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Comment by ComNik | Hacker News Reader

Clojure, with its dabbling in schemas and rule systems / logic programming (as a substitute for conditional statements) and thanks to its great tools (like figwheel or devcards), could really be establishing a cheaper, "as-good" alternative to static typing and full formal verification.

Instead of encoding constraints as type signatures, the Clojure folks (true to character) encode them in data. In my eyes a very interesting, pragmatic trade-off between expressiveness and automatic verifiability.

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> Instead of encoding constraints as type signatures, the Clojure folks (true to character) encode them in data.

I've never bought this line of thinking when the "Clojure folks" also say "code is data", because then a syntactic type signature you write down is also just data. If you want to consume the type signature (or spec signature or whatever) as syntax (as a sibling comment suggests), use a macro or any other program which consumes programs.

The thing I do see as being important is not how you write these things down, but whether they have an accessible representation at run/read/compile/whenever time.

Lastly, I strongly disagree these approaches are meant as a replacement for static typing or formal verification because (as far as I can tell) a value can be checked against a spec on demand, but makes zero guarantees as to what that value does in other parts of a program. This is also in contrast to Racket contracts, which will give you the correct blame information between parties using and not using contracts.

You make a very good point, the comparison was flawed there.

> The thing I do see as being important is not how you write these things down, but whether they have an accessible representation at run/read/compile/whenever time.

This is a better way to put it. The second important factor for me is expressiveness. With spec or any other contracts-like system one gets the full power of the language to express constraints. Of course type systems are not artificially restricted in this regard, they simply make a different trade-off.

I hope my comment did not come off as a riff on static vs dynamic typing, and I don't think any contract system is meant to replace type systems. Until expressing all important program specifications formally becomes viable for everyone (maybe through this work https://www.math.ias.edu/vladimir/current_work?), a less-formal, dynamic approach seems very attractive.

I hear you on expressiveness of using the language. Of course (and as you mention) the strength of restricting the expressiveness is that it becomes decidable in some amount of acceptable time for most programs.

I agree with your last sentiment but I don't think it has to be any less "formal" than something like Coq. I just think we shouldn't be so concerned with proving all properties of a program before running. And then we can turn the knob to adjust how much to prove during vs. before running.

> a value can be checked against a spec on demand, but makes zero guarantees as to what that value does in other parts of a program

The spec is just metadata. How it's used is part of the tooling. I see nothing precluding using specs in tools that employ and analyze that information in different ways.

Yep you are totally correct. I only point this out because the tool presented here does not do the same thing as other contract systems, but could (as you say) be used as a building block to get there.

Important to note, that as data, they can be consumed by other systems for other purposes. For example, it's feasible to consume a spec within Alloy and use a SAT solver (which is made even easier given the split on keyset). This pushes spec from design and requirements validation, into runtime constraints, and on through to tests and verification.

> dabbling in schemas and rule systems / logic programming (as a substitute for conditional statements

That sounds interesting. Could you elaborate or post a link?

Quick refs to logic programming and pattern matching in Clojure.

core.logic https://github.com/clojure/core.logic core.match https://github.com/clojure/core.match

I assume @ComNik is referring to core.schema and core.logic[1] here, respectively? (if not, sorry @ComNik)

1: https://clojure.github.io/core.logic/

As a side note in his talk "Simple Made Easy" (https://www.infoq.com/presentations/Simple-Made-Easy, around minute 42) Rich Hickey mentions, that conditional statements are complex, because they spread (business-)logic throughout the program.

As a simpler (in the Hickey-sense) alternative, he lists rule systems and logic programming. For example, keeping parts of the business logic ("What do we consider an 'active' user?", "When do we notify a user?", etc...) as datalog expressions, maybe even storing them in a database, specifies them all in a single place. This helps to ensure consistency throughout the program. One could even give access to these specifications to a client, who can then customise the application directly in logic, instead of chasing throughout the whole code base.

Basically everyone involved agrees on a common language of predicates explicitly, instead of informally in database queries, UI, application code, etc...

But Hickey also notes that this thinking is pretty "cutting-edge" and probably not yet terribly practical.

It can work. My current company uses a rule system to represent most of our business logic since it is so dynamic. The downside is that we have to rebuild the entire graph into memory (times the number of threads, times the number of app servers) every time anything changes (which is constant).

Facebook wrote about rebuilding a similar system in Haskell that only changes memory incrementally, so it's definitely possible to do better.

Interesting note, thank you. Are you referring to "Sigma" https://code.facebook.com/posts/745068642270222/fighting-spa... ?

That's the one.

Cheaper? How so?

Full, formal verification of complex systems (especially in the distributed case) requires a lot of modelling effort, beyond what is needed for "mere" implementation. A lot of important constraints cannot even be expressed in most type systems. And if one uses a specialised modelling language for the proof, the actual implementation might introduce bugs. Also relevant to real, shipping software: specifying a program in such that detail as required for automatic verification makes it even harder to change (which, given infinite time and money, is indeed a very very good property!).

This just goes to say, that a type system can be very helpful, but is ultimately just a part of regular testing.

So for most companies and most developers, anything that aides in keeping documentation up-to-date, writing or generating tests and helping developers understand what they are reading is probably a better ROI.

If static type systems cannot do everything, it does not make sense to me to conclude that they shouldn't then be used for anything. Type systems aren't part of "regular testing", they're part of compile-time error checking.

That was not the intension behind my comment at all. I rely on type systems a lot, myself.

I was trying to note that Clojure, as a dynamic language, is making a (to my eyes) very interesting choice, of doubling down on these dynamic methods of verification. For some uses, I can see this as the better choice, for others it isn't and won't be.

Sure, and I can't fault anyone for wanting less dangerous dynamic languages. But to me it's kind of like wanting less dangerous cigarettes.

When you prove that a language like Clojure (untyped, but with dispatch that is no more dynamic than that of many modern typed languages) combined with a construct like specs is significantly less safe than any typed language, then you can make that statement. It's not like untyped and typed are two binary categories; otherwise, I could say that wanting less dangerous typed languages as opposed to formal tools are just like wanting less dangerous cigarettes. These ideas exist on a spectrum.

As I commented elsewhere, Clojure very deliberately adopts the stance that type systems like Haskell's are valuable but heavyweight, and chose development speed and concision over types.

In my own work, I chose Clojure because it mirrored my experiences: that type systems weren't worth the trade-offs for me and my field.

Would I prefer stronger typing in different domains (longer project times available, greater consequences of failure like medical devices, etc.)? Sure, and I have in the past. But it's not the case that dynamic languages are always the incorrect choice, like smoking cigarettes.

Or like leaving the house without a suit of plate mail.

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