Files

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

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.

Info The use-keyword ensures the StreamReader is correctly disposed of when it is no longer needed.

Next We use a while-loop to continue iterating over the lines in the file while they can be read. We read all the lines and print them.

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.

Here We use Seq.toList to get a list from the array. And with Seq.where we get a sequence of only capitalized lines in the list.

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" uppercaseABC
abc
Cat
Dog
bird
fish
["ABC"; "abc"; "Cat"; "Dog"; "bird"; "fish"]
seq ["ABC"; "Cat"; "Dog"]

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#.