Files. In files we find persistent data. Files introduce complexity and bugs. A file may be unavailable, have invalid formatting, or be corrupted.

Open System.IO. In the System.IO namespace we find many useful types like StreamReader. Methods like File.ReadAllLines are also included.

StreamReader. Let us begin with a StreamReader example. We introduce a type called Data. In the Read function, a member of Data, we open a StreamReader and read in a file's lines.

open System.IO

type Data() =
    member x.Read() =
        // Read in a file with StreamReader.
        use stream = new StreamReader @"C:\programs\file.txt"

        // Continue reading while valid lines.
        let mutable valid = true
        while (valid) do
            let line = stream.ReadLine()
            if (line = null) then
                valid <- false
            else
                // Display line.
                printfn "%A" line

// Create instance of Data and Read in the file.
let data = Data()
data.Read()
carrot
squash
yam
onion
tomato
"carrot"
"squash"
"yam"
"onion"
"tomato"

File.ReadAllLines. This is a simpler way to read all the lines in a file, but it may be less efficient on large files. ReadAllLines returns an array of strings.

open System
open System.IO

let lines = File.ReadAllLines(@"C:\programs\file.txt")

// Convert file lines into a list.
let list = Seq.toList lines
printfn "%A" list

// Get sequence of only capitalized lines in list.
let uppercase = Seq.where (fun (n : String) -> Char.IsUpper(n, 0)) list
printfn "%A" uppercase
ABC
abc
Cat
Dog
bird
fish
["ABC"; "abc"; "Cat"; "Dog"; "bird"; "fish"]
seq ["ABC"; "Cat"; "Dog"]

Summary. In F# we have access to the .NET Framework's IO library. This enables efficient and well-tested use of files. With StreamReader we iterate over the lines in a file.

Although we can use constructs like the while-loop, if and else, a simpler approach is to use methods like File.ReadAllLines. The returned array can be used easily in F#.