asdf.fs

open System

type Location =
| City of string * float
| State of string * Location list
| Country of string * Location list

let data = [Country("United States", [State("Wasington", [City("Anacortes", 32.0); City("Seattle", 523.0)])])]

let getTotalPopulation locations =
    let rec getPopulation item =
        let doListOfItems items = items |> List.map (getPopulation) |> List.fold (+) 0.0
        match item with
        | City(_, pop)      -> pop
        | State(_, list)    -> doListOfItems list
        | Country(_, list)  -> doListOfItems list
    locations |> List.map (getPopulation) |> List.fold (+) 0.0

Console.WriteLine(getTotalPopulation data)

Console.ReadKey() |> ignore

asdf2.fs

type Shape = 
| Rectangle of float * float
| Square of float
| Circle of float

let getArea = function
    | Square(width)             -> width * width
    | Circle(rad)               -> Math.PI * rad * rad
    | Rectangle(width, height)  -> width * height

let shapes = [Rectangle(3.0, 4.0); Square(3.0); Circle(2.0); Rectangle(412.34, 322.2)]
let totalArea = shapes |> List.map getArea |> List.fold (+) 0.0

asdf3.fs

let cities = [("Seattle", 23); ("Anacortes", 2); ("Whoa", 3232)]

let output =
    cities
    |> List.map (fun (name, pop) -> sprintf "%s has a population of %i" name pop)
    |> List.fold (fun a s -> a + s + "\n") ""

Console.WriteLine(output)

let totalPopulation = cities |> List.fold (fun a (_, pop) -> a + pop) 0

printfn "Total population: %i" totalPopulation