Flatten arbitrarily nested lists of integers

FlattenNestedLists.fsx

// Data structures

type Node =
| Int of int
| Nodes of Node list


// Behavior

module Node =
    // private helper
    let rec private flattenNodesRec nodes result =
        match nodes with
        | [] -> result
        | x::xs ->
            let newsubnodes, newresult =
                match x with
                | Int v -> xs, result @ [v]
                | Nodes subnodes -> subnodes @ xs, result
            flattenNodesRec newsubnodes newresult // Tail-call position

    // flattenNodes will take a list of nodes and return a single list of int's
    // It is tail-call optimized
    let flattenNodes nodes = flattenNodesRec nodes []


// Test

let input = [
    Nodes [
        Int 1
        Int 2
        Nodes [
            Int 3
        ]
    ]
    Int 4
]

let output = Node.flattenNodes input

let expected = [ 1; 2; 3; 4 ]

let testPassed = (output = expected)