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But I have an impression that Common Lisp doesn't have a type system, whereas this languages strives to have all "levels" of it's metaprogramming typed, so it's

But I have an impression that Common Lisp doesn't have a type system, whereas this languages strives to have all "levels" of it's metaprogramming typed, so it's not like this language doesn't have any novelty.

lisper 4y ago
Common Lisp has a very sophisticated type system. The only thing it doesn't have (among things that are currently fashionable) is compile-time typing by default. Peridot is "novel" in that it introduces user-level compiler optimizations into a language that does have compile-time typing by default. But that's kind of like Chevrolet making a plug-in hybrid version of the C8 Corvette (which they recently announced they are going to do). Neither the C8 nor plug-in hybrids are new. A plug-in hybrid C8 will be new, and when it happens it will be a Big Deal to a niche market: the intersection of C8 fans and plug-in-hybrid fans. But it won't change the world. (For the record, I happen to be a member of the niche market to which a plug-in hybrid C8 will appeal, so I am very excited about it. So I get that some people may be excited about Peridot. I just think it's important to keep things in perspective.)
Mikeb85 4y ago
Having dynamic typing != no type system.
- lapinot 4y ago
  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

lisper 4y ago

Common Lisp has a very sophisticated type system. The only thing it doesn't have (among things that are currently fashionable) is compile-time typing by default. Peridot is "novel" in that it introduces user-level compiler optimizations into a language that does have compile-time typing by default. But that's kind of like Chevrolet making a plug-in hybrid version of the C8 Corvette (which they recently announced they are going to do). Neither the C8 nor plug-in hybrids are new. A plug-in hybrid C8 will be new, and when it happens it will be a Big Deal to a niche market: the intersection of C8 fans and plug-in-hybrid fans. But it won't change the world. (For the record, I happen to be a member of the niche market to which a plug-in hybrid C8 will appeal, so I am very excited about it. So I get that some people may be excited about Peridot. I just think it's important to keep things in perspective.)

Mikeb85 4y ago

Having dynamic typing != no type system.

lapinot 4y ago
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