F# Ununit - reunit inside a function

Question

This question is closely related to these ones (1, 2, 3)

I\'m using an external library which doens\'t (yet) handle units of measure. I want to be able to \

Answer 1

And just for fun, here is the opposite:

let deunit (fn:float<'u> -> float<'v>) (v:float) =
    let unit = box 1. :?> float<'u>
    fn(v * unit) |> float

Test :

#light

[]type mm

let reunit (fn:float -> float) (v:float<'u>) =
    let unit = box 1. :?> float<'u>
    unit * (fn(v/unit))

let deunit (fn:float<'u> -> float<'v>) (v:float) =
    let unit = box 1. :?> float<'u>
    fn(v * unit) |> float

let nounits v = v + 2.5            //function with no units
let withunits = reunit nounits     //make it handle units (run with next line)
withunits 2.5                  //try it -> 5.0

let newnounits = deunit withunits  //remove unit handling
newnounits 2.5                     //try it -> 5.0

let withunits2 = reunit newnounits //reunit to another function
withunits2 2.5               //try with different units

Weird stuff with that #"(£$! Value restriction error in there if you run let withunits = reunit nounits on it's own. So you have to run it with the line that uses withunits. I guess it's not surprising, you have to pass in the (v:float<'u>) to reunit for F# to be able to work out what 'u is. Possibly makes reunit of limited interest, I guess...

UPDATE: A slightly kooky workaround is to pass in 'model' values

let reunit2 (fn:float -> float) (model:float<'u>*float<'v>) =
    let unitin = box 1. :?> float<'u>
    let unitout = box 1. :?> float <'v>
    (fun v -> (fn(v / unitin)) * unitout)

let withunits3 = reunit2 nounits (0., 0.)
withunits3 3.5