Efficiency of Morse code

Program.fs

open System.IO

type MorseCodePoint = Dit | Dah
type MorseEncodedWord = { word: string; total: int; efficiency: float }

let codePointLength point = 
    match point with 
        | Dit -> 1
        | Dah -> 3

let alphabet = 
    Map.empty.
        Add('a', [ Dit; Dah ]).
        Add('b', [ Dah; Dit; Dit; Dit;]).
        Add('c', [ Dah; Dit; Dah; Dit;]).
        Add('d', [ Dah; Dit; Dit;]).
        Add('e', [ Dit;]).
        Add('f', [ Dit; Dit; Dah; Dit;]).
        Add('g', [ Dah; Dah; Dit;]).
        Add('h', [ Dit; Dit; Dit; Dit;]).
        Add('i', [ Dit; Dit;]).
        Add('j', [ Dit; Dah; Dah; Dah;]).
        Add('k', [ Dah; Dit; Dah;]).
        Add('l', [ Dit; Dah; Dit; Dit;]).
        Add('m', [ Dah; Dah;]).
        Add('n', [ Dah; Dit;]).
        Add('o', [ Dah; Dah; Dah;]).
        Add('p', [ Dit; Dah; Dah; Dit;]).
        Add('q', [ Dah; Dah; Dit; Dah;]).
        Add('r', [ Dit; Dah; Dit;]).
        Add('s', [ Dit; Dit; Dit;]).
        Add('t', [ Dah;]).
        Add('u', [ Dit; Dit; Dah;]).
        Add('v', [ Dit; Dit; Dit; Dah;]).
        Add('w', [ Dit; Dah; Dah;]).
        Add('x', [ Dah; Dit; Dit; Dah;]).
        Add('y', [ Dah; Dit; Dah; Dah;]).
        Add('z', [ Dah; Dah; Dit; Dit;])

let morseCharacterLength char =
    match alphabet.TryFind(char) with
        | Some value -> value |> List.sumBy codePointLength 
        | None -> 0

let computeWordEfficiency (word:string) = 
    let len = word.Length
    let chars = word.ToLowerInvariant().ToCharArray() |> Array.toList
    let total = chars |> List.map morseCharacterLength |> List.append([ len - 1]) |> List.sum

    { word = word; total = total; efficiency = float total / float len }

let readLines (input:TextReader) = 
    let readNext _ = input.ReadLine()
    let inputNotDone = (<>) null
    Seq.initInfinite readNext |> Seq.takeWhile inputNotDone

let source = new StreamReader(@"C:\Downloads\linuxwords.txt") 
let data = readLines source |> Seq.map computeWordEfficiency 

data |> Seq.maxBy (fun (item:MorseEncodedWord) -> item.efficiency) |> printfn "%A"