Question

I’ve written the following

[<Measure>]
type m

[<Measure>]
type s

[<Measure>]
type v = m/s

type Vector3<[<Measure>] 'a> =
    {
    X : float<'a>
    Y : float<'a>
    Z : float<'a>
    }
    static member (*)
        (v:Vector3<'a>,f:float<'b>):Vector3<'a*'b> =
        { X = v.X*f; Y = v.Y*f ; Z = v.Z * f}

Now I’m trying to use it this way:

let next_pos (position:Vector3<m> , velocity: Vector3<m/s> ,dt : float<s>  ->  Vector3<m>) =
     position + (velocity * dt)

It gives me a compiler error, but I’m pretty sure the measure unit are expressed right.
What’s my mistake?

Answer 1

The syntax you tried to use for specifying the return type was incorrect. It should look like this:

let next_pos (position:Vector3<m>, velocity:Vector3<m/s>, dt:float<s>) : Vector3<m> = 
  position + (velocity * dt) 

To specify that the function returns a value of type Vector3<m>, you need to add type annotation to the result, which is done by writing let foo <arguments> : T = <expr>. When adding type annotations to parameters, these need to be parenthesized (so the syntax is not ambiguous). As noted by Paolo in a comment, your use of -> was saying that dt is a function, because the annotation float<s> -> Vector3<m> was attached to the parameter dt.

To make the code compile, I also had to add an implementation of (+) operator to your Vector3, but I assume you have that already (and just left it out when posting the question).