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> But state transitions are pure! Deterministic state machines are nothing more than (obviously, "pure") functions from one program state to the next, and nonde

> But state transitions are pure! Deterministic state machines are nothing more than (obviously, "pure") functions from one program state to the next, and nondeterministic ones are binary relations! Okay, yes, but 99% of mathematics isn't deterministic state machines. So again, pure functions can model a lot of things, but deterministic state machines are foreign to the way people think about mathematics in most instances. > For example, look at the pure specification language (and proof assistant) TLA+. It is far simpler than Coq, just as pure, and requires no more than highschool math. Okay, that's completely fine as an example. I just mentioned Coq because it's one of the better known ones. > Coq and other PFP formalizations are built on the core idea of denotational semantics, an approximation which at times only complicates the very simple math of computation, requiring exotic math like category theory. I'm not sure where CiC (or any of the other recent type theories, including HOTT) requires category theory (which is hardly exotic in 2016). People who are doing meta-mathematics on these systems are interested in categorical models and such, but implementing and using these systems requires no category theory. Categories are not fundamental objects in these theories; you have to build them just like you do in material set theories, although some parts of that process are easier (especially in HOTT). But that is the reverse of the link that you're claiming. > The manipulation of formulas and proofs fits very well with the imperative approach, only no category theory is required, and core concepts such as simulation/trace-inclusion arise naturally without need for complex embeddings. Again, no category theory is required (though I'm still not sure why this is a bad thing?) to develop a prover like Coq or TLA+. If you're bringing simulation and trace-inclusion into this, then you're just saying the stateful, imperative approach is well adapted to working with stateful, imperative systems. I agree, but how exactly does that equate to that approach having any benefit whatsoever for formalizing the rest of mathematics?

pron 10y ago
> Okay, yes, but 99% of mathematics isn't deterministic state machines. So again, pure functions can model a lot of things, but deterministic state machines are foreign to the way people think about mathematics in most instances. Sorry, I wasn't clear. 99% of mathematics isn't Schrödinger's equation either, but Schrödinger's equation is still relatively simple math. State machines are simple math, but math isn't state machines. State machines are the concept that underlies computation, and simple math is used to reason about them. > I just mentioned Coq because it's one of the better known ones. ... and one of the least used ones, at least where software is concerned. There's a reason for that: it's very hard (let alone for engineers) to reason about computer programs in Coq; it's much easier (and done by engineers) to reason about computer programs in TLA+ (or SPIN or B-Method or Alloy). > which is hardly exotic in 2016 I think it's safe to say that most mathematicians in 2016 -- let alone software engineers -- are pretty unfamiliar with category theory, and have hardly heard of type theory. Engineers, however, already have nearly all the math they need to reason about programs and algorithm in the common mathematical way (they just may not know it, which is why I so obnoxiously bring it up whenever I can, to offset the notion that is way overrepresented here on HN that believes that "PFP is the way to mathematically reason about programs". It is just one way, not even the most common one, and certainly not the easiest one). > but implementing and using these systems requires no category theory. Right, but we're talking about foundational principles of computation. Those systems are predicated on denotational semantics, which is a formalization that identifies a computation with the function it computes (yes, some of those systems also have definitional equality, but still, denotational semantics is the core principle), rather than view the computation as built up from functions (in fact, this is precisely what monads do and why they're needed, as the basic denotational semantics fails to capture many important computations). This formalization isn't any better or worse (each could be defined in terms of the other), but it is more complicated, and is unnecessary to mathematically reason about programs. It does require CT concepts like monads to precisely denote certain computations. > If you're bringing simulation and trace-inclusion into this, then you're just saying the stateful, imperative approach is well adapted to working with stateful, imperative systems. There are no "imperative systems". Imperative/functional is a feature of the language used to describe a computation, not the computation itself (although, colloquially we say functional/imperative algorithms to refer to those algorithms that commonly arise when using the different linguistic approaches). The algorithm is always a state machine (assuming no textual deconstructionism) -- whether expressed in a language like Haskell or in a language like BASIC -- and that algorithm can be reasoned about with pretty basic math. And I am not talking about a "stateful" approach, but a basic mathematical approach based on state machines (a non-stateful pure functional program also ultimately defines a state machine). > I agree, but how exactly does that equate to that approach having any benefit whatsoever for formalizing the rest of mathematics? Oh, I wasn't talking about a new way to formalize the foundation of mathematics (which, I've been told, is the goal of type theory), nor do I think that a new foundation for math is required to mathematically reason about computation (just as it isn't necessary to reason about physics). I just pointed out that algorithms have a very elegant mathematical formulation in "simple" math, which is unrelated to PFP. This formulation serves as the basis for most formal reasoning of computer programs.
- jroesch 10y ago
  I feel like you have confused quite a few concepts here. First, type theory based proof assistants (like Coq) and model checkers are very different tools with very different guarantees. Most model checkers are used for proving properties about finite models and can not reason about infinite structures. A type theory with inductive types gives one a mechanism for constructing inductive structures, and the ability to write proofs about them. I am not sure how you have equated denotational semantics and type theory but there is no inherent connection, denotational semantics are just one way to give a language semantics. One can use them to describe the computational behavior of your type theory, but they are not fundamental. Category theory and type theory have a cool isomorphism between them, but otherwise can be completely silo'd you can be quite proficient in type theory and never touch or understand any category theory at all. On the subject of TLA+, Leslie Lamport loves to talk about "ordinary mathematics" but he just chose a different foundational mathematics to build his tool with, type theory is an alternative formulation in this regard. One that is superior in some ways since the language for proof, and programming is one and the same and does not require strange stratification or layering of specification and implementation languages. Another issue with many of these model based tools is the so called "formality gap" building a clean model of your program and then proving properties is nice, but without a connection to your implementation the exercise has questionable value. Sure, with distributed systems for example, writing out a model of your protocol can help find design bugs, but it will not stop you from incorrectly implementing said protocol. In the distributed systems even with testing, finding safety violations in practice is hard, and many of them can occur silently. Proof assistants like Coq make doing this easier since your implementation and proof live side by side, and you can reason directly about your implementation instead of a model. If you don't like dependently typed functional languages you can check out tools like Dafny which provide a similar work style, but with more automation and imperative programming constructs. > This formulation serves as the basis for most formal reasoning of computer programs. On this statement I'm not sure what community you come from, but much of the work going on in the research community is using things like SMT which exposes SAT and a flavor of first order logic, an HOL based system like Isabelle, or type theory, very few people use tools like set theory to reason about programs. > Engineers, however, already have nearly all the math they need to reason about programs and algorithm in the common mathematical way (they just may not know it, which is why I so obnoxiously bring it up whenever I can, to offset the notion that is way overrepresented here on HN that believes that "PFP is the way to mathematically reason about programs". Finally this statement is just plain not true, abstractly its easy to hand way on paper about the correctness of your algorithm. I encourage you to show me the average engineer who can pick up a program and prove non-trivial properties about its implementation, even on paper. I wager even proving the implementation of merge sort correct would prove too much. I've spent the last year implementing real, low-level systems using type theory, and this stuff is hard if you can show me a silver bullet I would be ecstatic, but any approach with as much power and flexibility is at least as hard to use. Its not that "PFP" (and its not PFP, its type theory that makes this possible) is the "right way" to reason about programs but that it makes it possible to reason about programs. For example, how do you prove a loop invariant in Python? how would you even start? I know of a few ways to do this, but most provide a weaker guarantee then you would type theory version would, and requires a large trusted computing base.

pron 10y ago

> Okay, yes, but 99% of mathematics isn't deterministic state machines. So again, pure functions can model a lot of things, but deterministic state machines are foreign to the way people think about mathematics in most instances. Sorry, I wasn't clear. 99% of mathematics isn't Schrödinger's equation either, but Schrödinger's equation is still relatively simple math. State machines are simple math, but math isn't state machines. State machines are the concept that underlies computation, and simple math is used to reason about them. > I just mentioned Coq because it's one of the better known ones. ... and one of the least used ones, at least where software is concerned. There's a reason for that: it's very hard (let alone for engineers) to reason about computer programs in Coq; it's much easier (and done by engineers) to reason about computer programs in TLA+ (or SPIN or B-Method or Alloy). > which is hardly exotic in 2016 I think it's safe to say that most mathematicians in 2016 -- let alone software engineers -- are pretty unfamiliar with category theory, and have hardly heard of type theory. Engineers, however, already have nearly all the math they need to reason about programs and algorithm in the common mathematical way (they just may not know it, which is why I so obnoxiously bring it up whenever I can, to offset the notion that is way overrepresented here on HN that believes that "PFP is the way to mathematically reason about programs". It is just one way, not even the most common one, and certainly not the easiest one). > but implementing and using these systems requires no category theory. Right, but we're talking about foundational principles of computation. Those systems are predicated on denotational semantics, which is a formalization that identifies a computation with the function it computes (yes, some of those systems also have definitional equality, but still, denotational semantics is the core principle), rather than view the computation as built up from functions (in fact, this is precisely what monads do and why they're needed, as the basic denotational semantics fails to capture many important computations). This formalization isn't any better or worse (each could be defined in terms of the other), but it is more complicated, and is unnecessary to mathematically reason about programs. It does require CT concepts like monads to precisely denote certain computations. > If you're bringing simulation and trace-inclusion into this, then you're just saying the stateful, imperative approach is well adapted to working with stateful, imperative systems. There are no "imperative systems". Imperative/functional is a feature of the language used to describe a computation, not the computation itself (although, colloquially we say functional/imperative algorithms to refer to those algorithms that commonly arise when using the different linguistic approaches). The algorithm is always a state machine (assuming no textual deconstructionism) -- whether expressed in a language like Haskell or in a language like BASIC -- and that algorithm can be reasoned about with pretty basic math. And I am not talking about a "stateful" approach, but a basic mathematical approach based on state machines (a non-stateful pure functional program also ultimately defines a state machine). > I agree, but how exactly does that equate to that approach having any benefit whatsoever for formalizing the rest of mathematics? Oh, I wasn't talking about a new way to formalize the foundation of mathematics (which, I've been told, is the goal of type theory), nor do I think that a new foundation for math is required to mathematically reason about computation (just as it isn't necessary to reason about physics). I just pointed out that algorithms have a very elegant mathematical formulation in "simple" math, which is unrelated to PFP. This formulation serves as the basis for most formal reasoning of computer programs.

jroesch 10y ago
I feel like you have confused quite a few concepts here. First, type theory based proof assistants (like Coq) and model checkers are very different tools with very different guarantees. Most model checkers are used for proving properties about finite models and can not reason about infinite structures. A type theory with inductive types gives one a mechanism for constructing inductive structures, and the ability to write proofs about them. I am not sure how you have equated denotational semantics and type theory but there is no inherent connection, denotational semantics are just one way to give a language semantics. One can use them to describe the computational behavior of your type theory, but they are not fundamental. Category theory and type theory have a cool isomorphism between them, but otherwise can be completely silo'd you can be quite proficient in type theory and never touch or understand any category theory at all. On the subject of TLA+, Leslie Lamport loves to talk about "ordinary mathematics" but he just chose a different foundational mathematics to build his tool with, type theory is an alternative formulation in this regard. One that is superior in some ways since the language for proof, and programming is one and the same and does not require strange stratification or layering of specification and implementation languages. Another issue with many of these model based tools is the so called "formality gap" building a clean model of your program and then proving properties is nice, but without a connection to your implementation the exercise has questionable value. Sure, with distributed systems for example, writing out a model of your protocol can help find design bugs, but it will not stop you from incorrectly implementing said protocol. In the distributed systems even with testing, finding safety violations in practice is hard, and many of them can occur silently. Proof assistants like Coq make doing this easier since your implementation and proof live side by side, and you can reason directly about your implementation instead of a model. If you don't like dependently typed functional languages you can check out tools like Dafny which provide a similar work style, but with more automation and imperative programming constructs. > This formulation serves as the basis for most formal reasoning of computer programs. On this statement I'm not sure what community you come from, but much of the work going on in the research community is using things like SMT which exposes SAT and a flavor of first order logic, an HOL based system like Isabelle, or type theory, very few people use tools like set theory to reason about programs. > Engineers, however, already have nearly all the math they need to reason about programs and algorithm in the common mathematical way (they just may not know it, which is why I so obnoxiously bring it up whenever I can, to offset the notion that is way overrepresented here on HN that believes that "PFP is the way to mathematically reason about programs". Finally this statement is just plain not true, abstractly its easy to hand way on paper about the correctness of your algorithm. I encourage you to show me the average engineer who can pick up a program and prove non-trivial properties about its implementation, even on paper. I wager even proving the implementation of merge sort correct would prove too much. I've spent the last year implementing real, low-level systems using type theory, and this stuff is hard if you can show me a silver bullet I would be ecstatic, but any approach with as much power and flexibility is at least as hard to use. Its not that "PFP" (and its not PFP, its type theory that makes this possible) is the "right way" to reason about programs but that it makes it possible to reason about programs. For example, how do you prove a loop invariant in Python? how would you even start? I know of a few ways to do this, but most provide a weaker guarantee then you would type theory version would, and requires a large trusted computing base.