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An introduction to Units of Measure by Andrew Kennedy

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Andrew Kennedy has published the first part of a tutorial introduction to Units of Measure in F#.

Do you remember NASA's Mars Climate Orbiter? It was lost in September 1999 because of a confusion between metric and so-called " English " units of measurement . The report into the disaster made many recommendations. In all likelihood, the accident could have been prevented if the NASA engineers had been able to annotate their program code with units, and then employed static analysis tools or language-level type-checking to detect and fix any unit errors.

Of course, the languages that NASA used had no language or tool support for units. Many people have suggested ways of extending programming languages with support for static checking of units-of-measure. It's even possible to abuse the rich type systems of existing languages such as C++ and Haskell to achieve it, but at some cost in usability. More recently, Sun's Fortress programming language has included type system support for dimensions and units. It also happens to have been the subject of my PhD thesis from a few years ago. So it's particularly exciting for me that a feature which I studied in theory in 1995 will now get used in practice in F#.

As recently announced in the September 2008 F# CTP (Community Technical Preview), the F# programming language now has full support for static checking and inference of units-of-measure. In this series of articles I'll gently introduce the feature. (If you're not familiar with F#, look here .) We've already been testing out the units-of-measure feature inside Microsoft and I'm amazed at the diversity of applications that are turning up. Of course, there are the obvious applications to scientific computing, and games (which are all about physics, after all), but we're seeing applications in machine learning , finance, search (think click rates, etc) and others.

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