Best Strategy for throwing/returning from an assignment switch/

Developers Language Support
1

What is the best way to throw or return from an assignment switch? I want to do something like:

   func pick(request : MyRequest) : Result<MyVariant, Text>{
             let config = switch(request.config){
                    case(#variant1(data)){#internal;};
                    case(#variant2(pullRequest)){#internal2}; };
                    case(#badVariant){return #err("Never should be here"}; // won't compile due to a type error
                    // or case(#badVariant){throw #err("Never should be here"}; but returns an Error type and doesnt work
                };
         return #ok(config);
     }
2

I would skip the assignment and instead return #ok(…) inside of the case:

func pick(request : MyRequest) : Result<MyVariant, Text> {
  switch(request.config) {
    case(#variant1(data)) { #ok(#internal); };
    case(#variant2(pullRequest)) { #ok(#internal2); };
    case(#badVariant) { #err("Never should be here"; };
  };
};
3

That is a good strategy if I’m ready to return…and my example was perhaps a bit simplistic. In the real world I’m setting up this variant to go into a function call further down in the function, so I can’t return right away in every scenario.

4

In that case I would keep what I did but create a let binding for the result and then use map.

In languages with special syntax like do notation or Rust’s ? you can use that instead.

5

If you don’t need the Result type immediately, then you can use do?.

func pick(request : MyRequest) : Result<MyVariant, Text>{
   let result = do? {
     let config = switch (request.config) {
        case (#variant1(data)) { #internal };
        case (#variant2(pullRequest)) { #internal2 };
        case (#badVariant) { null! };
     };
     // ...do something with config
     // ...produce result
  };
  return Result.fromOption(result, "error message");
}

At some point, we’d like to generalise do? to enable producing something like Result directly, but that’s not there yet.

1 Like
6

.fromOption! Perfect. I need to re read the Base Libraries now that I actually know motoko. :joy: