Next Steps

We are now ready write our first real program!

We will write a simple variant of the venerable wordcount (wc) program from UNIX.

We will use the opportunity to illustrate how to use algebraic effects in Flix.

use Fs.FileRead

def wc(file: String): Unit \ { FileRead, IO } =
    match FileRead.readLines(file) {
        case Ok(lines) =>
            let totalLines = List.length(lines);
            let totalWords = List.sumWith(numberOfWords, lines);
            println("Lines: ${totalLines}, Words: ${totalWords}")
        case Err(_) =>
            println("Unable to read file: ${file}")
    }

def numberOfWords(s: String): Int32 =
     s |> String.words |> List.length

def main(): Unit \ { FileRead, IO } =
    wc("Main.flix")

The program works as follows:

We define a wc function that takes a filename and reads all lines from the file using the FileRead effect.

If the file is successfully read, we calculate:

• The number of lines using List.length.
• The number of words by summing the results of applying numberOfWords to each line.

The results are printed to the terminal using println.

If the file cannot be read, an error message is printed instead.

The wc function’s type and effect signature specifies the {FileRead, IO} effect set, indicating these effects are required. Both FileRead and IO have default handlers, so no explicit handler calls are needed in main.