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Units of Measure in F#

One of the nice features of F#, is the support for units of measure. This allows you to associate a unit of measure(e.g. liters, centimeters,...) to a type(typically floating point, integral and decimal types). Now the compiler becomes your friend and helps you avoid the unit mismatch errors that can occur.

With the compile time checking you now exactly the expected input or output type.

Built-in units

For some of the common unit types Microsoft has a unit library available in the FSharp.Data.UnitSystems.SI namespace. It includes SI units in both their symbol form (like m for meter) in the UnitSymbols subnamespace, and in their full name (like meter for meter) in the UnitNames subnamespace.

Here you can find the list of available units: https://fsharp.github.io/fsharp-core-docs/reference/fsharp-data-unitsystems-si-unitsymbols.html

To convert a unitless value to a value that has units, the easiest way is to multiply by a 1 or 1.0 value that is annotated with the appropriate units.

Custom units

You are not limited to the built-in types and can create your own using the following syntax:

[<Measure>] type unit-name [ = measure ]

Here is an example:

It is also possible to derive a unit from an existing unit type:

More information:

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