Mean Squarred Errors

Errors.fs

//Mean Absolute Error
let mae (ratings:float list)(predictions:float list) =
        (List.zip ratings predictions |> List.sumBy (fun t -> abs (fst t - snd t)))
                            /float ratings.Length

//Normalized Mean Absolute Error
let nmae (ratings:float list)(predictions:float list) =
   let rMax = ratings |> List.max
   let rMin = ratings |> List.min
   (mae ratings predictions )/(rMax - rMin)

//Root mean squared error
let rmse(ratings:float list)(predictions:float list) =
    sqrt( ( List.zip ratings predictions
                  |> List.map (fun t -> fst t - snd t)
                         |> List.sum )
         /(float predictions.Length ))