为什么一个返回int的函数被报告为类型a'->int

我有以下代码,它报告错误。不过我不理解错误信息。

module MonochromeHandler

open System.Drawing

let private transparent = Color.FromArgb(0, 0, 0, 0)

let private calculateAlpha currentAlpha greyValue =
    match currentAlpha - greyValue with
        | value when value < 0 -> 0
        | value -> value

let private actionPixel(pixelColour:Color) =
    match (pixelColour.A, pixelColour.R, pixelColour.G, pixelColour.B) with
        | (0uy, _, _, _) -> transparent
        | (alpha, red, green, blue) when red = blue && red = green && red <> 255uy ->
            let newAlpha = calculateAlpha((int)alpha, (int)red)
            Color.FromArgb(newAlpha, 0, 0, 0)
        | _ -> pixelColour

我在“ Color.FromArgb(newAlpha, 0, 0, 0) “行

Error   1   This expression was expected to have type
    int    
but here has type
    'a -> int

该函数接受我提供的两个参数,并返回一个int。那么,为什么newAlpha的类型不是“ int “为什么” 'a -> int "?

可能与此有关,因为我已经声明 calculateAlpha 没有括号或逗号作为参数,我对F#的理解(显然不够)是,我应该能够这样使用它:

let newAlpha = calculateAlpha (int)alpha (int)red

如果我这样做,我会得到一个不同的错误 calculateAlpha (int)alpha 部分:

Error   1   This value is not a function and cannot be applied

为什么我不能以这种方式调用函数?