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Actually no, not in programming language theory. What researchers call 'type system' is much more precise than what programmers call 'type system'. In the PL fi

Actually no, not in programming language theory. What researchers call 'type system' is much more precise than what programmers call 'type system'. In the PL field, the behavior of a term is traditionally split into static behavior (described by types) and dynamic behavior (described by reduction/evaluation). What programmers call 'dynamic type systems' is just a way to instrument the runtime with dynamic data-shape checks. Note that most dynamic languages do have a non-trivial type system, where functions are typed with their number of arguments (eg static scoping and other structure-related static checks). Languages like bash otoh do really have a trivial type system, whith every term merrily going into the evaluator without any static/typing well-formedness analyzis. You might also be interested in gradual typing, which is about deriving systematic dynamic checks from a type system and then augmenting a given type system with an 'any' type to go into untyped (ie dynamically checked) mode.

lisper 4y ago
> dynamic data-shape checks What is the difference between a "data-shape check" and a type system?
- lapinot 4y ago
  I'm assuming you why want me to explain what the difference between a static data-shape check and a type system is (since i'm already saying type systems are about static information). For sure there is a non-empty intersection in some way, but the two are differently structured. For one 'data' is a runtime concept (some programs might be data, but some others are computations). So already a data-shape check isn't always doable statically like you would at runtime. And the other way around, some information might be statically available but not materialized in any kind of data shape, like type parameters for polymorphic functions, lifetimes in rust or in general irrelevant proof-terms in dependently-typed languages. Perhaps you would be satisfied if i said that type-systems are a shape-check for terms and not for data.
  - lisper 4y ago
    No, that would not satisfy me. It's not that I don't understand the concept you are trying to convey, or even that I disagree with it. What I object to is the co-opting of the word "type" to mean something other than it means in common usage. I understand that "type system" is a term of art within a certain research community, but this is not that community. HN is much broader than that, and within the context of that audience, "type system" already has an established meaning that encompasses things that happen both at compile time and at run time, and which applies to both terms and data [1]. > 'data' is a runtime concept (some programs might be data, but some others are computations) And how do you distinguish between runtime and non-runtime? It's true that most languages have a distinction between compile-time and run-time but this is purely artificial, a design choice, not a reflection of any deep underlying mathematical truth. Common Lisp notably does not have a sharp distinction between the two. Compilation can trigger macro-expansion, which can in turn trigger arbitrary computations. This process can even loop back on itself: the computations triggered by a macro expansion can themselves invoke the compiler, and can even define new macros. So when you say "Common Lisp does not have a type system" what you really mean is "the structure of Common Lisp is different from what is currently being studied by a group of researchers who have chosen to call the object of their study 'type theory'." The problem with phrasing it the way that you do is that it implies that Common Lisp is lacking something (a type system, according to your narrow definition of what a type system is). It isn't. Common Lisp isn't lacking anything, it is just structured differently. But this is a feature, not a bug, because if you want to avail yourself of the technology of what in academic circles is called "type systems" in Common Lisp you very easily can. But you don't have to. In fact it is so-called "strongly typed" language that are lacking something, namely, the freedom to write code that does not conform to the constraints of the "type system". [1] https://en.wikipedia.org/wiki/Type_system https://en.wikipedia.org/wiki/Type_system
    - lapinot 4y ago
      :) i was waiting for you to come all lisp on me! (eh i just realized your pseudo) [sorry for the long msg, i like this topic!] But this is just all staging! I would never say that lisp is not typed! Only perhaps streaming lisp interpreters (perhaps like lisp machines, don't know them very well) could be said truly untyped (actually: trivially statically typed). Dynamic vs static is more precisely seen as a sequentiality between stages, and you can have several stacked stages and even looping (as in lisp). But the distinction (in lisp still) between something that is quoted (a next-stage computation encapsulated into what is currently data) and current-stage computation is pretty clear. Some function bodies eval quoted stuff and some build quoted computations and you can statically see that: you can type it (using a quoted vs normal modality on top of any type system you already have). We could also talk about erlang. In fact the trend is that for every convoluted language semantic that has traditionally been seen as dynamic/permissive/untyped, the research oriented world has come up with type systems explaining these languages. I talked about gradual typing earlier in the thread. There is static typing for memory state, complexity constraints, timing, validating interface checks, for well-typed macros and syntax reflection. Typing in some way is just structuring the mathematical semantic of your language. It can always be done, even after the fact, usually in tons of different ways. I see where you come from, but still, actually i do believe there is a "deep underlying mathematical truth" in the sense that this sharp definition (typing is static) will help you to understand things because it's a deep concept. And as any deep concepts usually when you think you hit it's limit, you understand that you can generalize it into something that is more abstract but still has the core properties. The concept is still there, just said in more precise terms (more precise, because you're taking more parameters and leaving more irrelevant choices open in the concept, hence more abstract). > In fact it is so-called "strongly typed" language that are lacking something, namely, the freedom to write code that does not conform to the constraints of the "type system". This is wrong if by "strongly typed" you mean the term of art: you'd have to be willing to go out of your way to write a compiler for a language without any appearance stuff that could be qualified static typing. Static scoping and dependency analysis is a strong type system where types are the number/name of free variables. Compilation of call-conventions, memory layout and memory allocation interfaces can be done at scale with type systems (the llvm one is well-known, you'd be quite in the minority saying that llvm constrains you not to write your "untypable" self-mutating program). For most part, in practice, type systems are not a feature of the language but of the compiler, as for tons of cool systems, types can be infered (by a complete deciding procedure). Static type systems are just the fact that you will annotate programs with some local invariants, whatever they might be, to ease manipulating and reasoning on some form of term representation (AST, call-graph, ..).
Mikeb85 4y ago
> most dynamic languages do have a non-trivial type system Lol

lisper 4y ago

> dynamic data-shape checks What is the difference between a "data-shape check" and a type system?

lapinot 4y ago
I'm assuming you why want me to explain what the difference between a static data-shape check and a type system is (since i'm already saying type systems are about static information). For sure there is a non-empty intersection in some way, but the two are differently structured. For one 'data' is a runtime concept (some programs might be data, but some others are computations). So already a data-shape check isn't always doable statically like you would at runtime. And the other way around, some information might be statically available but not materialized in any kind of data shape, like type parameters for polymorphic functions, lifetimes in rust or in general irrelevant proof-terms in dependently-typed languages. Perhaps you would be satisfied if i said that type-systems are a shape-check for terms and not for data.
- lisper 4y ago
  No, that would not satisfy me. It's not that I don't understand the concept you are trying to convey, or even that I disagree with it. What I object to is the co-opting of the word "type" to mean something other than it means in common usage. I understand that "type system" is a term of art within a certain research community, but this is not that community. HN is much broader than that, and within the context of that audience, "type system" already has an established meaning that encompasses things that happen both at compile time and at run time, and which applies to both terms and data [1]. > 'data' is a runtime concept (some programs might be data, but some others are computations) And how do you distinguish between runtime and non-runtime? It's true that most languages have a distinction between compile-time and run-time but this is purely artificial, a design choice, not a reflection of any deep underlying mathematical truth. Common Lisp notably does not have a sharp distinction between the two. Compilation can trigger macro-expansion, which can in turn trigger arbitrary computations. This process can even loop back on itself: the computations triggered by a macro expansion can themselves invoke the compiler, and can even define new macros. So when you say "Common Lisp does not have a type system" what you really mean is "the structure of Common Lisp is different from what is currently being studied by a group of researchers who have chosen to call the object of their study 'type theory'." The problem with phrasing it the way that you do is that it implies that Common Lisp is lacking something (a type system, according to your narrow definition of what a type system is). It isn't. Common Lisp isn't lacking anything, it is just structured differently. But this is a feature, not a bug, because if you want to avail yourself of the technology of what in academic circles is called "type systems" in Common Lisp you very easily can. But you don't have to. In fact it is so-called "strongly typed" language that are lacking something, namely, the freedom to write code that does not conform to the constraints of the "type system". [1] https://en.wikipedia.org/wiki/Type_system https://en.wikipedia.org/wiki/Type_system
  - lapinot 4y ago
    :) i was waiting for you to come all lisp on me! (eh i just realized your pseudo) [sorry for the long msg, i like this topic!] But this is just all staging! I would never say that lisp is not typed! Only perhaps streaming lisp interpreters (perhaps like lisp machines, don't know them very well) could be said truly untyped (actually: trivially statically typed). Dynamic vs static is more precisely seen as a sequentiality between stages, and you can have several stacked stages and even looping (as in lisp). But the distinction (in lisp still) between something that is quoted (a next-stage computation encapsulated into what is currently data) and current-stage computation is pretty clear. Some function bodies eval quoted stuff and some build quoted computations and you can statically see that: you can type it (using a quoted vs normal modality on top of any type system you already have). We could also talk about erlang. In fact the trend is that for every convoluted language semantic that has traditionally been seen as dynamic/permissive/untyped, the research oriented world has come up with type systems explaining these languages. I talked about gradual typing earlier in the thread. There is static typing for memory state, complexity constraints, timing, validating interface checks, for well-typed macros and syntax reflection. Typing in some way is just structuring the mathematical semantic of your language. It can always be done, even after the fact, usually in tons of different ways. I see where you come from, but still, actually i do believe there is a "deep underlying mathematical truth" in the sense that this sharp definition (typing is static) will help you to understand things because it's a deep concept. And as any deep concepts usually when you think you hit it's limit, you understand that you can generalize it into something that is more abstract but still has the core properties. The concept is still there, just said in more precise terms (more precise, because you're taking more parameters and leaving more irrelevant choices open in the concept, hence more abstract). > In fact it is so-called "strongly typed" language that are lacking something, namely, the freedom to write code that does not conform to the constraints of the "type system". This is wrong if by "strongly typed" you mean the term of art: you'd have to be willing to go out of your way to write a compiler for a language without any appearance stuff that could be qualified static typing. Static scoping and dependency analysis is a strong type system where types are the number/name of free variables. Compilation of call-conventions, memory layout and memory allocation interfaces can be done at scale with type systems (the llvm one is well-known, you'd be quite in the minority saying that llvm constrains you not to write your "untypable" self-mutating program). For most part, in practice, type systems are not a feature of the language but of the compiler, as for tons of cool systems, types can be infered (by a complete deciding procedure). Static type systems are just the fact that you will annotate programs with some local invariants, whatever they might be, to ease manipulating and reasoning on some form of term representation (AST, call-graph, ..).

Mikeb85 4y ago

> most dynamic languages do have a non-trivial type system Lol