The compiler would enforce Number-ness every time I try and run a function that takes a number, right?

Wouldn’t I still have to check for NoScore?

> a sum type with units of Score(Int) and NoScore won't allow assignments of any other "null" instances.

I get this part - I wouldn’t be able to assign ‘NewBornBaby’ (my name null) to ‘NoScore’ (my score null)

> The compiler would enforce Number-ness every time I try and run a function that takes a number, right?

> Wouldn’t I still have to check for NoScore?

No, because you differentiate between the sum type (e.g. Maybe in Haskell) and the number type at compile time. It's a small distinction - there will still be one or two places where you ask "is this a Maybe-score, or a Just-Score, or a No-Score", but the upside is that in all places you are very clear if a No-Score is possible, and you can't confuse the value with the error signal.

I.e. if you pass maybe-scores to something that computes the mean, you'll get a compiler error. The writer of the mean function doesn't need to care you've overloaded an error value onto your numbers.

The compiler support is the important part. Languages like C/C++ know sum-types just fine. They usually surface as sentinel values (NULL for ptrs, -1 for ints, etc) or unions with type fields. The stdlib offers it as std::optional<T>. As you progress along that spectrum, you get increasing compiler support there, as well.

One could even argue that sentinel values are a slightly better choice than Go's pairs, because they are closer to being sum-types than the strict product type that is go's (result, error) tuple - at least sentinels can't be simultaneously carrying a valid value and an error.

> I.e. if you pass maybe-scores to something that computes the mean, you'll get a compiler error. The writer of the mean function doesn't need to care you've overloaded an error value onto your numbers.

If I pass a null into something that calculates an average, taking numbers, the TS compiler will complain now. The writer of mean() (assuming mean() is typed) doesn’t have to know anything about my code.

This only works for non-sentinels. I.e. if "-1" happens to be the value indicating NoScore, I don't think the TS compiler can catch that?

Nobody is discussing using a magic number for null, nor is that an accepted practice in TS.

>Wouldn’t I still have to check for NoScore?

That's right, and the compiler will reject programs where you don't do this. It's a set of safety rails for your code. You pay a dev-/compile-time cost in exchange for your programs not exploding at runtime.

Yes but that’s no less work than checking for null in TypeScript. The parent said:

> In your example, you still have to manually check if there's a value every time, but this is not compiler-enforced.

Sure, it's the same amount of work, but you're forced to do it and the compiler will be able to reject invalid programs. In languages that allow null (including TS, it doesn't 100% enforce the stuff that Haskell, etc. does), you can skip that check, saving some work I suppose, at the risk of your code exploding at runtime. Having stack traces which jump across 50,000 lines of code because someone forgot to check for null somewhere sucks a lot.

TS wouldn’t let you skip that check though.