4 ms·

This 'flow typing' / TypeGuard approach is really awesome and probably the best part about languages like typescript/mypy. I wish that other languages would fol

This 'flow typing' / TypeGuard approach is really awesome and probably the best part about languages like typescript/mypy. I wish that other languages would follow suit so that I can incrementally narrow the types in my program through conditionals.

leni536 4y ago
While it is cool and useful, it is unsound. https://mypy-play.net/?mypy=latest&python=3.10&flags=strict&gist=6e2532267d6ecca04896c1223473b3f9 https://mypy-play.net/?mypy=latest&python=3.10&flags=strict&... https://www.online-python.com/V5YRZJAWlk https://www.online-python.com/V5YRZJAWlk
- staticassertion 4y ago
  Nice, that's a cool example. But that's not fundamental to the concept of flow typing, it's just a mypy limitation.
  - planede 4y ago
    And a typescript limitation? https://www.typescriptlang.org/play?#code/MYGwhgzhAEDC0G8BQ1XQB4C5oDsCuAtgEYCmATtAD654gjQC80ADANxIC+SSAZnjsAAuASwD2OaD1GiAFMGywAlNgBuo4QBNEKNMAB06RjTrsuvfkLESiYMnIXKaxctrTRhPaHIOMGTfHSKrm5oZCSCeGQSbDqoXG5SssCK7G5hEVHQ+uim3CDhGNj4zhRMNnY4JADucDKKKdzA4hCi+XogogDmMugpQA https://www.typescriptlang.org/play?#code/MYGwhgzhAEDC0G8BQ1...
    - staticassertion 4y ago
      Yes, it seems that the issue is essentially one of binding the type to a scope but then mutating within that scope. The type system could just "look" for those mutations, or the language could add mutability to the type system.
      - planede 4y ago
        > The type system could just "look" for those mutations That's a very big "just". Typescript and mypy don't look through function call boundaries, doing so would open a lot of problems, I believe. > or the language could add mutability to the type system. I agree there, the core issue is that the call to `foo` leaks a an object where the member is rebindable through assignment, but the "flow typing" assumes that doesn't happen. Ideally such a call should be a barrier for flow typing, unless there was a way to specify functions that don't modify their parameter this way (readonly parameters?). A similar problem but with a global instead: https://www.typescriptlang.org/play?#code/DYUwLgBAlhBcEDsCuBbARiAThAPopwwEAvBACwBMA3AFA0BmSCAxmFAPYIT3vsAUASngA3dlAAmEAN40Ic6CXyFaAXzqMWbThDQBDTIPjJ0WabPlR6EPjGJ2lwAWfkuImcEkxcADLRdqXHn4BP3l3ME8uKFU6UEgADyNUDGxSPQMQumZOAGd2UAA6YHYAcz54kKA https://www.typescriptlang.org/play?#code/DYUwLgBAlhBcEDsCuB... This should be handled differently. Every function should be assumed to modify globals, unless they can be marked to be pure.
        staticassertion 4y ago
        Yeah, that is definitely a big "just" - it's basically whole program type inference, which I expect to be very messy in Python, if not intractable. > unless there was a way to specify functions that don't modify their parameter this way (readonly parameters?). Right, so in a language with typed mutability (ie: Rust) or total immutability flow typing would be a lot easier to implement without these footguns. Python is such an insane language, I'm not surprised to see that you can subvert the type checker like this.
- IshKebab 4y ago
  In fairness, Dart took the fully-sound approach and while it is clearly better, it does make dealing with class member variables a massive pain for similar reasons. Also clearly a bit of unsoundness is way better than just no type annotations at all, which is the alternative.
  - planede 4y ago
    Interesting. It looks like it just doesn't allow narrowing types of class members and globals, sidestepping this whole issue. I like the Dart approach here. Users of the language can always bind a local variable to the thing you want a narrower type for, it's not the big of a hassle.
    - IshKebab 4y ago
      It is quite a hassle when every method looks like void foo() { var local_a = this.a; var local_b = this.b; ... this.a = local_a; this.b = local_b; } But I'd say it's still worth it for the soundness. And I have confidence they'll come up with some improvement. Maybe a way to opt out of setters/getters.

leni536 4y ago

While it is cool and useful, it is unsound. https://mypy-play.net/?mypy=latest&python=3.10&flags=strict&gist=6e2532267d6ecca04896c1223473b3f9 https://mypy-play.net/?mypy=latest&python=3.10&flags=strict&... https://www.online-python.com/V5YRZJAWlk https://www.online-python.com/V5YRZJAWlk

staticassertion 4y ago
Nice, that's a cool example. But that's not fundamental to the concept of flow typing, it's just a mypy limitation.
- planede 4y ago
  And a typescript limitation? https://www.typescriptlang.org/play?#code/MYGwhgzhAEDC0G8BQ1XQB4C5oDsCuAtgEYCmATtAD654gjQC80ADANxIC+SSAZnjsAAuASwD2OaD1GiAFMGywAlNgBuo4QBNEKNMAB06RjTrsuvfkLESiYMnIXKaxctrTRhPaHIOMGTfHSKrm5oZCSCeGQSbDqoXG5SssCK7G5hEVHQ+uim3CDhGNj4zhRMNnY4JADucDKKKdzA4hCi+XogogDmMugpQA https://www.typescriptlang.org/play?#code/MYGwhgzhAEDC0G8BQ1...
  - staticassertion 4y ago
    Yes, it seems that the issue is essentially one of binding the type to a scope but then mutating within that scope. The type system could just "look" for those mutations, or the language could add mutability to the type system.
    - planede 4y ago
      > The type system could just "look" for those mutations That's a very big "just". Typescript and mypy don't look through function call boundaries, doing so would open a lot of problems, I believe. > or the language could add mutability to the type system. I agree there, the core issue is that the call to `foo` leaks a an object where the member is rebindable through assignment, but the "flow typing" assumes that doesn't happen. Ideally such a call should be a barrier for flow typing, unless there was a way to specify functions that don't modify their parameter this way (readonly parameters?). A similar problem but with a global instead: https://www.typescriptlang.org/play?#code/DYUwLgBAlhBcEDsCuBbARiAThAPopwwEAvBACwBMA3AFA0BmSCAxmFAPYIT3vsAUASngA3dlAAmEAN40Ic6CXyFaAXzqMWbThDQBDTIPjJ0WabPlR6EPjGJ2lwAWfkuImcEkxcADLRdqXHn4BP3l3ME8uKFU6UEgADyNUDGxSPQMQumZOAGd2UAA6YHYAcz54kKA https://www.typescriptlang.org/play?#code/DYUwLgBAlhBcEDsCuB... This should be handled differently. Every function should be assumed to modify globals, unless they can be marked to be pure.
      - staticassertion 4y ago
        Yeah, that is definitely a big "just" - it's basically whole program type inference, which I expect to be very messy in Python, if not intractable. > unless there was a way to specify functions that don't modify their parameter this way (readonly parameters?). Right, so in a language with typed mutability (ie: Rust) or total immutability flow typing would be a lot easier to implement without these footguns. Python is such an insane language, I'm not surprised to see that you can subvert the type checker like this.
IshKebab 4y ago
In fairness, Dart took the fully-sound approach and while it is clearly better, it does make dealing with class member variables a massive pain for similar reasons. Also clearly a bit of unsoundness is way better than just no type annotations at all, which is the alternative.
- planede 4y ago
  Interesting. It looks like it just doesn't allow narrowing types of class members and globals, sidestepping this whole issue. I like the Dart approach here. Users of the language can always bind a local variable to the thing you want a narrower type for, it's not the big of a hassle.
  - IshKebab 4y ago
    It is quite a hassle when every method looks like void foo() { var local_a = this.a; var local_b = this.b; ... this.a = local_a; this.b = local_b; } But I'd say it's still worth it for the soundness. And I have confidence they'll come up with some improvement. Maybe a way to opt out of setters/getters.