Problem using F# match

gistfile1.fs

// this works

let makeImage bytes =
    match Array.length bytes with
        | 784 -> Image bytes |> Some
        | _ -> None


// but this does not. The none branch is never used.

let makeImage bytes =
    let numOfPixels = 784
    match Array.length bytes with
        | numOfPixels -> Image bytes |> Some
        | _ -> None
        
        
        

Line 14 is a typo I presume. You're naming the result of Array.length bytes instead of using the value you've assigned in line 12. The only way to make that work (that i'm aware of) is:

let makeImage bytes =
    let numOfPixels = 784
    match Array.length bytes with
        | x when x = numOfPixels -> Image bytes |> Some
        | _ -> None