F# – Converting to and from Units of Measure

Yan Cui

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If you’re reading this post, you probably know about F#’s Units of Measure already, it’s very useful when working with real-world units and adds extra safety to code that needs to work with and convert from one unit to another.

Here’s a quick snippet that shows you how to define and use units-of-measure:

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This code outputs the following, note the units associated with the float values:

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As you can see, units of measure can also be compounded by multiplication or division!

If you have a function that requires a int<m>, you won’t be able to call the function with a normal int, hence providing you with extra protection to ensure the correctness of your application because the unit of a numeric value is now a formal contract between a function and its caller:

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Having said that, there are cases where you want to be able to convert between an int and an int<m>. For instance, to provide better interoperability with other .Net languages, as units-of-measure only exists in F# there’s no way to create a numeric value with units-of-measure in C# (that I’m aware of anyway).

To convert from int<m> to int (or any other numeric type) is easy, just do a straight cast:

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Going the other way is slightly more tricky, you can’t use int<m> to cast an int to an int<m>, but you can either multiply the value with 1<m> or use the Int32WithMeasure method on the LanguagePrimitives core module:

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