Generic `TypeWithMeasure` and `TypeWithoutMeasure` functions for adding units of measure on any type

[cloudRoutine]

Idea 8401578: Generic TypeWithMeasure and TypeWithoutMeasure functions for adding units of measure on any type

Status : open
Submitted by exercitus vir on 6/13/2015 12:00:00 AM
2 votes

Currently, the Core.LanguagePrimitives module only provides the following functions to add units of measure to decimal, double (= float) and single (=float32), respectively:
DecimalWithMeasure : decimal -> decimal<'u>
Float32WithMeasure : float -> float<'u>
FloatWithMeasure : float32 -> float32<'u>
and the Core.Operators module overloaded functions for removing units of measure on decimal, double (= float) and single (=float32):
decimal : ^T -> decimal
float : ^T -> float
float32 : ^T -> float32
Even though all the other primitive numeric types are also annotated with [], you cannot use them to carry a unit of measure, because the needed conversion functions are missing. And since MeasureAnnotatedAbbreviation is a public attribute, we would should be able to add units of measure on any type.
So, I propose to add the following generic functions to the Core.LanguagePrimitives module as follows:
TypeWithMeasure<'T, 'U> : 'T -> 'U
TypeWithoutMeasure<'T, 'U>: 'U -> 'T
where 'U is 'T with the measure.
Currently, DecimalWithMeasure, Float32WithMeasure, FloatWithMeasure are all implemented using the same generic function:
let inline retype<'T,'U> (x:'T) : 'U = (# "" x : 'U #)
So TypeWithMeasure and TypeWithoutMeasure could simply reuse this function. I realize that retype might not be entirely safe, but implementors of custom types that can carry units of measure can then implement two type-safe functions:
//custom type
type SomeType = ...
//custom type that can carry measure
[]
type SomeType <[] 'm> = SomeType
//module with conversion functions for custom type
module SomeType =
let inline SomeTypeWithMeasure (withoutMeasure : SomeType) : SomeType<'m> = TypeWithMeasure withoutMeasure
let inline SomeWithoutMeasure (withMeasure : SomeType<'m>) : SomeType = TypeWithMeasure withMeasure

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