[ComNik]

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.

[aconz2]

> 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.

[ComNik]

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.

[pron]

> 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.

[ohpauleez]

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.

[llamaz]

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

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

[ComNik]

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.

[mhluongo]

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

[untothebreach]

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/

[pklausler]

Cheaper? How so?

[ComNik]

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.