TypedArrayIndexing.fs

[<Measure>] type Chicken
[<Measure>] type Cow

type Arr<[<Measure>] 'Measure>(v: array<'T>) =

    member _.Item
        with get (i: int<'Measure>) =
            v.[int i]


let chickenSizeArray =
    [|1.0 .. 10.0|]
    |> Arr<Chicken>

let cowSizeArray =
    [|1.0 .. 10.0|]
    |> Arr<Cow>


let chickenIndex = 1<Chicken>

// This works
let chickenSize = chickenSizeArray[chickenIndex]

// This won't compile
let cowSize = cowSizeArray[chickenIndex]

I have a warning on line 13: FS0064: This construct causes code to be less generic than indicated by the type annotations. The type variable 'T has been constrained to be type 'float'.
Like this it works:

[<Measure>] type ChickenIdx
[<Measure>] type CowIdx

type Arr<'T, [<Measure>] 'Measure>(v: array<'T>) =

    member _.Item
        with get (i: int<'Measure>) =
            v.[int i]


let chickenSizeArray =
    [|1.0 .. 10.0|]
    |> Arr<float, ChickenIdx>

let cowSizeArray =
    [|1.0 .. 10.0|]
    |> Arr<float, CowIdx>


let chickenIndex = 1<ChickenIdx>

// This works
let chickenSize = chickenSizeArray[chickenIndex]

// This won't compile
let cowSize = cowSizeArray[chickenIndex]

Yeah, I've updated my code since then. Good catch!