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    <p>Based on the following:</p> <p><a href="https://stackoverflow.com/questions/16016087/strange-behavior-of-f-records">Strange behavior of F# records</a> :</p> <blockquote> <p>It is important to realize that F# does not use structural typing (meaning that you cannot use record with more fields as an argument to a function that takes record with fewer fields). This might be a useful feature, but it does not fit well with .NET type system. This basically means you cannot expect too fancy things - the argument has to be a record of a well known named record type.</p> </blockquote> <p>nicolas:</p> <blockquote> <p>If you truly dont know beforehand which part of Large fn will use, then you have to give it all. this is the only semantically correct choice.</p> </blockquote> <p>I have come up with two possible solutions:</p> <p><strong>Solution 1</strong> </p> <p>I basically split the user function into two. The nice thing is that <code>reducedPostprocessFn</code> accepts only what it needs. This function is therefore easy to reason about and unit test. <code>postprocessFn</code> is so short that it is also simple to see what it does. I find this solution similar to active patterns presented by <code>Phillip Trelford</code>. I wonder what the advantage of active patterns is?</p> <pre><code>(* simulation *) type Large = {v1: int; v2: int} let simulation postprocessFn = let large = {v1 = 1; v2 = 2} postprocessFn large (* user *) let reducedPostprocessFn (v1: int) = printfn "%d" v1 let postprocessFn (large: Large) = reducedPostprocessFn large.v1 simulation postprocessFn </code></pre> <p><strong>Solution 2</strong></p> <p>This solution uses duck typing but:</p> <p><a href="http://msdn.microsoft.com/en-us/library/dd233203.aspx" rel="nofollow noreferrer">http://msdn.microsoft.com/en-us/library/dd233203.aspx</a></p> <blockquote> <p>Explicit Member Constraint: ... not intended for common use.</p> </blockquote> <p><a href="https://stackoverflow.com/questions/7065939/f-and-duck-typing">F# and duck-typing</a></p> <blockquote> <p>I just remembered this won't work for record types. Technically their members are fields, although you can amend them with members using with member .... </p> </blockquote> <p>So I used an ordinary class instead of records. Now I use only one function instead of two but, to be honest, duck typing in F# is just ugly.</p> <pre><code>(* simulation *) type Large(v1, v2) = member o.v1 = v1 member o.v2 = v2 let simulation postprocessFn = let large = Large(1, 2) postprocessFn large (* user 2 *) let inline postprocessFn small = let v = (^a: (member v1: int) small) printfn "%d" v simulation postprocessFn </code></pre>
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    1. COSolution 2 will work with records in F# 3.0
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