gistfile1.fs

module ComposableTesting
open System.Net
open System.IO
open System

let download (uri: Uri) = 
    use cl = new WebClient()
    cl.DownloadString uri

let parse (line: string) =
    line.Split [|';'|]
    |> Seq.skip 1
    |> Seq.take 3

let write file (lines: string seq) =
    use sw = new StreamWriter (path=file)
    lines |> Seq.iter sw.WriteLine
    
// we can write only end-to-end tests for it, which will download actual 
// data then saves it into a real file on disk
let processUri uri =
    uri
    |> download
    |> parse
    |> write "file.txt"

// there's no way to test it since the calls to processUri is hardcoded
let processUris file =
    File.ReadLines file
    |> Seq.map (fun x -> Uri x)
    |> Seq.iter processUri

// yes, we can write (integration) tests for it
let processUris' file f =
    File.ReadLines file
    |> Seq.map (fun x -> Uri x)
    |> Seq.iter f

// we can easily write unit/property-based tests for it, but...
let processUris'' reader file f =
    reader file
    |> Seq.map (fun x -> Uri x)
    |> Seq.iter f

// nice
processUris "src.txt"
// ugly
processUris' "src.txt" processUri
// even uglier...
processUris'' File.ReadLines "src.txt" processUri