F# call .Net Function with generic params - wont compile

Question

F# Beginner here, I want to draw a Polygon using ImageSharp. The build fails with:

No overloads match for method 'DrawPolygon'. The available overloads are shown below. error FS0001: This expression was expected to have type 'Image' but here has type 'a * 'b'.

    let renderRect(img:Image<Rgba32>) =

        let points: PointF list = [
            new PointF(float32 2.0, float32 8.0);
            new PointF(float32 4.0, float32 1.0);
            new PointF(float32 1.0, float32 7.0);
            new PointF(float32 5.0, float32 2.0);
        ]

        img.Mutate (fun x -> x.DrawPolygon(Rgba32.White, 1.0, points))
        img

The signature of the method i want to call:

static IImageProcessingContext<TPixel> DrawPolygon<TPixel>(this IImageProcessingContext<TPixel> source, TPixel color, float thickness, params PointF[] points) where TPixel : struct, IPixel<TPixel>

score 4 · Accepted Answer · answered Aug 21 '18 at 19:19

I see several issues:

As Aaron said, you're passing a list where an array is required.
The second argument to DrawPolygon should be a float32, not a float. Note that rather than calling the float32 function, you can just use a literal of the correct type by adding a suffix: 1.0f.
The second argument to Mutate needs to be an Action<_>, so it shouldn't return anything; this means you should ignore the result of the DrawPolygon call rather than returning it. You can achieve this by putting |> ignore after the call.

Here's a version of your code with these issues addressed:

let renderRect(img:Image<Rgba32>) =

    let points = [|
        PointF(2.0f, 8.0f);
        PointF(4.0f, 1.0f);
        PointF(1.0f, 7.0f);
        PointF(5.0f, 2.0f);
    |]

    img.Mutate (fun x -> x.DrawPolygon(Rgba32.White, 1.0f, points) |> ignore)
    img

Since you didn't include a full repro (or even the list of suggested overloads from the error message!) it's hard to know if there's anything else that needs to be changed.

score 3 · Answer 2 · answered Aug 21 '18 at 17:32

You are passing a list of points, but the signature of the method expects an array of points. I would suggest either passing the points in-line as params, or defining points as an array:

let points = 
    [|
        new PointF(float32 2.0, float32 8.0)
        new PointF(float32 4.0, float32 1.0)
        new PointF(float32 1.0, float32 7.0)
        new PointF(float32 5.0, float32 2.0)
    |]

F# call .Net Function with generic params - wont compile

2 Answers2