract v2 : int let fn (value: Small) = printfn "%d" value.v1 let large = { new Large with member small.v1 = 5 member large.v2 = 6 } fn large </code></pre> <p>With interfaces the <code>Large</code> type can now be composed from 1 or more <code>Small</code> types. Functions can expect only the <code>Small</code> type they need, and the caller can simply pass a <code>Large</code> type to each function.</p> <p>Depending on the scenario you might also consider having a <code>Large</code> record type that implements one or more <code>Small</code> interfaces.</p> <pre><code>type Small = abstract v1 : int type Large = { v1: int; v2: int } with interface Small with member this.v1 = this.v1 let fn (value: Small) = printfn "%d" value.v1 let large = { v1 = 5; v2 = 6 } fn large </code></pre> <p>Yet another option is to define a union for all the possible <code>Small</code> items, and define <code>Large</code> as a list:</p> <pre><code>type Item = | V1 of int | V2 of int type Large = Item list let (|Small|) (large:Large) = large |&gt; List.pick (function V1(v1) -&gt; Some(v1) | _ -&gt; None) let fn (Small value) = printfn "%d" value let large = [V1(5);V2(6)] fn large </code></pre> <p><a href="http://msdn.microsoft.com/en-gb/library/dd233248.aspx" rel="nofollow">Active patterns</a> can be used to extract <code>Small</code> types from the data. An example of this kind of scheme is the <a href="http://www.fixprotocol.org/" rel="nofollow">FIX protocol</a> which composes large financial data blocks from small tagged data blocks.</p>
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    1. POSub-Records using F#
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    1. USPhillip Trelford
    1. USPhillip Trelford
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    1. COI have completely rewritten the question to make it more clear. Is your answer still valid?
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      1. USOldrich Svec
    2. COBut here you assume that you know about the `Small` type. But in reality you do not know about it. Imagine that you have a lot of data and the user picks a few of it and use it (save it etc.) and the rest of the data is lost for ever. So I do not know what `Small` will be - just that it is a sub-data of `Large`.
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    3. CO@OldrichSvec I've added another option, given your comment above you might want to consider composing `Large` from a list of `Small` types rather than using records
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      1. USPhillip Trelford
 

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