Comment by wolfgang42 | Hacker News Reader

Comment by wolfgang42 | Hacker News Reader

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> avoiding using PL/type-system-related terms...tell the user with easy to understand the sentence.

What I meant by “making type errors clear” wasn’t so much the text of the error messages, but rather making the cause of the error more obvious. Here’s a very simplified example of the sort of thing I mean:

  function foo() {return '1.2'}
  const bar = foo() + 1
  const baz = bar.toFixed()

ESInfer complains on line 2, and TypeScript on line 3 (I guess it understands that `+` will coerce the type), but from my perspective the mistake is on line 1, where I forgot parseFloat(). Without annotations, the problem manifests as individual errors somewhere downstream of every call site; if I annotate foo() as returning a number, the type checker will immediately point me at the problem. In fact when I get a type error from tsc that I don’t immediately understand, my first step is usually to start scattering annotations around to help it narrow down where its take on the situation differs from mine.

Basically, neither ESInfer nor TypeScript give me great diagnostics for this code as-is, but TS lets me have a ‘conversation’; it has the tools I need to help it help me find the problem. ESInfer (in my admittedly cursory understanding) seems like it’s missing that ability; I can imagine a heuristics that would help it better explain the situation (e.g. showing the entire data flow that lead to a mismatch, rather than claiming the error is at a single point) but I haven’t thought about it very much so I was wondering if you had any particular ideas around this situation.

(Incidentally, you can find the syntax for HN comments at <https://news.ycombinator.com/formatdoc>. It’s linked from the FAQ, which is itself linked at the bottom of the page, though that’s not very obvious.)

  foo() + 1

ESInfer complains about this because you are adding a string with a number, which sometimes will result in an unexpected value in runtime, and you hardly find it out. So ESInfer, on purpose, disallows this kind of type casting automatically.

As for the error message:

> expecting possible candidates all failed: candidate number failed: expecting number but got string. candidate string failed: expecting string but got number. but got number.

It's saying two possible situations: #1 number + number and #2 string + string are all failed, which is precisely the error. However, this may be a lack of readability if someone is the first time using ESInfer. I'll write some wikis on how to read error messages efficiently as a starting point and, finally and hopefully, evolve a better error reporting system.

So if you ask me how to deal with this kind of code with ESInfer, I'd suggest making the compiler happy by converting the string to a number or the number to a string.

(BTW, try to use the format as suggested, hope it works :D)

I think the parent is raising the point that knowing that there's a problem is only half of the issue you need to solve. Being able to assert that certain types only appear in certain places is still important: if I pass the wrong type as a function argument, I don't know whether there's a bug somewhere upstream where that data came from, or whether there's a bug in the function where I'm using a parameter incorrectly. Or if I'm not returning where I should be, or returning where I shouldn't be.

Did you mean something like an error stack?

That'll be significantly helpful if the last error is not apparent enough to solve the error. I'll investigate this and see if there's any performance impact on the inferring process.

TypeScript can infer all the types but it still requires the coder to specify types in certain places because of the exact issue that is being shown here.

The types create an API contract that keeps errors localized. ESInfer raises an error at the call site, which is harder to understand what is going on. Also, lack of types will cause the API contract to suddenly change in unexpected ways.

I commend you for creating ESInfer, but there is a reason this approach is usually not taken.

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