[phaedrus]

I once attended a talk by someone who is or was big in the node.js world. He opened with the premise, "a static type check is just a stand-in for a unit test."

I wanted to throw a shoe at him. A static type check doesn't stand in for "a" unit test; static typing stands in for an unbounded number of unit tests.

Put another way, this common misconception by users of languages like Javascript and Python that unit testing is just as good as type checking (plus more flexible) is a confusion between the "exists" and "for all" logical operators.

[kccqzy]

Plus, it is simply more enjoyable to design the types in your program than to write unit tests. The fun factor comes from operating on a higher level of abstraction and engages more of your brain’s puzzle-solving mode than just writing unit tests. Making yourself think about “for all x” rather than a concrete x forces your brain to consider deeply the properties of x being used.

[zahlman]

> it is simply more enjoyable to design the types in your program than to write unit tests.

I have tried both and I have no idea what you're talking about.

> Making yourself think about “for all x” rather than a concrete x forces your brain to consider deeply the properties of x being used.

The entire point of dynamic typing is that you can think about interfaces rather than concrete types, which entails deep consideration of the properties of the object (semantics of the provided interface).

[array_key_first]

That's not the entire point of dynamic typing, because all the interface stuff comes from statically typed languages. Some* dynamic languages borrowed it, but most use "implicit" interfaces - where the interface is whatever kind of works, I guess.

[zahlman]

> because all the interface stuff comes from statically typed languages.

No, it doesn't. It comes from theory that came after the languages.

> Some* dynamic languages borrowed it, but most use "implicit" interfaces

An implicit interface is an interface, and is exactly the sort of thing I'm talking about in GP. The point is that you think about the object in terms of its capabilities, rather than some proven-up-front categorization that it fits into. What it does, not what it is.

[maleldil]

You can achieve this with structural subtyping, such as Go interfaces and Python protocols. Whether that is desirable is a different question.

[andrewl-hn]

> "a static type check is just a stand-in for a unit test."

This is not an original argument. Rich Hickey made a similar argument in his "Simple made easy" talk in 2011, though his focus was on a fact that every bug that easiest in a software system has passed unnoticed through both a type checker and a test suit. And even before that similar ideas of test suits being a suitable replacement for a type checker have percolated through Python and Ruby communities, too.

I distinctly remember that the "tests makes static type checks unnecessary" was in fact so prevalent in JavaScript community that TypeScript had really hard time getting adoption in its first 3-4 years, and only the introduction of VSCode in 2015 and subsequent growth of its marketshare over Atom and SublimeText got more people exposed to TypeScript and the benefits of a type checker. Overall it took almost 10 years for Typescript to become the "default" language for web projects.

[imiric]

Agreed.

Besides, it's not like types don't matter in dynamically typed languages. The (competent) programmer still needs to keep types in their head while programming. "Can this function work with a float, or must I pass an int?" "This function expects an iterable, but what happens if I pass a string?" Etc.

I started my career with JavaScript and Python, but over the years I've come to the conclusion that a language that hides types from programmers and does implicit conversion magic in the background does not deliver a better DX. It might make the language more approachable initially, and the idea of faster prototyping might be appealing, but it very quickly leads to maintenance problems and bugs. Before type hinting tools for Python became popular, I worked on many projects where `TypeError` was the #1 exception in Sentry by a large margin.

Gradual and optional typing is better than nothing, but IME if the language doesn't require it, most programmers are lazy and will do the bare minimum to properly add type declarations. Especially with things like TypeScript, which makes many declarations difficult to read, write, and understand.

I think that type inference is a solid middle ground. Types are still statically declared, but the compiler is smart enough to not bother the developer when the type is obvious.

[zahlman]

> Before type hinting tools for Python became popular, I worked on many projects where `TypeError` was the #1 exception in Sentry by a large margin.

My experience is radically different. `ValueError` is far more common in my un-annotated Python, and the most common cause of `TypeError` anyway is the wrong order or number of arguments after a refactoring.

[imiric]

Hhmm I could be misremembering if it was `ValueError` or `TypeError`. This was a few years ago. I know that typing issues were always the most frequent in any Python project I have worked on.

How is your experience different?

[seg_lol]

Hundreds of unit tests replace a type.

Start using properties and it is in the thousands.

Most code should be typed. Python is great for prototypes, but once the prototype gels, you need types.

[lelanthran]

I've always hated Python. Could never enjoy it at all. Pretty much the same poor DX as PHP, Javascript, Ruby, etc.

Finally set up neovim with pyright; use types on every single fucking thing, and now I love Python[1].

Can't wait to see TC39 become a reality (learned about it just this past week on HN, actually). Maybe I'll enjoy Javascript too.

--------------------

[1] Within reason; the packaging experience is still extremely poor!

[lenkite]

Didn't get the bit about TC39 - it is just a group. It has nothing to do with types in JS.

[lelanthran]

Sorry, I should have been clearer about which proposal I was talking about:

https://github.com/tc39/proposal-type-annotations

[tasuki]

I've never seen types used correctly in Python. A `tuple` is not even a type! It's a type constructor!

[baq]

I’ve been doing Python and Typescript professionally, Python for almost two decades, Typescript for last 5 years and I can very confidently say that it doesn’t matter.

Besides, you see to be confusing Python run-time with Python typecheck-time, theoretically unfortunate, but again practically irrelevant distinction. (Unfortunate since Python typecheck is basically a different language than Python execution; irrelevant, because the right subsets of both align well.)

[zahlman]

The distinction you are trying to make is nonsensical in Python's object model. Types are inherently callable, and calling them constructs (i.e. instantiates) the type (normally; this can be overridden, by design). There is also no type->kind->category hierarchy; `type` itself is an object, which is its own type.

When you're at a level of theory where terms like "type constructor" are natural, it's unreasonable to expect any of it to be applicable to Python. This is why the Haskell people speak of dynamically-typed languages in the Python mold as "untyped" regardless of their attitude towards implicit casts.

And I love it, and have been using it for decades, and write beautiful things where the annotations hardly ever seem worth the effort — perhaps for documentation, but not for a static checker. Then I look at other, newer Pythonistas trying to figure out how to write complex generic type expressions (and sacrificing backwards compatibility as they keep up with the churn of Python figuring out how to offer useful annotation syntax) and deal with covariance vs contravariance etc. and I just smile.

[lelanthran]

Most of the types are "not even types". It's why you can pass a type as a value (not a string) to a function.

If the type was a type, you'd not be able to use it as a value.

[shermantanktop]

A unit test is a functional assertion. A type is a semantic construct that can provide that, but it provides a lot more.

As a trivial example, if I create a type alias from “string” to “foobarId,” I now (assuming a compliant language) can prevent code that consumes foobarIds from accidentally consuming a string.

[baq]

Good that Python supports types then

[Groxx]

Python has formalized magic comments.

You can run a third party linter on those comments, but you must hope that they're correct. There are usually some checks for that, but they're only reliable in trivial cases.

This is not static typing any more than "you can use emscripten to transpile JavaScript to C" means that JavaScript is a low level language with native assembly support. It's a huge step forward from "no system at all" and I'm thrilled it exists, but it's hardly the same thing.

[kccqzy]

They are originally magic comments, unformalized. The feature is originally known as annotations and you can basically put anything there.

    def f(a: "something like an int", b: "another int-like thing"): pass

    def g(a: sys.stdin, b: sys.stderr): pass

[andrewl-hn]

It's actually remarkable how with the success of TypeScript so many other dynamic languages switched to gradual typing.

Erlang and Clojure were the early ones, TypeScript followed, and now Python, Ruby, and even Perl have ways to specify types and type check your programs.

[lelanthran]

> Good that Python supports types then

"Optional typing" is not the same as "Static typing".

Great, my program will crash, because I forgot to opt-in to typing :-/

[paulddraper]

C has void pointers.

[lelanthran]

> C has void pointers.

And? Void pointers are not the default type :-/

With Python I have to do extra work to get type errors.

With C I have to do extra work to hide the type errors.

I am battling to understand the point you are making.

[paulddraper]

I believe that is in fact the exact return type of malloc.

You have to do extra work to type it.

I’m not aware of any other statically typed language that does that.

Hopefully that helps you in your battle.

[array_key_first]

He's probably conflating static and strong typing.

C is statically typed, but weakly typed - you need to throw away types to do a bunch of run of the mill things. Python is dynamically typed, but strongly typed, where it will just fail if typed don't resolve.

C# and C++ are both statically typed and strongly typed, although C# more than C++ in practice.

[baq]

Hard to argue with an argument which isn’t internally coherent.

[SR2Z]

Poorly, though, and with lots of edge cases and foot guns to make you miserable once your code calls out to numpy or gets some JSON.

[tasuki]

Tell me more please: how does one use types in Python? Unfortunately I write Python professionally these days (it is the language that has all the libraries) and hate it with a passion.

[seg_lol]

You will never get good if you continue to hate it. You don't have to like it, but hating it creates a mental block.

[baq]

mypy is the simplest/oldest option, look into pyright and ty, too.

install uv and then

    uvx mypy

should do the rest.

[tasuki]

> I wanted to throw a shoe at him.

You should have!

[lesuorac]

If you care about the type of a parameter you can just add an assertion in the method /s

[pmontra]

Good luck using static typing to model many real world unit tests for the programming languages people use most. I start with an easy example: those records should be sorted by date of birth. We can move on to more complicated scenarios.

[jodrellblank]

> "records should be sorted by date of birth."

What's wrong with C#'s:

    System.Collections.Generic.SortedList<DoBDateTime, PersonRecord>

?

[paulddraper]

No one claims that types are a stand in for all unit tests.

They stand in for the banal unit tests.

[Jaxan]

The comment didn’t claim that types are a stand in for tests either! IMO, they are orthogonal.

[zahlman]

The comment explicitly set out to refute the idea "...that unit testing is just as good as type checking" by describing the former as simply inferior.

[Veserv]

No. They refuted the claim that "a static type check is just a stand-in for a unit test". That is a claim that you can just remove your type checks and replace them with unit tests at no loss. The comment stated that removing a type check just so you can replace it with a unit test is inferior. The prior state was already pre-supposed to have a type check/type checkable condition that you could replace.

That is the literal converse of the claim in the response to that comment arguing that the comment stated that all unit tests can be replaced with type checks. Those are not at all the same claim.

To make it even more clear the comment said: I saw a talk that said Type Check -> Unit Test. I said that is silly.

Response said: Unit Test -> Type Check is not reasonable. So clearly your claim that Type Check -> Unit Test is silly is wrong.

[zahlman]

> A static type check doesn't stand in for "a" unit test; static typing stands in for an unbounded number of unit tests.

You have conflated "a static type check" with "static typing". Unit tests stand in, in the same way, for an unbounded number of states of real-world input. They're simply being subjected to a trial verification system rather than a proof system. It turns out that writing proofs is not very many people's idea of a good time, even in the programming world. And the concept of "type" that's normally grokked is anemic anyway.

> Put another way...

Rhetoric like this is unconvincing and frankly insulting. You pass off your taste and opinion as fact, while failing to understand opposed arguments.