Defer in Julia

In Julia, we can use the finally block to ensure that a piece of code is executed at the end of a function, regardless of how the function exits. This is similar to the defer keyword in other languages.

Let’s see how we can use finally to create a file, write to it, and then close it:

using Printf

function main()
    # Open the file and ensure it's closed at the end
    f = open("/tmp/defer.txt", "w")
    try
        writeFile(f)
    finally
        closeFile(f)
    end
end

function writeFile(f::IOStream)
    println("writing")
    println(f, "data")
end

function closeFile(f::IOStream)
    println("closing")
    close(f)
end

# Helper function to create the file
function createFile(path::String)
    println("creating")
    open(path, "w")
end

main()

In this example, we use a try-finally block to ensure that the file is closed after we’re done writing to it, even if an error occurs during the writing process.

The createFile function opens a file for writing and returns the file handle. The writeFile function writes some data to the file, and the closeFile function closes the file.

In the main function, we open the file using createFile, then use a try-finally block to ensure that closeFile is called after writeFile, regardless of whether writeFile completes successfully or throws an error.

Running this program will produce the following output:

creating
writing
closing

This confirms that the file is created, written to, and then closed in the correct order.

It’s important to note that in Julia, you don’t typically need to manually close files as the garbage collector will eventually close them for you. However, it’s still good practice to close files explicitly, especially when dealing with large files or when you need to ensure the file is closed immediately after use.

In more complex scenarios, you might want to use Julia’s do block syntax, which automatically closes the file when the block ends:

open("/tmp/defer.txt", "w") do f
    writeFile(f)
end

This achieves the same result as our try-finally block, but in a more concise and idiomatic Julia style.