Programming Flix

Exceptions

In Flix, we can catch Java exceptions using the try-catch construct. The construct is similar to the one in Java, but the syntax is slightly different.

For example:

import java.io.BufferedReader
import java.io.File
import java.io.FileReader
import java.io.FileNotFoundException
import java.io.IOException

def main(): Unit \ IO = 
    let f = new File("foo.txt");
    try {
        let r = new BufferedReader(new FileReader(f));
        let l = r.readLine();
        println("The first line of the file is: ${l}");
        r.close()
    } catch {
        case _: FileNotFoundException => 
            println("The file does not exist!")
        case ex: IOException => 
            println("The file could not be read!");
            println("The error message was: ${ex.getMessage()}")
    }

Here the calls new FileReader(), r.readLine(), and r.close() can throw IOExceptions. We use a try-catch block to catch these exceptions. We add a special case for the FileNotFoundException exception.

Note: Flix programs should not use exceptions: it is considered bad style. Instead, programs should use the Result[e, t] type. The try-catch construct should only be used on the boundary between Flix and Java code.

Note: Flix does not (yet) support a finally block.

Note: In Flix a function can contain at most one try-catch block.