Comment by catnaroek | Hacker News Reader

Comment by catnaroek | Hacker News Reader

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Agreed. These days, the most popular dependently typed languages are based on some variant of Martin-Löf type theory, in which arbitrary mathematical propositions are expressible at the type level. I'm not convinced that these designs offer a good power/cost ratio. For example, these languages often have very limited type inference.

In practice, most of the properties that a programmer (rather than, say, a mathematician using a proof assistant) would want to enforce mechanically (say, invariants of data structures) are expressible as first-order statements (lambda-bound variables are never applied). This strongly suggests that the ergonomics of dependent types could be significantly improved by deliberately limiting their expressive power. By the way, the idea isn't new: the Damas-Milner type system, which is most certainly known and proven technology, is a subset of System F-omega's type system for which type inference is possible, at the reasonable (IMO) price of not having higher-ranked or higher-kinded types. A dependently typed language with a first-order type level would benefit from reusing the basic architecture of a Damas-Milner type checker: a constraint generator and a constraint solver powered by a first-order unification engine.

> By the way, the idea of deliberately limiting the expressive power of types isn't new: the Damas-Milner type system, which is most certainly known and proven technology, is a first-order subset of System F-omega's type system for which type inference is possible. A dependently typed language with a first-order type level would benefit from reusing the basic architecture of a Damas-Milner type checker: a constraint generator and a constraint solver powered by a first-order unification engine.

Have you seen languages like DML (let's you capture linear arithmetic properties only and automates the proof of these)?

https://www.cs.bu.edu/~hwxi/DML/DML.html

> Dependent ML (DML) is a conservative extension of the functional programming language ML. The type system of DML enriches that of ML with a restricted form of dependent types. This allows many interesting program properties such as memory safety and termination to be captured in the type system of DML and then be verified at compiler-time.

What program properties are useful to capture and what properties you can mostly automate is a really interesting problem.

> Have you seen languages like DML (let's you capture linear arithmetic properties only and automates the proof of these)?

Yes. But what I have in mind is a little bit more ambitious. I want dependent types restricted to be parameterized by syntactic values (in the sense of ML's value restriction) of what Standard ML calls `eqtype`s. For example, correctly balanced red-black trees would be:

    (* clearly eqtypes *)
    datatype Color = R' | B'
    datatype Nat = Z | S : Nat -> Nat
    
    datatype Tree =
      | Empty : Tree (B', Z, 'a)
      | R : Tree (B', 'h, 'a) * 'a * Tree (B', 'h, 'a) -> Tree (R', 'h, 'a)
      | B : Tree ('c, 'h, 'a) * 'a * Tree ('c, 'h, 'a) -> Tree (B', S 'h, 'a)

Any examples? I'm curious for examples of a nontrivial type that would catch lots of common programming errors where the proofs can be automated. I see lots of new dependently typed languages but not much interest in addressing the proof automation aspect.

I'd love to give more concrete examples, but I'm slightly hampered by the fact the type system I want isn't actually implemented anywhere. I have a rough sketch of the design of the type system I want, and I've been looking for a computer scientist, logician or mathematician to help me polish the design and prove that it is, in fact, type safe. But I couldn't find anyone. :-|

Why not just learn Coq or Agda yourself? There are also sorts of introductions for simple type systems that I'm sure you can build off of.

I have no idea how to implement fresh unification variable generation (my type system is a superset of Damas-Milner, kind of like how the calculus of constructions is a superset of System F-omega) in Coq or Agda. Everywhere I've seen it implemented, it involves mutable state. So, “in principle”, I could fake it using something like the state monad, and plumbing around the collection of unification variables generated so far, but in practice reasoning about this seems like a pain.

You could take a look at Coquand (2002) “A formalised proof of the soundness and completeness of a simply typed lambda-calculus with explicit substitutions”.

https://www.dropbox.com/s/xmj7yv1i1moe0ag/Coquand2002.pdf

Drop in to #dependent on Freenode IRC, and let’s chat.

You don't need type theory to verify program properties, people have been using first-order methods for decades to prove properties about programs. For example there was a line of work in ACL2 that verified a microprocessor implementation, as well as plenty of modern work using tools like SMT to automatically prove program properties, see Dafny, F* for language based approaches. Though there is plenty of language agnostic approaches as well. My colleagues at UW have a paper in this year's OSDI verifying crash consistency for a realistic file system with no manual proof.

Nice. SMT can be (and indeed has been) integrated into type-based approaches to program verification as well.

Yeah I think SMT is really the state-of-the-art right now in this area. Leo De Moura (one of the main authors of Z3) has been working on Lean for the past couple of years. There is a small group of us (5~) who have been working on a big release for the past couple of months. The goal is to bring the easy of use of SMT automation to type theory, so you can get both the ability to do induction and automation.

Lean: http://leanprover.github.io/

I wish you guys good luck. Freedom from Coq-style proof scripts should be the goal.

If (like me) you didn't know what the abbreviation SMT stands for -- I looked it up and it is Shiver Me Timbers http://encyclopedia.thefreedictionary.com/Shiver+Me+Timbers Seems to be some kind of pirate speak.

Only kidding! It stands for Satisfiability Modulo Theories https://en.wikipedia.org/wiki/Satisfiability_modulo_theories

“In computer science and mathematical logic, the satisfiability modulo theories (SMT) problem is a decision problem for logical formulas with respect to combinations of background theories expressed in classical first-order logic with equality. Examples of theories typically used in computer science are the theory of real numbers, the theory of integers, and the theories of various data structures such as lists, arrays, bit vectors and so on. SMT can be thought of as a form of the constraint satisfaction problem and thus a certain formalized approach to constraint programming.”

I'm not sure I'm any the wiser after that …

A notable building block appears to be SMT-LIB http://smtlib.cs.uiowa.edu/index.shtml

Type inference isn't the most important, most languages that employ type inference are moving towards features sets make full inference hard, if not impossible. Not to mention you can get pretty good "inference" in dependently typed programming languages just by designing an appropriate higher order unification procedure. I have never met a daily user of these tools that find this to be the bottleneck.

In practice you spend way more time writing proofs in these system then doing anything else. There is evidence that we don't need to limit the expressive power of our logics, just design appropriate automation for the logic. Lean, a new ITT based theorem prover from MSR, has been focused on building automation for type theory. It already has promising results that formal methods techniques can be scaled to type theory (see the website for more information on recent work).

> Type inference isn't important, most languages that employ type inference are moving towards features sets make full inference hard, if not impossible.

What I'm saying is that I don't like this direction, because type inference is very important to me.

> Not to mention you can get pretty good "inference" in dependently typed programming languages just by designing an appropriate higher order unification procedure.

Such higher-order unification procedures must always be tailored towards specific use cases, since higher-order unification is in general undecidable. Effectively implementing a different type checking algorithm for every program I write doesn't feel like good use of my time.

What I want is a type system that is more honest about how much it can help me: it only promises to enforce properties expressible as first-order statements, but offers much better inference, and the remainder of my proof obligation is hopefully small enough that I can fulfill it manually.

> In practice you spend way more time writing proofs in these system then doing anything else.

Indeed, and that's what I'd like to fix.

I like your proposal of first-order, dependent types. Shocked I havent heard of it before now given advantages. I hope someone helps you build it.

On related note, what do you think about the Shen LISP approach where Sequent Calculus can be used for advanced typing? Does it have the expressibility and decidability benefits you described? Hadn't seen many specialists talk about it so still an unknown to me. If it does, I figure some default type systems could be created for a specific style of Shen and/or specific libraries. Or something similar done in a ML.

http://www.shenlanguage.org/learn-shen/types/types_sequent_c...

I'm not familiar with Shen, but, if I recall correctly, its type system is Turing-complete, so it's probably more complicated than I'd be comfortable with.

IMO, the topmost priority in a type system designed for programmers ought to be automation, not expressiveness. Programmers don't spend their time proving deep theorems. Most of what a programmer does is actually fairly boring, but it ought to be done correctly, reliably and within a reasonable time frame.

I totally agree. They're not going to use it if we make them work for it. Neither work hard or even lightly. Needs to be no harder than basic annotations but preferably closer to existing, static typing. Let them focus on functions instead of proofs.

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