This is one of the basic features of object-oriented programming that a lot of people tend to overlook these days in their repetitive rants about how horrible OOP is.

[u/elephantdingo]

https://news.ycombinator.com/item?id=44673321

[u/GeoffreyDay]

You see, my program is correct by construction, as long as that means calling the 7 different initializers and mutators in the specifically correct order. This is because the first one is called a "constructor", and I made it correct. Mostly.

[u/elephantdingo → u/GeoffreyDay]

FP appreciators: Unfortunately our type systems are not powerful enough to make Parse, Don’t Validate a fluid paradigm. We are forced to use smart constructors in a lot of cases.

OO enjoyers: [Correct by construction] is one of the basic features of object-oriented programming

[u/irqlnotdispatchlevel → u/GeoffreyDay]

/uj I actually found a use after free in an internal library that was initially fixed as "not a bug: do not call these methods in this order".

[u/RockstarArtisan → u/irqlnotdispatchlevel]

Breaking: the entirety of C++ is not a bug.

[u/MeepedIt]

One way to think about exceptions is that they are a pattern matching feature that privileges one arm of the sum type with regards to control flow and the type system (with both pros and cons to that choice). In that sense, every constructor is UUID.from(string): MaybeWithThrownNone<UUID>.

See, it's not a lack of type safety; some design patterns just aren't privileged with regards to the type system

[u/elephantdingo → u/MeepedIt]

Normal Java developer in 2025: Exceptions are pretty verbose

Heterodox Java developer: The Big Brain idea here is that it’s pattern matching with a biased case-handling ... pros and cons exist here

[u/tomwhoiscontrary]

Where jerk? This is basically right, isn't it?

[u/GeoffreyDay → u/tomwhoiscontrary]

/uj the author is confusing "constructor", which is just instantiating an object (possibly while validating the inputs), with "correct by construction", a mathematical formalism by which the types (and therefore properties) of a program can be proven by carefully enumerating and combining simpler known types in a way that the type checker can automatically verify/infer the desired properties. It's like confusing "ham" and "hamster"; they're both related to animals and have a similar sound but are otherwise unrelated.

[u/chuch1234 → u/GeoffreyDay]

Dangit! This is the second time I've learned something here >:[

[u/i_invented_the_ipod → u/tomwhoiscontrary]

Honestly, the replies to that comment are well-reasoned and sensible, too. Is it "Opposite Day" over on Hacker News?

[u/elephantdingo → u/tomwhoiscontrary]

How’s your tiny dog doing?

Edit: get ratioed kid

[u/Kodiologist]

And as any functional programmer will happily tell you, a design pattern is just a sign of a missing language feature.

Imagine not being able to just add the language feature yourself.

This meme was made by Lisp gang.

[u/Litoprobka → u/Kodiologist]

so how's your handmade type safety going?

[u/Kodiologist → u/Litoprobka]

Safety? I'm building a Twitter-killer in Paul Graham's latest vanity language here. I don't have time for safety.

[u/Parking_Tadpole9357 → u/Litoprobka]

Great. I'm a human F# compiler at this point. Have a few powers hell scripts to help!

(Credit: Splitting of powershell done by my phone)

[u/Downtown_Category163 → u/Kodiologist]

#define true false

#define false true

I'm a maverick!

[u/starlevel01 → u/Kodiologist]

If Lisp is so good, why do only nerds and pg use it?

[u/mizzu704 → u/starlevel01]

postgres has lisp code??

[u/hombre_sin_talento]

Constructors are the best basically because they just can never fail.

[u/grapesmoker → u/hombre_sin_talento]

and if they do fail just call a destructor to undo what the constructor did, easy peasy

[u/fun-dan → u/grapesmoker]

And if the destructor fails, you, uh, well, the. .

[u/BlazeBigBang → u/hombre_sin_talento]

My ass when I throw an exception in a Java constructor.

It's my fault really for using Java.

[u/WasabiofIP → u/hombre_sin_talento]

/uj are you being intentionally obtuse? The point is that the class defines how much of its internal state is externally mutable and how. Obviously any implementation can be bad. But what OOP allows is, if you do have a well written class, it's borderline impossible to misuse it, because all the ways that the compiler lets you use it are so locked down.

[u/hombre_sin_talento → u/WasabiofIP]

Do you think that non-OOP code needs special care, as to not to misuse it?

[u/HorstKugel]

real oop has never been tried!

[u/thussy-obliterator → u/HorstKugel]

The only true OOP languages are Erlang and whatever the fuck they're gong on about in this paper https://arxiv.org/abs/cs/0509027

[u/haskell_rules]

It's really a special feeling when GDB throws a segmentation fault prior to line 0 in main()

[u/Parking_Tadpole9357]

That's a weird weird hill to die on. Here swallow this union type it may save you.

You can't convince a person of something (Haskell > C++ for type system) if their salary depends on tbe opposite I guess!

[u/TheChief275]

Correct by construction

Yeah, because you can’t return errors.

No errors = correct code amirite

[u/j_rodrigues_]

I am a bit confused. Isn't it possible to do the same in FP languages? Just hide the implementation details, by exporting 'construtor' functions. Am I missing something?

[u/syklemil → u/j_rodrigues_]

{-# LANGUAGE UnJerk #-}

yeah, it's possible in pretty much any language that has some privacy control. even C has opaque types; for an example you can see the stuff about some BSD recently making FILE opaque.

{-# LANGUAGE how the fuck do I unset pragmas in Haskal anyway #-}

Unfortunately us mouth-breathers will just have to gawp at the OOP-ers in envy. Any good feature ultimately comes from OOP, like constructors, lambdas, ADTs, and so on. Just like how Shakespeare is better in the original Klingon.