Discussion on: Is “Defensive Programming” actually healthy?

@yaser

the compilers' job is to check syntax and such

I completely disagree here, ideally the compiler would check all your app logic too, the more you can get checked by the compiler, the less tests you need to maintain. Languages like Haskell are popular just because the compiler can help you a lot.

I trust my tests more than the compiler

Tests can never show the absence of bugs, only the presence. Having a type system cut the possible inputs down to (in this case) a finite amount of values is far more valuable than testing the 4 values you mention in your unit tests.

@Cubicle

The problem with that dictionary solution is that it relies on throwing an error at runtime

Not with TypeScript, the following code will throw a compiler error if you change the TrafficLight type, without adding something to the object:

type TrafficLight = "red" | "yellow" | "green";
type TrafficAction = "stop" | "go" | "pause";

type TrafficResponse = {
    [k in TrafficLight]: TrafficAction;
};

function respondToTrafficSignal(signal: TrafficLight): TrafficAction {
    const mapping: TrafficResponse = {
        red: "stop",
        yellow: "pause",
        green: "go"
    };
    return mapping[signal];
}