F# Ununit - reunit inside a function

问题

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 'ununit' values before I pass them in, then 'reunit' them when I get the results back.

The catch is that I'd like to avoid being forced to declare WHICH units in advance.

Example snippet

let ExternalNonUnitAwareFunction s = s + 1.

let MyUnitAwareClient (s:float<'u>) =  //'
    //1. this option "flattens" to no unit, or fixes to first inferred unit
    //let (unit:float<'u>) = 1.0<_>  
    //2. this works fine, except for 0!
    let unit = s / (float s) 
    s |> float |> ExternalNonUnitAwareFunction |> (*) unit

I haven't managed to work out how to handle this one...

Update If I have understood correctly, the final version of F# will include functions to do this.

回答1:

For now, boxing and casting appears to work:

let MyUnitAwareClient (s:float<'u>) =  
  let result = s |> float |> ExternalNonUnitAwareFunction
  (box result :?> float<'u>)

I wouldn't be surprised if the units of measure stuff goes through some further changes before release, though, which might break this. You can also make a more generic version, such as:

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

EDIT

There is now a FloatWithMeasure function to 'cast to units':

http://msdn.microsoft.com/en-us/library/ee806527(VS.100).aspx

回答2:

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

[<Measure>]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<mm>                  //try it -> 5.0<mm>

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

let withunits2 = reunit newnounits //reunit to another function
withunits2 2.5<mm^2>               //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.<mm>, 0.<mm^2>)
withunits3 3.5<mm>

来源：https://stackoverflow.com/questions/1398611/f-ununit-reunit-inside-a-function

标签

units-of-measurement