open System.IO

type PostCodeResult = { PostCode: string; Valid: bool }
let linesToFile filename (lines: string seq) =
    use writer = File.CreateText(filename)
    for line in lines
        do writer.WriteLine(line)

let formatLine pcr = pcr.PostCode + "-" + (string pcr.Valid) + "\n"

let postCodeValidationResults = [ { PostCode = "12345"; Valid = true }; { PostCode = "asdf"; Valid = false }]

let dataForFile = postCodeValidationResults |> Seq.map formatLine
dataForFile |> linesToFile "postcodes.txt"