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Comment by whilenot-dev | Hacker News Reader

How do you type the min > max constraint though?

If we're trying to solve problems with good design, use endpoint1 and endpoint2 and then the function sorts them. Having max and min is itself a bad design choice, the function doesn't need the caller to work that out. Why should the caller have to order the ends of the interval? It adds nothing but the possibility of calling the function wrong. So in this this case:

    export function clamp(value: number, endpoint1: number, endpoint2: number): number {
      return Math.min(Math.max(value, Math.min(endpoint1, endpoint2)), Math.max(endpoint1, endpoint2));
    }

So an implicit fallback, but make it explicit through good design. Haven't even thought about this as a principle, since type checking persuades me to avoid anything implicit, thank you!

That would lead to unpleasant surprises. When calling the function from some loop and when the bounds are inclusive, it's pretty common for (correct) edge cases to exist where you'd call the function with end===start-1. The function would do the right thing by returning an empty set. You'd get duplicate/unexpected records in some cases, that may be hard to debug.

It seems like your approach is just trying to ignore programmer errors, which is rarely a good idea.

I have no horse in the race and would usually just implement my clamp function the way the article does. However, if the clamp function clamping a number is an unpleasant surprise, I'm not going to accept that it is the fault of the clamp function. This hypothetical loop is buggy code and should be rewritten to expect clamp to clamp.

It is a special type of madness if we're supporting a reliance on implementation specific failure modes of the clamp function when someone calls it with incoherent arguments.

> This hypothetical loop is buggy code and should be rewritten to expect clamp to clamp.

But it makes it harder for the developer to recognize that the code is buggy. More feedback to the developer allows them to write better code, with less bugs.

Your argument could be made in the same way to claim that static typing is bad; because the caller should be calling it with the right types of values in the first place.

> But it makes it harder for the developer to recognize that the code is buggy. More feedback to the developer allows them to write better code, with less bugs.

But the feedback is unrelated to the bug, the bug here is that the programmer doesn't understand what the word "clamp" means and is trying to use the function in an incorrect way. Randomly throwing an exception on around 50% of intervals doesn't help them understand that, and the other 50% of the time they're still coding wrong and not getting any feedback. I'm not against the clamp function doing whatever if people want it to, it can make coffee and cook pancakes when we call it for all I care. But if it just clamps that is probably better. It isn't a bug if I call clamp and don't get pancakes. It also isn't a bug if I call clamp and it remains silent on the fact that one argument is larger than another one.

Feedback has to be relevant. It'd be like having a type system that blocks and argument that isn't set to a value. If the programmer provides code that has bugs, it'll give them lots of feedback. But the bug and the error won't be related and it is effectively noise.

This maps poorly to the mathematical concept of a closed interval [a, b], which can be written a ≤ x ≤ b for a set of x. An interval where a > b is usually a programming error.

To ensure only valid intervals are supported at the type system level, the function could perhaps be redefined as:

    function clamp(n: number, i: Interval<number>): number

Of course, you need to deal with the distinction between closed and open intervals. Clamping really only makes sense for closed ones.

It maps very well onto the mathematical concept of a closed interval [a, b] where a and b are endpoints of the interval though. You're adding a constraint for no logical reason and it happens to be very hard to represent in a basic type system.

> An interval where a > b is usually a programming error.

If you want it to be, sure. Anything can be a programming error if the library author feels like it. We may as well put all sorts of constraints on clamp, it is probably an error if the caller uses a large number or a negative too. It is still bad design in a theoretical sense - the clamp function throws an error despite there being an obvious non-error return value. It isn't hard to meaningfully clamp 2 between 4 and 3.

Well, if your language has a sufficiently strong type system (namely, dependent types), you can take proofs of some properties as arguments. Example in Lean:

  def clamp (value min max : Float) {H : min < max} : Float := ...

Sure, but the author picked TypeScript nonetheless. TypeScript is not a runtime, but a mere type checker - JavaScript is the runtime and a highly dynamic language. This detail got somehow completely lost in the article, but is IMHO the main culprit why such validations aren't bad, or sometimes even preferred.

The article also skipped over the following related topics:

  - When would you wrap errors from lower levels as your own?
  - What does "parse don't validate" mean when a TypeScript library gets transpiled to JavaScript?

You don’t need a runtime for dependent types. After type checking the types get erased during compilation.

Nobody would question that, but publishing a JavaScript library means that anyone using plain JavaScript can make use of it. Even though you aren't ever in control of the toolchain of your library's users, it's still your responsibility - as library author - to take that differences into account. If you'd transpile your library from Idris to JavaScript and publish it, these validations just can't be neglected at runtime. Type systems are just another model of the world at runtime.

In a compiled language, it takes one or two machine instructions to test

    assert!(b >= a);

Works in C, C++, Go, Rust...

Amusingly, nowhere in the original article is it mentioned that the article is only about Javascript.

Languages should have compile time strong typing for at least the machine types: integers, floats, characters, strings, and booleans. If user defined types are handled as an "any" type resolved at run time, performance is OK, because there's enough overhead dealing with user defined structures that the run time check won't kill performance.

(This is why Python needs NumPy to get decent numeric performance.)

Sure, use macros in function bodies. That won't affect the function signature in any meaningful way for the type checker and remains a check at runtime only, doesn't it?

It seems like the point of the article was to not do that though, contrary to my own opinion, and I just wonder why...

Many libraries throw an exception, panic, or silently swap the parameters at runtime.

To detect this at compile time, you would need either min and max to be known at compile time, or a type system that supports value-dependent types. None of the popular language support this. (My language named 'Bau', which is not popular of course, support value-dependent types to avoid array-bound checks.)

You define an Interval type, and check the constraint in its constructor.

You don't need to. One if statement to check that is not a problem. The problem occurs when you have a bunch of other ifs as well to check all kinds of other stuff that a type system would handle for you like nullability, incorrect types etc.

Personally I just write JS like a typed language. I follow all the same rules as I would in Java or C# or whatever. It's not a perfect solution and I still don't like JS but it works.

Don't send `start` and `end`; send `start` and `lengthOfInterval`. (Whether that's a good idea in a given API is another question.)

That trades "min <= max" with "min + interval <= MAXINTEGER":

  if(number < min) return min;
  else if(number < min + interval) return number; // "if(number < max)"
  else return min + interval; // "return max"

If min and max aren’t user inputs maybe we should trust the developer that they know what they are doing.

Some languages can do it, but most can't do you either throw an error or do something reasonable. In this case just returning min would be reasonable.

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