Huffman coding in F#

huffman.example.fs

let ct = createCodeTree "abcdefghijklmnopqrstuvxyz "
let encoded = fastEncode ct ("simon says fsharp rocks" |> List.ofSeq)
let decoded = decode ct encoded
printfn "%A" encoded
printfn "%A" decoded

huffman.fs

type CodeTree =
    | Branch of CodeTree * CodeTree * list<char> * int
    | Leaf of char * int

let chars tree =
    match tree with
        | Branch(_,_,chars,_) -> chars
        | Leaf(char,_) -> [ char ]

let weight tree =
    match tree with
        | Branch(_,_,_, weight)
        | Leaf(_, weight) -> weight

let createCodeTree text =
    let rec combineTrees trees = 
        
        let makeCodeTree l r =
            Branch(l,r, chars l @ chars r, weight l + weight r)
        match trees with
            | fst :: snd :: rest -> combineTrees (makeCodeTree fst snd :: rest |> List.sortBy weight)
            | _ -> trees
    let orderedLeafList =
        text 
            |> Seq.countBy id
            |> Seq.sortBy snd
            |> Seq.map (fun f -> Leaf(fst f, snd f))
            |> List.ofSeq
    orderedLeafList
        |> combineTrees
        |> List.head

let decode tree bits =
    let rec doDecode _tree bits chars =
        match _tree with
            | Branch(l, r, _, _)  ->
                match bits |> List.head with
                    | 0 -> doDecode l bits.Tail chars
                    | 1 -> doDecode r bits.Tail chars
                    | _ -> raise (ArgumentOutOfRangeException "Invalid bit in bits")
            | Leaf(c, _) when bits.IsEmpty -> c :: chars
            | Leaf(c, _) -> doDecode tree bits (c :: chars)
    doDecode tree bits [] |> List.rev

let encode tree text = 
    let hasCharInBranch tree c =
      match tree with
        | Branch(_, _, cs, _) -> cs |> List.exists (fun _c -> _c = c)
        | Leaf(char, _) -> char = c

    let rec doEncode _tree (chars : list<char>) bits =
      if chars.IsEmpty then bits
      else
        match _tree with 
          | Branch(left, right, _, _) ->
            if hasCharInBranch left chars.Head then
              doEncode left chars (0 :: bits)
            else
              doEncode right chars (1 :: bits)
          | Leaf(c, _) -> doEncode tree chars.Tail bits

    doEncode tree text [] |> List.rev

let fastEncode tree =
    let codeMap = chars tree
                        |> Seq.map (fun c -> (c, encode tree [c]))
                        |> Map.ofSeq
    fun text -> List.foldBack (fun c s -> Map.find c codeMap @ s) (text |> List.ofSeq) []