Typechecking is undecidable when 'type' is a type (1989) [pdf]

2/2/2026 at 12:50:48 AM

This stack exchange answer talks about the importance of decidability in type checking

https://langdev.stackexchange.com/a/2072

My interpretation

Decidability is of academic interest, and might be a hint if something is feasible.

But there are (1) ways of sidestepping undecidability, e.g. A valid C++/Rust program is one for which the typechecker terminates in x steps without overflowing the stack

And (2) things which are decidable, but physically impossible to calculate, e.g the last digit of the 10^10^10 th prime

What matters is being able to reject all incorrect programs, and accept most human written valid programs

by pvillano

2/2/2026 at 6:37:52 AM

Decidability of a type system is like well-typedness of a program. It doesn't guarantee it's sensible, but not having the property is an indicator of problems.

by penteract

2/2/2026 at 7:59:25 AM

I'm not entirely smart enough to connect all of these things together but I think there is a kind of subtlety here thats being stepped on.

1. Complete, Decidable, Well founded are all distinct things.

2. Zig (which allows types to be types) is Turing complete at compile time regardless. So the compiler isn't guaranteed to halt regardless and it doesn't practically matter.

3. The existance of a set x contains x is not enough by itself to create a paradox and prove false. All it does is violate the axiom of foundation, not create a russles paradox.

4. The axiom of foundation is a weird sort of arbitrariness in that it implies this sort of DAG nature to all sets under set membership operation.

5. This isn't nessesarily some axiomatically self evident fact. Aczel's anti foundation axiom works as well and you can make arbitrary sets with weird memberships if you adopt that. https://en.wikipedia.org/wiki/Aczel%27s_anti-foundation_axio...

6. The Axiom of Foundation exists to stop you from making weird cycles, but there is parallel to the axiom of choice, which directly asserts the existance of non computable sets using a non algorithmicly realizable oracle anyway....

by kingstnap

2/2/2026 at 8:50:06 AM

Your other points are more relevant to the content of the article, but point 2. relates the practical consequences of undecidable type-checking, so I'll reply to that.

I don't have a problem with compile time code execution potentially not terminating, since it's clear to the programmer why that may happen. However, conventional type checking/inference is more like solving a system of constraints, and the programmer should understand what the constraints mean, but not need to know how the constraint solver (type checker) operates. If it's undecidable, that means there is a program that a programmer knows should type check, but the implementation won't be happy with; ruining the programmer's blissful ignorance of the internals.

by penteract

2/2/2026 at 11:45:15 AM

> 2. Zig (which allows types to be types) is Turing complete at compile time regardless. So the compiler isn't guaranteed to halt regardless and it doesn't practically matter.

Being Turing complete at compile time causes the same kinds of problems as undecidable typechecking, sure. That doesn't make either of those things a good idea.

> 3. The existance of a set x contains x is not enough by itself to create a paradox and prove false. All it does is violate the axiom of foundation, not create a russles paradox.

A set that violates an axiom is immediately a paradox from which you can prove anything. See the principle of explosion.

> 4. The axiom of foundation is a weird sort of arbitrariness in that it implies this sort of DAG nature to all sets under set membership operation.

Well, sure, that's what a set is. I don't think it's weird; quite the opposite,

> 5. This isn't nessesarily some axiomatically self evident fact. Aczel's anti foundation axiom works as well and you can make arbitrary sets with weird memberships if you adopt that.

I don't think this kind of thing is established enough to say that it works well. There aren't enough people working on those non-standard axioms and theories to conclude that they're practical or meet our intuitions.

> 6. The Axiom of Foundation exists to stop you from making weird cycles, but there is parallel to the axiom of choice, which directly asserts the existance of non computable sets using a non algorithmicly realizable oracle anyway....

The Axiom of Foundation exists to make induction work, and so does the Axiom of Choice. They both express a sense that if you can start and you can always make progress, eventually you can finish. It's very hard to prove general results without them.

by lmm

2/3/2026 at 6:39:01 PM

But like, of all the expressive power vs analyzability trade-offs you can make, there's a huge leap in expressive power when you give away decidability.

Undecidability is not a sign that the foundation has cracks (not well founded), but it might be a sign that you put the foundation on wheels so you can drive it at highway speeds, with all the dangers that entails.

It's not a trade everyone would make, but the languages I prefer do.

by pvillano

2/1/2026 at 8:48:29 PM

This sounds close to Russell's "class of all classes" paradox. Is it?

by Animats

2/1/2026 at 9:28:05 PM

Yes the type theoretic analog to Russel's (set theoretic) paradox is Girard's (as mentioned in the abstract) paradox.

by enricozb

2/1/2026 at 9:38:18 PM

This is incorrect. The set paradox it’s analogous to is the inability to make the set of all ordinals. Russel’s paradox is the inability to make the set of all sets.

by srcreigh

2/1/2026 at 10:18:33 PM

Since we're being pedantic, Russel's paradox involves the set of all sets that don't contain themselves.

by kibwen

2/1/2026 at 11:25:09 PM

Technically speaking, because it's not a set, we should say it involves the collection of all sets that don't contain themselves. But then, who's asking...

by iterance

2/2/2026 at 1:09:59 AM

The paradox is about the set of all sets that do not contain themselves, or a theorem that such a set does not exist.

In the context of the paradox you need to call it a set, otherwise it would not be a paradox

by afiori

2/1/2026 at 11:50:07 PM

I'm asking. What prevents that collection from being a set?

by nurettin

2/2/2026 at 12:16:42 AM

This is the easiest of the paradoxes mentioned in this thread to explain. I want to emphasize that this proof uses the technique of "Assume P, derive contradiction, therefore not P". This kind of proof relies on knowing what the running assumptions are at the time that the contradiction is derived, so I'm going to try to make that explicit.

Here's our first assumption: suppose that there's a set X with the property that for any set Y, Y is a member of X if and only if Y doesn't contain itself as a member. In other words, suppose that the collection of sets that don't contain themselves is a set and call it X.

Here's another assumption: Suppose X contains itself. Then by the premise, X doesn't contain itself, which is contradictory. Since the innermost assumption is that X contains itself, this proves that X doesn't contain itself (under the other assumption).

But if X doesn't contain itself, then by the premise again, X is in X, which is again contradictory. Now the only remaining assumption is that X exists, and so this proves that there cannot be a set with the stated property. In other words, the collection of all sets that don't contain themselves is not a set.

by scapp

2/2/2026 at 12:53:52 AM

Let R = {X \notin X}, i.e. all sets that do not contain themselves. Now is R \in R? Well this is the case if and only if R \notin R. But this clearly cannot be.

Like the barber that shaves all men not shaving themselves.

by whiterock

2/2/2026 at 12:46:32 AM

They redefined sets specifically to exclude that construction and related ones.

by empath75

2/2/2026 at 12:49:14 AM

The paradox. If you create a set theory in which that set exists, you get a paradox and a contradiction. So the usual "fix" is to disallow that from being a set (because it is "too big"), and then you can form a theory which is consistent as far as we know.

by lmm

2/2/2026 at 9:18:29 AM

Perhaps I overstated how related the two were. I was pulling mostly from the Lean documentation on Universes [0]

> The formal argument for this is known as Girard's Paradox. It is related to a better-known paradox known as Russell's Paradox, which was used to show that early versions of set theory were inconsistent. In these set theories, a set can be defined by a property.

[0]: https://lean-lang.org/functional_programming_in_lean/Functor...

by enricozb

2/2/2026 at 2:33:36 AM

It is Burali-Forti 1897, predating Russel's paradox.

by hackandthink

2/2/2026 at 2:12:41 AM

Does Rice's theorem cover this?

> [ all non-trivial semantic properties of programs are undecidable ]

https://en.wikipedia.org/wiki/Rice's_theorem

Found here:

From Sumatra to Panama, from Babylon to Valhalla

https://www.youtube.com/watch?v=bE1bRbZzQ_k&t=48m27s

by jeberle

2/2/2026 at 5:08:28 AM

No. Being well typed is not a semantic property of of a program - in a language where it makes sense to talk about running badly typed code, a piece of code that starts with an infinite loop may be well or badly typed after that point with no observable difference in program behaviour.

There are decidable type systems for Turing complete languages (many try to have this property), and there are languages in which all well typed programs terminate for which type checking is undecidable (System F without all type annotations).

by penteract

2/1/2026 at 10:21:14 PM

And still this type system could be the base for a very interesting and powerfull programming language imo.

by randomNumber7

2/1/2026 at 11:01:41 PM

Yeah we don't throw programming languages away because they are undecidable.

So why would we throw type systems away if they are undecidable?

by amelius

2/2/2026 at 12:44:54 AM

My 2¢ from an interactive theorem proving perspective: In so-called computational type theory, typechecking is indeed allowed to be undecidable, and you get a lot of cool expressive power (e.g. well-behaved quotients and subtypes) as a result. This was one of the big ideas behind NuPRL back in the day, and Istari[0] more recently.

[0] www.istarilogic.org

by BalinKing

2/1/2026 at 11:04:46 PM

Decidability isn't even that useful. If typechecking takes a million years, that's also bad. What you want is guarantees that correct programs typecheck quickly. Without this, you end up in swift land, where you can write correct code that can't be typechecked quickly and the compiler has to choose between being slow or rejecting your code

by ChadNauseam

2/1/2026 at 11:23:17 PM

> What you want is guarantees that correct programs typecheck quickly.

In practice there's wealth of lemmas provided to you within the inference environment in a way standard library functions are provided in conventional languages. Those act like a memoization cache for the purpose of proving your program's propositions. A compiler can also offer a flag to either proceed with ("trust me, it will infer in time") or reject the immediately undecidable stuff.

by instig007

2/1/2026 at 11:49:45 PM

Presumably with the rather unpleasant side-effect of compiles that may never finish. :-P

by rerdavies

2/2/2026 at 11:50:58 AM

Yes, but all practical programming languages have that problem anyway, in practice if not in theory.

by lmm

2/1/2026 at 11:53:56 PM

This really isn't a big deal as it resolved via type universes.

by solomonb

2/2/2026 at 5:06:54 AM

This seems strange to me, but it's hours past my bed time and I haven't tried reading lambda calculus or lambda-star calculus theory in about 20 years.

Many programming languages have a variant or object type. In C#, any instance of a class will also say that it is of type System.Object. That does nearly make that a type of all types.

There is some nuance and special cases. Like any null is considered a null instance of any nullable object, but you're also not permitted to ask a null value what type it is. It just is a null. Similarly, C# does differentiate between a class and an instance of a class. Both a class and an instance are of a given type, but a class is not an instance of a class.

Presumably the difference is either in one of those nuances, or else in some other axiomatic assertion in the language design that this paper is not making.

Or else I'm very much missing what the author is driving at, which at this time of the morning seems equally possible.

by da_chicken

2/2/2026 at 11:50:12 AM

> Many programming languages have a variant or object type. In C#, any instance of a class will also say that it is of type System.Object. That does nearly make that a type of all types.

That's a type of all values, not a type of all types.

In richer programming languages, you can do operations on types much like you can do operations on values. E.g. in C# you can write "List<String>", but you can't do something like "var x = List; var y = String; x<y>". (Which might seem pointless, but it allows you to do things like write a conversion from x<Int> to x<String> that you can reuse with different x types). So then you need a type for x and y, which is conventionally type, and you have to ask what the type of type is. (And if it's type, then that's very natural and makes some things very easy - but at the cost of making typechecking undecidable, as per the article).

by lmm

2/2/2026 at 6:13:06 AM

> Similarly, C# does differentiate between a class and an instance of a class. Both a class and an instance are of a given type, but a class is not an instance of a class.

Incidentally one of the things I missed from Delphi in C#. In Delphi you can declare

  type TWidgetClass = class of TWidget;

Then you can declare variables of TWidgetClass, and assign TWidget or any subclass to such variables, and call class methods using it, like say the constructor.

  var widgetClass := TTextWidget;
  var myWidget := widgetClass.Create();

After which myWidget holds and instance of TTextWidget. Very handy for factory pattern code, but also other stuff. Of course after C# got generics it alleviated some of this.

by magicalhippo

2/2/2026 at 6:02:53 AM

System.Object is a type of all objects, NOT a type of all types. That is, all objects are System.Object. If one has a class Foo, then every object of type Foo is also of type System.Object. But what is the type of Foo itself? Not of an instance of Foo, but Foo itself. In the languages this paper is about, the type system can recurse upon itself, allowing types themselves to have type. It’s sort of like C#’s System.Type, except System.Type is a class that holds type information for runtime type inspection, whereas the type of types in this paper is for compile-time generics that can abstract over not just types but the types of types as well.

by lalaithion

2/1/2026 at 11:13:59 PM

Type systems aren't magic. They do stop all incorrect programs from running, but also the majority of correct programs too.

by noosphr

2/1/2026 at 11:25:07 PM

Luckily most everyday programs are typeable.

by anon291

2/2/2026 at 2:43:47 AM

And you don't need cut in logic either: https://philpapers.org/rec/BOODEC

It's just that your typeable program might take more data to store than there are bits in the universe.

I'm not saying that types are bad. They aren't.

I'm saying they aren't magic and they come with a trade off.

by noosphr

2/2/2026 at 5:08:12 AM

Types are the closest thing to a free lunch that exists in this fallen world. Yes they're not perfect because nothing is, but their cost/benefit is astonishingly good.

by lmm

2/2/2026 at 5:44:51 AM

A shoe five sizes too small is very comfortable for someone who cut off his toes.

by noosphr

2/2/2026 at 6:17:42 AM

It would be a mistake to conclude that this means shoes are pointless or there's no such thing as a shoe that fits well.

by lmm

2/2/2026 at 7:16:39 AM

Indeed, there are other invariants to type systems that are much better than cutting off your toes.

by noosphr

2/1/2026 at 11:47:58 PM

... for some spectactularly inconsistent and arbitrary definition of "correct program".

by rerdavies

2/2/2026 at 2:18:48 AM

Yes, like giving the correct result for every possible input.

by noosphr

2/1/2026 at 10:15:55 PM

It feels like this is unsurprising, given we already have Goedel's theorems and halting theorems. Any system of self-describing complexity ends up in this territory.

by moomin

2/1/2026 at 11:46:05 PM

Not getting it. Why would you want to do this? And why is no distinction made between `typeof(type)` and `type`? And doesn't the entire problem go away if you distinguish between `typeof(type)`, which is a value whose type is `type`?

by rerdavies

2/2/2026 at 12:55:52 AM

> Why would you want to do this?

It makes life much easier when you want to use fancier types. E.g. if you want to be able to have ListOfLength[4], it's much nicer to be able to use normal 4 which you can use normal arithmetic on (and therefore say that when you append ListOfLength[x] to ListOfLength[y] you get ListOfLength[x + y]), than to have to encode everything in types and make it some kind of ListOfLength[SpecialTypeLevel4] (and then when you append the two lists you get a ListOfLength[TypeLevelAdditionIsCommutative[TypeLevelAdd[x, y]]#Result] or something).

To make that work you have to be able to use values as type parameters, i.e. types, so you have to be able to have e.g. types of type int, as well as types of type type, and it all gets a lot simpler and easier to work with if you just say that types have type type.

> And why is no distinction made between `typeof(type)` and `type`?

Well that's the whole point, to say that type is of type type.

> And doesn't the entire problem go away if you distinguish between `typeof(type)`, which is a value whose type is `type`?

No, because why would you ever use it as a value? The whole point of typeof is that it gives you a type that you can use as a type.

by lmm

2/1/2026 at 9:05:13 PM

Hum... I'm getting 403, forbiden. Is it down?

by marcosdumay

2/1/2026 at 11:51:24 PM

Working for me, but very very slow to load (which I assume is most of the way to 403).

by rerdavies

2/1/2026 at 9:19:53 PM

still working for me

by zem

2/1/2026 at 8:50:11 PM

Similar to the difficulty in finding Title Search companies on Indeed.

by cwmoore

2/2/2026 at 1:07:32 AM

[dead]

by operogg