14 ms·

The Rust borrow checker from a different perspective

173 points by utaal 8y ago

wilkystyle 8y ago
As someone who is not familiar with Rust at all, wow—what refreshingly helpful error messages!
- nnethercote 8y ago
  Yes, the compiler's error messages are extremely good. They often give correct and useful suggestions on how to change code to make it compile. This can be invaluable when dealing with trickier corners of the language such as explicit lifetimes.
- krackers 8y ago
  Can someone chime in as to why C++ compilers in particular are notorious for extremely verbose yet unhelpful error messages? Is it something endemic to parsing the language that there can't be errors as helpful as Rust's?
  - oddity 8y ago
    The short answer, devoid of some interesting nuance, is that C++’s template system is similar to an untyped functional programming language, so the errors you get are the compiler’s attempt to make sense of the large tree of nonsense it couldn’t reduce. C++ also suffers from years of backwards-compatibility twister creating abundant cases of ambiguity if you do something wrong. The compiler will do it’s best, but if your code suddenly parses wildly differently because your semicolon is missing, oh well. Rust is far simpler in that regard, so there are fewer instances where your code might accidentally make sense to the compiler if you have an error.
    - bluejekyll 8y ago
      > Rust is far simpler in that regard, so there are fewer instances where your code might accidentally make sense to the compiler if you have an error. I say this from a place of love for Rust, but the first time you run into a massive generic type tree error thats root cause is a type parameter three levels deep not implementing Send (or some other trait) it takes your breath away. There are some great jokes about this in Deisel and Futures for example. It makes Java generics look like a baby just starting to craw. The error messages are good, you just need to go grab a coffee while you read through the book on type theory that’s been dumped to your screen.
      - marcosdumay 8y ago
        Similar stuff happens on Haskell too. I guess that since the type system is an unityped logic programming language, it is an even worse place than C++ templates. It is a smaller problem on practice only because the type declarations are much shorter programs than C++ templates.
    - fmap 8y ago
      Let me just stress the first point, because that's exactly where Rust and C++ differ: C++ templates really are untyped. There is no way to check whether a template definition is correct. A C++ compiler has to first expand the templates ("complete monomorphisation") and then perform type checking. Any errors you get will be in terms of the template instantiations, not your original code. Unlike C++, Rust has a type system for the full language. In particular, traits are type checked once and you get errors in terms of the code you wrote yourself. There are other compilers that use complete monomorphisation, such as the MLton compiler for Standard ML, where you don't hear horror stories about terrifying error messages because your code was checked before being specialized. I want to stress that this is a terrible design decision in terms of usability, because it is incredibly attractive from an implementation perspective. Parametric types are a delicate issue in an imperative language and always end up rejecting some perfectly fine programs. Monomorphisation both eliminates this issue and potentially leads to more efficient code, but your type errors will suffer. I could say more about this - there are more trade-offs involved - but actually working with template heave C++ code is a better argument than anything I could say. ...That said, the examples in the blog post actually don't use any complicated machinery, and the quality of the compiler errors just comes down to very good engineering on the part of the Rust development team. :)
  - nnethercote 8y ago
    As well as templates, C++ has function overloading, so when a function is named in an error message it has to show all the arguments as well. And if it can't find a function variant that matches a call, it'll print out a "did you mean this one?" message for all of them.
    - masklinn 8y ago
      And of course C++ harks back to an era where error messages were not considered much of a concern. It took the Clang project for C compilation errors to start improving (including kicking gcc's ass), and C is not the most difficult language to parse/validate.
  - twic 8y ago
    I suspect it's the inverse: the Rust compiler team has specifically put a lot of effort into good error messages, whereas for C++ compiler developers it's been a low priority. Some of the inspiration came from Elm [1], which has famously good error messages. [1] https://blog.rust-lang.org/2016/08/10/Shape-of-errors-to-come.html https://blog.rust-lang.org/2016/08/10/Shape-of-errors-to-com...
  - abacate 8y ago
    C++ template system is too generic (Turing complete), and it is usually the one to blame for the verboseness of error messages. Something like Concepts would probably make it more like a proper "generics" mechanism and allow better error messages - ie, "this type does not implement EqualityTrait" instead of "signature mismatch <detailed and very verbose function declaration 1> and <detailed and very verbose function declaration 2>". Despite this, newer g++ versions have better error messages overall - but sometimes you are stuck with an older version of the compiler as well.
i_feel_great 8y ago
How do you get back something owned by something else?
- Munksgaard 8y ago
  You don't, unless they pass it to you (as a return value or function argument).
  - alrz 8y ago
    Or if the borrow doesn't outlive the source e.g. if a local goes out of scope.
- i_feel_great 8y ago
  I don't see that big a deal. In a language, say Ada, I would pass in an "IN" parameter, which cannot be altered in a procedure (and by default in a function). In other languages I would keep the scope/lifetime extremely small, and just not mutate or leak memory. This is just plain ol' good programming practice IMHO. Is there another more compelling use case. In concurrent applications perhaps?
  - oconnor663 8y ago
    Yes, being able to pass ownership of a Vec to another thread is important, because otherwise the two threads have to coordinate over when to call its destructor.
- twic 8y ago
  Return it from a method, or store it into a field of an object you already own.
ammaskartik 8y ago
Great write up, when I started coding in Rust, I sometimes used to get frustrated, but then realised how helpful the checker is and really made me think about structure and mechanics of my code, making me a better developer.
Annatar 8y ago
From that writeup, it seems like one is forced to program Rust in an object oriented manner. Is it possible to program it in a functional manner like Lisp, or a procedural manner like BASIC or assembler?
- zkomp 8y ago
  Just to be clear, rust is not object oriented. You do not use inheritance nor do you have objects. There was a recent post comparing rust to haskell https://www.fpcomplete.com/blog/2018/10/is-rust-functional https://www.fpcomplete.com/blog/2018/10/is-rust-functional. And tldr is yes it lends itself to a functional style...
  - lucozade 8y ago
    > And tldr is yes it lends itself to a functional style Rust does a good job of supporting a functional style efficiently. It's also considered idiomatic to code functionally. Probably the biggest, obvious exception is that Rust embraces mutability because it's statically safe to do so i.e. you aren't as susceptible to the traditional disadvantages of mutability. > You do not use inheritance nor do you have objects That's only partially true though. Rust does support interface inheritance and polymorphism in its trait system. And enums are a closed form of sub-typing. RAII is also idiomatic and I'd argue that many would class that as object-oriented. The way I'd characterise the OO approach in Rust is that the team looked at the experiences of using OO in other languages and picked out what they considered the valuable pieces and left the dangerous ones (like implementation inheritance).
    - masklinn 8y ago
      > That's only partially true though. Rust does support interface inheritance and polymorphism in its trait system. It supports no more than Haskell's typeclasses (rather less in fact), are typeclasses an object system? > And enums are a closed form of sub-typing. Rust enums are pretty standard sum types, a staple of statically typed functional languages. > RAII is also idiomatic and I'd argue that many would class that as object-oriented. That's defensible but setting one hell of a low bar on the concept of object orientation.
bluejekyll 8y ago
I just sent this around to my entire (mainly Java) team. I think it might be the best intro to describe the fundamental memory model that I’ve read. The walk through this basic example, showing interface changes that enforce different aspects of the code, are wonderful. It took me a long time to grok some of these basic concepts, and this explains them so concisely. I might use this as an intro to Rust for anyone who starting to learn the language. Kudos to the author for expressing these concepts in such elegance.
fmap 8y ago
That's a nice way of introducing type state programming in an affine programming language! In general though, type state programming would be even nicer in a linear language. For example, in the http server I could start writing a response without ever writing a body. Rust would happily accept this code, because Rust is always allowed to throw away resources. An affine language controls the duplication of resources, while a linear language controls both duplication and destruction of resources. In a linear Rust, a data structure would have to implement Drop in order to be silently deleted and in the http server "HttpResponseWritingHeaders" should not implement Drop.
- deleted 8y ago
  [deleted]

173 points by utaal 8y ago

wilkystyle 8y ago

As someone who is not familiar with Rust at all, wow—what refreshingly helpful error messages!

nnethercote 8y ago
Yes, the compiler's error messages are extremely good. They often give correct and useful suggestions on how to change code to make it compile. This can be invaluable when dealing with trickier corners of the language such as explicit lifetimes.
krackers 8y ago
Can someone chime in as to why C++ compilers in particular are notorious for extremely verbose yet unhelpful error messages? Is it something endemic to parsing the language that there can't be errors as helpful as Rust's?
- oddity 8y ago
  The short answer, devoid of some interesting nuance, is that C++’s template system is similar to an untyped functional programming language, so the errors you get are the compiler’s attempt to make sense of the large tree of nonsense it couldn’t reduce. C++ also suffers from years of backwards-compatibility twister creating abundant cases of ambiguity if you do something wrong. The compiler will do it’s best, but if your code suddenly parses wildly differently because your semicolon is missing, oh well. Rust is far simpler in that regard, so there are fewer instances where your code might accidentally make sense to the compiler if you have an error.
  - bluejekyll 8y ago
    > Rust is far simpler in that regard, so there are fewer instances where your code might accidentally make sense to the compiler if you have an error. I say this from a place of love for Rust, but the first time you run into a massive generic type tree error thats root cause is a type parameter three levels deep not implementing Send (or some other trait) it takes your breath away. There are some great jokes about this in Deisel and Futures for example. It makes Java generics look like a baby just starting to craw. The error messages are good, you just need to go grab a coffee while you read through the book on type theory that’s been dumped to your screen.
    - marcosdumay 8y ago
      Similar stuff happens on Haskell too. I guess that since the type system is an unityped logic programming language, it is an even worse place than C++ templates. It is a smaller problem on practice only because the type declarations are much shorter programs than C++ templates.
  - fmap 8y ago
    Let me just stress the first point, because that's exactly where Rust and C++ differ: C++ templates really are untyped. There is no way to check whether a template definition is correct. A C++ compiler has to first expand the templates ("complete monomorphisation") and then perform type checking. Any errors you get will be in terms of the template instantiations, not your original code. Unlike C++, Rust has a type system for the full language. In particular, traits are type checked once and you get errors in terms of the code you wrote yourself. There are other compilers that use complete monomorphisation, such as the MLton compiler for Standard ML, where you don't hear horror stories about terrifying error messages because your code was checked before being specialized. I want to stress that this is a terrible design decision in terms of usability, because it is incredibly attractive from an implementation perspective. Parametric types are a delicate issue in an imperative language and always end up rejecting some perfectly fine programs. Monomorphisation both eliminates this issue and potentially leads to more efficient code, but your type errors will suffer. I could say more about this - there are more trade-offs involved - but actually working with template heave C++ code is a better argument than anything I could say. ...That said, the examples in the blog post actually don't use any complicated machinery, and the quality of the compiler errors just comes down to very good engineering on the part of the Rust development team. :)
- nnethercote 8y ago
  As well as templates, C++ has function overloading, so when a function is named in an error message it has to show all the arguments as well. And if it can't find a function variant that matches a call, it'll print out a "did you mean this one?" message for all of them.
  - masklinn 8y ago
    And of course C++ harks back to an era where error messages were not considered much of a concern. It took the Clang project for C compilation errors to start improving (including kicking gcc's ass), and C is not the most difficult language to parse/validate.
- twic 8y ago
  I suspect it's the inverse: the Rust compiler team has specifically put a lot of effort into good error messages, whereas for C++ compiler developers it's been a low priority. Some of the inspiration came from Elm [1], which has famously good error messages. [1] https://blog.rust-lang.org/2016/08/10/Shape-of-errors-to-come.html https://blog.rust-lang.org/2016/08/10/Shape-of-errors-to-com...
- abacate 8y ago
  C++ template system is too generic (Turing complete), and it is usually the one to blame for the verboseness of error messages. Something like Concepts would probably make it more like a proper "generics" mechanism and allow better error messages - ie, "this type does not implement EqualityTrait" instead of "signature mismatch <detailed and very verbose function declaration 1> and <detailed and very verbose function declaration 2>". Despite this, newer g++ versions have better error messages overall - but sometimes you are stuck with an older version of the compiler as well.

i_feel_great 8y ago

How do you get back something owned by something else?

Munksgaard 8y ago
You don't, unless they pass it to you (as a return value or function argument).
- alrz 8y ago
  Or if the borrow doesn't outlive the source e.g. if a local goes out of scope.
i_feel_great 8y ago
I don't see that big a deal. In a language, say Ada, I would pass in an "IN" parameter, which cannot be altered in a procedure (and by default in a function). In other languages I would keep the scope/lifetime extremely small, and just not mutate or leak memory. This is just plain ol' good programming practice IMHO. Is there another more compelling use case. In concurrent applications perhaps?
- oconnor663 8y ago
  Yes, being able to pass ownership of a Vec to another thread is important, because otherwise the two threads have to coordinate over when to call its destructor.
twic 8y ago
Return it from a method, or store it into a field of an object you already own.

ammaskartik 8y ago

Great write up, when I started coding in Rust, I sometimes used to get frustrated, but then realised how helpful the checker is and really made me think about structure and mechanics of my code, making me a better developer.

Annatar 8y ago

From that writeup, it seems like one is forced to program Rust in an object oriented manner. Is it possible to program it in a functional manner like Lisp, or a procedural manner like BASIC or assembler?

zkomp 8y ago
Just to be clear, rust is not object oriented. You do not use inheritance nor do you have objects. There was a recent post comparing rust to haskell https://www.fpcomplete.com/blog/2018/10/is-rust-functional https://www.fpcomplete.com/blog/2018/10/is-rust-functional. And tldr is yes it lends itself to a functional style...
- lucozade 8y ago
  > And tldr is yes it lends itself to a functional style Rust does a good job of supporting a functional style efficiently. It's also considered idiomatic to code functionally. Probably the biggest, obvious exception is that Rust embraces mutability because it's statically safe to do so i.e. you aren't as susceptible to the traditional disadvantages of mutability. > You do not use inheritance nor do you have objects That's only partially true though. Rust does support interface inheritance and polymorphism in its trait system. And enums are a closed form of sub-typing. RAII is also idiomatic and I'd argue that many would class that as object-oriented. The way I'd characterise the OO approach in Rust is that the team looked at the experiences of using OO in other languages and picked out what they considered the valuable pieces and left the dangerous ones (like implementation inheritance).
  - masklinn 8y ago
    > That's only partially true though. Rust does support interface inheritance and polymorphism in its trait system. It supports no more than Haskell's typeclasses (rather less in fact), are typeclasses an object system? > And enums are a closed form of sub-typing. Rust enums are pretty standard sum types, a staple of statically typed functional languages. > RAII is also idiomatic and I'd argue that many would class that as object-oriented. That's defensible but setting one hell of a low bar on the concept of object orientation.

bluejekyll 8y ago

I just sent this around to my entire (mainly Java) team. I think it might be the best intro to describe the fundamental memory model that I’ve read. The walk through this basic example, showing interface changes that enforce different aspects of the code, are wonderful. It took me a long time to grok some of these basic concepts, and this explains them so concisely. I might use this as an intro to Rust for anyone who starting to learn the language. Kudos to the author for expressing these concepts in such elegance.

fmap 8y ago

That's a nice way of introducing type state programming in an affine programming language! In general though, type state programming would be even nicer in a linear language. For example, in the http server I could start writing a response without ever writing a body. Rust would happily accept this code, because Rust is always allowed to throw away resources. An affine language controls the duplication of resources, while a linear language controls both duplication and destruction of resources. In a linear Rust, a data structure would have to implement Drop in order to be silently deleted and in the http server "HttpResponseWritingHeaders" should not implement Drop.

deleted 8y ago
[deleted]