Defer in Standard ML

Standard ML doesn’t have a direct equivalent to Go’s defer keyword. However, we can achieve similar functionality using higher-order functions and closures. Here’s how we can implement a similar concept:

(* File: defer.sml *)

(* Import necessary modules *)
open TextIO;

(* Function to create a file *)
fun createFile filename =
    let
        val _ = print "creating\n"
        val file = openOut filename
    in
        file
    end;

(* Function to write to a file *)
fun writeFile file =
    let
        val _ = print "writing\n"
        val _ = output(file, "data\n")
    in
        ()
    end;

(* Function to close a file *)
fun closeFile file =
    let
        val _ = print "closing\n"
    in
        closeOut file
    end;

(* Main function *)
fun main () =
    let
        val filename = "/tmp/defer.txt"
        val file = createFile filename
        
        (* Define a function to be executed at the end *)
        fun cleanup () = closeFile file
        
        (* Execute the main logic *)
        val _ = writeFile file
        
        (* Execute the cleanup function *)
        val _ = cleanup ()
    in
        ()
    end;

(* Run the main function *)
val _ = main ();

In this Standard ML implementation, we’ve created functions that mimic the behavior of the Go example:

1. createFile opens a file for writing.
2. writeFile writes data to the file.
3. closeFile closes the file.

The main function demonstrates how to use these functions in a way similar to the Go example:

1. We create the file using createFile.
2. We define a cleanup function that will close the file.
3. We write to the file using writeFile.
4. Finally, we call the cleanup function to close the file.

While Standard ML doesn’t have a built-in defer mechanism, this approach achieves a similar result by defining and calling a cleanup function at the end of the main function.

To run this program:

$ sml defer.sml
creating
writing
closing

This output confirms that the file is created, written to, and then closed, mirroring the behavior of the original Go program.

Note that error handling in Standard ML is typically done using exceptions, which we’ve omitted here for simplicity. In a real-world application, you’d want to add proper exception handling, especially when dealing with file I/O operations.