Functional Programming style in F#

Question

I'm normally a C# developer, but I've started to learn F# and I want to make sure I'm writing code in a functional way that suits the language. I've quickly pieced this together with my knowledge from reading Functional Programming for the Real World and various material from the Web.

Please look and provide feedback on style, layout or anything you want (Program.fs is where the meat is):

// DataUtils.fs
module DataUtils

open System.IO
open System.Runtime.Serialization.Json
open System.Text

let GetJsonFromObject<'T> (obj:'T) =
    use ms = new MemoryStream()
    (new DataContractJsonSerializer(typeof<'T>)).WriteObject(ms, obj)
    Encoding.Default.GetString(ms.ToArray())

let GetObjectFromJson<'T> (json:string) : 'T =
    use ms = new MemoryStream(ASCIIEncoding.Default.GetBytes(json))
    let obj = (new DataContractJsonSerializer(typeof<'T>)).ReadObject(ms)
    obj :?> 'T

---

// Web.fs
module Web

open System
open System.Net
open System.IO

let GetJsonFromWebRequest url =

    let req = WebRequest.Create(new Uri(url)) :?> HttpWebRequest
    req.ContentType <- "application/json"

    let res = req.GetResponse () :?> HttpWebResponse
    use stream = res.GetResponseStream()
    use sr = new StreamReader(stream)
    let data = sr.ReadToEnd()
    data

---

// Model.fs
module Model

open System.Runtime.Serialization

[<DataContract>]
type exchangeRate = {
    [<field: DataMember(Name = "to")>]
    toCurrency:string;
    [<field: DataMember(Name = "rate")>]
    rate:decimal;
    [<field: DataMember(Name = "from")>]
    fromCurrency:string }

---

// Program.fs
module Program

open System
open Model

let getExchangeRate fromCurrency toCurrency =
    let url = String.Format("http://rate-exchange.appspot.com/currency?from={0}&to={1}", fromCurrency, toCurrency)
    let json = Web.GetJsonFromWebRequest url
    let rate:exchangeRate = DataUtils.GetObjectFromJson json
    rate

let rec displayExchangeRate currencies =
    match currencies with
    | [] -> []
    | (a, b)::tl ->
        let r = getExchangeRate a b
        printfn "%s -> %s = %A" a b r.rate
        displayExchangeRate tl

let currencies = [
    ("GBP", "EUR")
    ("GBP", "USD")
    ("EUR", "GBP")
    ("EUR", "USD")
    ("USD", "GBP")
    ("USD", "EUR") ]

displayExchangeRate currencies |> ignore

printfn "Press any key to exit"
let input = Console.ReadKey()

As you can see it's not doing an awful lot at the moment, just getting some exchange rates from a REST service and displaying them. The next step would be to add exchange rates to the database. I'd normally use Entity Framework for this, but given that Functional Programming is partly about reducing the amount of state that is held, is EF even the way to go?

Answer 1

It looks good! Just a couple of points.

• Instead of

  let data = sr.ReadToEnd()
  data
  

  you can write

  sr.ReadToEnd()
  

• sprintf is more idiomatic than String.Format.

• displayExchangeRate would be better written as a loop or using List.iter (or Seq.iter).

• DataContractJsonSerializer.WriteObject uses Encoding.UTF8, but you're decoding with Encoding.Default, which returns the OS's current ANSI code page.

Answer 2

You can destructure tuples in the function signature. So there is no need for the match in displayExchangeRate, you can just do this:

let displayExchangeRate (ccyA, ccyB) =
    let r = getExchangeRate ccyA ccyB
    printfn "%s -> %s = %A" ccyA ccyB r.rate

Answer 3

Your getCombinations function can perhaps be written a little more expressively as a list comprehension:

let getCombinations' list =
    [ for x in list do
      for y in list do
      if x <> y then yield (x, y) ]