Exercise 1a: ‘Hurl’

Hurl is an interesting programming language where exception handling is the only allowed control flow. Here’s an example of a program in Hurl:

let factorial = func(n) {
  try {
    hurl n == 0;
  } catch (true) {
    hurl 1;
  } catch (false) {
    let next = 1;       // mutable
    try {
      factorial(n - 1);
    } catch into next;
    hurl n * next;
  };
};

try {
  factorial(10);
} catch as x {
  println("factorial(10) = ", x);
};

Goal: Translate the program above into Effekt!

Here are some smaller steps:

Define a hurl effect operation. Note that you need to be able to hurl both booleans and numbers. We’d recommend making the effect parametric in what you’re throwing (“hurling”).
Transcribe the factorial function using your very own hurl operation.
Transcribe the final try { ... } catch as x { ... } section into the result()

// 1. define the effect operation and any useful definitions here


// 2. put your code for factorial here
//    The type of factorial should be `Unit` and it should use the effect operation you defined


// 3. put the try { ... } catch as x { ... } here
//    (something like `try { factorial(10); "" } with ...` in Effekt)
//    You should return the string "factorial(10) = ..." here
//
// Don't change the return type or effect set please!
def result(): String / {} = {
  <> // TODO
}

Here’s a REPL:

result()

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