Recover in Standard ML

In Standard ML, we don’t have built-in exception handling mechanisms like panic and recover. However, we can simulate similar behavior using exception handling. Here’s an example that demonstrates a similar concept:

(* This function raises an exception *)
fun mayRaise () =
    raise Fail "a problem"

(* Main function *)
fun main () =
    let
        (* Exception handler *)
        fun handler exn =
            case exn of
                Fail msg => print ("Recovered. Error:\n" ^ msg ^ "\n")
              | _ => raise exn  (* Re-raise other exceptions *)
    in
        (* Use exception handling to catch and recover from exceptions *)
        (mayRaise () handle exn => handler exn);
        
        (* This code will not run if mayRaise raises an exception *)
        print "After mayRaise()\n"
    end

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

In this Standard ML example:

1. We define a mayRaise function that raises a Fail exception, similar to the panic in the original example.

2. The main function contains an exception handler handler that catches exceptions. This is analogous to the deferred function with recover in the original code.

3. We use a handle expression to catch any exception raised by mayRaise. If an exception is caught, it’s passed to the handler function.

4. The handler function checks if the caught exception is a Fail exception. If so, it prints a message similar to the original example. For other types of exceptions, it re-raises them.

5. The print "After mayRaise()\n" line demonstrates that execution would stop at the point of the exception and not reach this line if an exception is raised.

To run this program, you would typically save it in a file (e.g., recover.sml) and use an Standard ML interpreter or compiler. For example, using the Standard ML of New Jersey (SML/NJ) interpreter:

$ sml recover.sml
Recovered. Error:
a problem

This example demonstrates exception handling in Standard ML, which provides functionality similar to the recover mechanism in the original language. While the syntax and exact mechanisms differ, the core concept of catching and handling exceptions remains the same.