F# has a units of measure capability, described at http://msdn.microsoft.com/en-us/library/dd233243.aspx as follows:

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

This allows units to be defined such as:

type [<Measure>] USD
type [<Measure>] EUR

And code to be written as:

let dollars = 25.0<USD>
let euros   = 25.0<EUR>

// Results in an error as the units differ
if dollars > euros then printfn "Greater!"

It also handles conversions (I'm guessing that means Measure has some functions defined that let Measures be multiplied, divided and exponentiated):

// Mass, grams.
[<Measure>] type g
// Mass, kilograms.
[<Measure>] type kg

let gramsPerKilogram: float<g kg^-1> = 1000.0<g/kg>

let convertGramsToKilograms (x: float<g>) = x / gramsPerKilogram

My instincts tell me it should be possible to implement a similar capability in Haskell, but I've not been able to find any examples of how to do it.

Edit: oh my word it's a huge can of worms! There's a research paper at http://research.microsoft.com/en-us/um/people/akenn/units/CEFP09TypesForUnitsOfMeasure.pdf. I'm guessing it's more than a few lines of code to implement the whole thing. Summer project anyone? :)

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