Defer in Chapel

use IO;
use FileSystem;

// Suppose we wanted to create a file, write to it,
// and then close when we're done. Here's how we could
// do that with a try-finally block.

proc main() {
    var f = createFile("/tmp/defer.txt");
    try {
        writeFile(f);
    } finally {
        closeFile(f);
    }
}

proc createFile(p: string): file {
    writeln("creating");
    var f = open(p, iomode.cw);
    return f;
}

proc writeFile(f: file) {
    writeln("writing");
    f.writeln("data");
}

// It's important to check for errors when closing a
// file, even in a finally block.
proc closeFile(f: file) {
    writeln("closing");
    try {
        f.close();
    } catch e {
        stderr.writef("error: %s\n", e.message());
        exit(1);
    }
}

In Chapel, there isn’t a direct equivalent to Go’s defer keyword. However, we can achieve similar functionality using a try-finally block. The finally clause ensures that the code within it is executed, regardless of whether an exception is thrown or not.

In this example, we’ve created a program that demonstrates file operations with proper cleanup:

1. We define a main() procedure that serves as the entry point of our program.

2. Inside main(), we create a file using the createFile() procedure.

3. We use a try-finally block to ensure that the file is closed after we’re done writing to it. The writeFile() procedure is called in the try block, and closeFile() is called in the finally block.

4. The createFile() procedure creates a new file and returns a file handle.

5. The writeFile() procedure writes some data to the file.

6. The closeFile() procedure closes the file. It’s wrapped in a try-catch block to handle any potential errors during the closing operation.

To run this program, you would save it as a .chpl file (e.g., file_operations.chpl) and compile it using the Chapel compiler:

$ chpl file_operations.chpl
$ ./file_operations
creating
writing
closing

This demonstrates how we can ensure proper resource cleanup in Chapel, similar to using defer in Go. The try-finally block guarantees that the file will be closed, even if an error occurs during the write operation.