F#'s standard library provides robust support for input and output operations. These libraries leverage the underlying .NET Framework classes for file access, console interaction, and network communication, while offering a more functional and idiomatic F# experience.

Core Concepts and Modules

The primary module for I/O operations in F# is System.IO, which is accessible directly. F# enhances this with convenience functions and types.

File Operations

Reading and writing to files is a common task. F# offers functions to handle this efficiently.

Reading from Files

You can read the entire content of a file as a string, or read it line by line.

open System.IO

// Read the entire content of a file
let fileContent = File.ReadAllText("myFile.txt")
printfn "%s" fileContent

// Read lines from a file
let lines = File.ReadAllLines("myFile.txt")
for line in lines do
    printfn "Line: %s" line

Writing to Files

Writing data to files can be done by appending or overwriting content.

open System.IO

// Write a string to a file, overwriting existing content
File.WriteAllText("output.txt", "This is the first line.\nThis is the second line.")

// Append a string to a file
File.AppendAllText("output.txt", "\nThis line was appended.")

Console Input/Output

Interacting with the console is essential for many command-line applications.

Reading from Console

Console.ReadLine() reads a line of text from the standard input stream.

open System

printf "Enter your name: "
let name = Console.ReadLine()
printfn "Hello, %s!" name

Writing to Console

printfn is the idiomatic F# way to write formatted output to the console, followed by a newline. printf writes without a newline.

open System

let pi = 3.14159
printfn "The value of Pi is approximately %.3f" pi
printf "This is on the same line."
printfn " This is too."

Working with Streams

For more granular control over I/O, you can work directly with streams like StreamReader and StreamWriter.

open System.IO

try
    use reader = new StreamReader("myFile.txt")
    use writer = new StreamWriter("copy.txt")

    let mutable line = reader.ReadLine()
    while line <> null do
        writer.WriteLine(line)
        line <- reader.ReadLine()
    printfn "File copied successfully."
except
    | ex -> printfn "An error occurred: %s" ex.Message

Directory Operations

Managing directories is also part of I/O. The System.IO.Directory class provides methods for this.

open System.IO

// Get current directory
let currentDirectory = Directory.GetCurrentDirectory()
printfn "Current directory: %s" currentDirectory

// Create a new directory
let newDir = "myNewFolder"
if not (Directory.Exists(newDir)) then
    Directory.CreateDirectory(newDir)
    printfn "Directory '%s' created." newDir

// List files in a directory
let files = Directory.GetFiles(".")
printfn "Files in current directory:"
for file in files do
    printfn "- %s" file