Defer in Wolfram Language

The concept of “defer” in Wolfram Language can be achieved using Catch and Throw. Here’s how we can implement a similar functionality:

(* Defer is used to ensure that a function call is
   performed later in a program's execution, usually for
   purposes of cleanup. In Wolfram Language, we can use
   Catch and Throw to achieve similar behavior. *)

(* 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 Catch and Throw. *)

createFile[path_] := Module[{stream},
  Print["creating"];
  stream = OpenWrite[path];
  If[FailureQ[stream],
    Throw[stream, "FileError"]
  ];
  stream
]

writeFile[stream_] := (
  Print["writing"];
  WriteString[stream, "data\n"]
)

closeFile[stream_] := Module[{},
  Print["closing"];
  Close[stream]
]

main[] := Catch[
  Module[{file},
    file = createFile["/tmp/defer.txt"];
    Throw[closeFile[file], "Defer"];
    writeFile[file];
  ],
  "Defer"
]

(* Run the main function *)
main[]

In this Wolfram Language implementation:

1. We use Catch and Throw to simulate the defer behavior. The Throw with the “Defer” tag is used to ensure that the file is closed at the end of the function.

2. The createFile function opens a file for writing and returns the stream. If there’s an error, it throws a “FileError”.

3. The writeFile function writes data to the file.

4. The closeFile function closes the file stream.

5. In the main function, we use Catch to wrap our operations. We create the file, set up the deferred close operation with Throw, and then write to the file.

6. The Catch will catch the “Defer” throw at the end of the function, ensuring that the file is closed.

Running the program confirms that the file is created, written to, and then closed:

creating
writing
closing

This implementation provides a way to ensure that cleanup operations (like closing a file) are performed even if an error occurs during the main operation. However, it’s worth noting that Wolfram Language has built-in functions like OpenWrite and Close that handle file operations safely, so this level of manual management is often unnecessary in practice.