11 ms·

Functional Language Features in Rust – Iterators and Closures

72 points by nanxor 10y ago

computerphage 10y ago
For context: there's a rewrite of the rust book in progress for a while now. This is a chapter of that book.
- steveklabnik 10y ago
  Yes. Co-author here, happy to answer any questions or hear feedback on the text, as usual. Incidentally, I just finished the build system stuff today to get the new book shipped on doc.rust-lang.org; that slates it to land in Rust 1.18. It will still be a draft, but this is one more step towards it being finished.
theossuary 10y ago
Please don't take over my Cmd+Left arrow key for going back, Chrome already took away my backspace :(
- steveklabnik 10y ago
  This is an upstream issue with https://github.com/azerupi/mdBook https://github.com/azerupi/mdBook, you should file a bug!
  - killercup 10y ago
    Couldn't find an open issue, but agreed that navigation should not be triggered when a modifier key is pressed, so I opened one: https://github.com/azerupi/mdBook/issues/224 https://github.com/azerupi/mdBook/issues/224
- jay-anderson 10y ago
  I totally agree. Is there a more general solution beside pages deciding to behave? Would making some select list of key combinations not overridable help? Or would that break too many pages?
- mattbreeden 10y ago
  Check out: https://chrome.google.com/webstore/detail/backspace-to-go-back/nlffgllnjjkheddehpolbanogdeaogbc?hl=en https://chrome.google.com/webstore/detail/backspace-to-go-ba... I've never had any issues with it.
- deleted 10y ago
  [deleted]
dozzie 10y ago
Iterators of this form are not functional construct, they are a pattern from object-oriented programming.
- ben0x539 10y ago
  The way iterators are used in Rust is reminiscent of the use of lazy lists in, like, Haskell, enabling a distinctly functional-ish style.
- steveklabnik 10y ago
  This is why we say > We're going to sidestep the issue of what, exactly, functional programming is or is not, and instead show off some features of Rust that are similar to features in many languages referred to as functional. Many people think of "programming with higher order functions" to be a feature of functional languages, and that may be wrong, but it's also okay. The goal is to teach the stuff, not worry about what exacty "functional" means.
  - dozzie 10y ago
    OK, but if you avoided calling non-functional patterns part of functional programming, you would avoid invoking the topic of "what constitutes functional programming" altogether.
    - steveklabnik 10y ago
      What specifically does "functional programming" mean to you and why do these features not qualify?
      - frankmcsherry 10y ago
        > In computer science, functional programming is a programming paradigm—a style of building the structure and elements of computer programs—that treats computation as the evaluation of mathematical functions and avoids changing-state and mutable data. from https://en.wikipedia.org/wiki/Functional_programming https://en.wikipedia.org/wiki/Functional_programming I totally agree that neither iterators nor 2/3s of closures are "functional programming". That's fine, though, because I really enjoy the sane way that it all was done. If you check out https://doc.rust-lang.org/std/iter/trait.Iterator.html https://doc.rust-lang.org/std/iter/trait.Iterator.html and search for `FnMut`, you see that most of the iterator methods are side-effectful. That is cool and fun and very useful, but it isn't functional programming. Edit. The term you are probably looking for (guessing) is "higher-order programming": https://en.wikipedia.org/wiki/Higher-order_programming https://en.wikipedia.org/wiki/Higher-order_programming
bigger_cheese 10y ago
That was really well written. As I was reading through the example code in the section about closures let add_one = |x| x + 1; let five = add_one(4); I thought to myself "hmm I wonder what happens if I were to call the code with a float rather than an int?" Answered my question a few paragraphs down. Kudos. Is it possible to write a generic closure? i.e something that would add 1 to any numeric type.
- steveklabnik 10y ago
  Thanks! To answer your last question, not yet. There was an RFC for this, but it was recently postponed; closures rely heavily on traits and we are in the process of re-doing the internals of the trait system. If I remember the reason correctly.
  - eddyb 10y ago
    Indeed, the way you would be generic over generic closures would be "type HRTB" (e.g. F: for<T: Clone> Fn(&T) -> T), which depends on the trait system overhaul, just like ATC.
- Manishearth 10y ago
  > Answered my question a few paragraphs down. Kudos. I absolutely love it when a blog post, talk, or docs like these do that. Especially during talks. Often the speaker will say something and it will immediately pop up questions in my mind which I really really want to ask but of course I should wait till the end. And then they answer it in the next slide. Perfect :)
- TheCycoONE 10y ago
  The whole new Rust Book is quite well written. It was my first resource when I read it a few months ago and I found it easy to understand everything it covered - even lifetimes which I completely failed to get from the original book or other online resources. I look forward to the rest of the chapters being filled in.
  - deleted 10y ago
Dowwie 10y ago
very well written
wahern 10y ago
How would one explicitly allocate space for a closure? (That is, in such a way that one could handle allocation failure.) I can't find any examples nor any hints about what the syntax might look like.
- steveklabnik 10y ago
  Closures are stack allocated unless you wrap them in something like a Box, so it's up to you. They're no different than any other value.
curun1r 10y ago
Under the "Improving our I/O Project" section, there's this little paragraph: It would be nice if we could use ? on the Option returned from next, but ? only works with Result values currently. Even if we could use ? on Option like we can on Result, the value we would get would be borrowed, and we want to move the String from the iterator into Config. It seems remiss to not mention that Option/Result have combinators/adapters too, no? let search = match args.next() { Some(arg) => arg, None => return Err("Didn't get a search string"), }; becomes: let search = args.next().ok_or("Didn't get a search string")?; I'm not the most fluent in Rust, so please tell me whether the above two snippets are not equivalent in some way. It might actually be a nice segue into a page on the combinators of Option and Result, since functional-style Rust seems to benefit from liberal use of them.
- steveklabnik 10y ago
  This is a great suggestion! I'm on mobile right now, but I'll file an issue when I get home, or if you feel like it, please do!
- pitaj 10y ago
  Please don't quote with code blocks.
  - curun1r 10y ago
    Umm...If I don't quote the entire code snippet, I'll end up with just the indented lines being quoted...the joys of HN formatting.
    - pitaj 10y ago
      I was speaking with reference to "It would be nice..." which you quoted using a pre/code block (prepending four spaces). Please only use that for actual code, and use `>` at the start of the line to signify a quote, like so: > Umm...If I don't quote the entire code snippet, I'll end up with just the indented lines being quoted...the joys of HN formatting. This allows for responsive formatting on different screen sizes, and is especially beneficial for mobile users.
bmh100 10y ago
This is beautiful. Higher order functions and closures are a big reason why I enjoy functional programming languages. To see these applied with method chaining is very exciting in a systems programming language.

72 points by nanxor 10y ago

computerphage 10y ago

For context: there's a rewrite of the rust book in progress for a while now. This is a chapter of that book.

steveklabnik 10y ago
Yes. Co-author here, happy to answer any questions or hear feedback on the text, as usual. Incidentally, I just finished the build system stuff today to get the new book shipped on doc.rust-lang.org; that slates it to land in Rust 1.18. It will still be a draft, but this is one more step towards it being finished.

theossuary 10y ago

Please don't take over my Cmd+Left arrow key for going back, Chrome already took away my backspace :(

steveklabnik 10y ago
This is an upstream issue with https://github.com/azerupi/mdBook https://github.com/azerupi/mdBook, you should file a bug!
- killercup 10y ago
  Couldn't find an open issue, but agreed that navigation should not be triggered when a modifier key is pressed, so I opened one: https://github.com/azerupi/mdBook/issues/224 https://github.com/azerupi/mdBook/issues/224
jay-anderson 10y ago
I totally agree. Is there a more general solution beside pages deciding to behave? Would making some select list of key combinations not overridable help? Or would that break too many pages?
mattbreeden 10y ago
Check out: https://chrome.google.com/webstore/detail/backspace-to-go-back/nlffgllnjjkheddehpolbanogdeaogbc?hl=en https://chrome.google.com/webstore/detail/backspace-to-go-ba... I've never had any issues with it.
deleted 10y ago
[deleted]

dozzie 10y ago

Iterators of this form are not functional construct, they are a pattern from object-oriented programming.

ben0x539 10y ago
The way iterators are used in Rust is reminiscent of the use of lazy lists in, like, Haskell, enabling a distinctly functional-ish style.
steveklabnik 10y ago
This is why we say > We're going to sidestep the issue of what, exactly, functional programming is or is not, and instead show off some features of Rust that are similar to features in many languages referred to as functional. Many people think of "programming with higher order functions" to be a feature of functional languages, and that may be wrong, but it's also okay. The goal is to teach the stuff, not worry about what exacty "functional" means.
- dozzie 10y ago
  OK, but if you avoided calling non-functional patterns part of functional programming, you would avoid invoking the topic of "what constitutes functional programming" altogether.
  - steveklabnik 10y ago
    What specifically does "functional programming" mean to you and why do these features not qualify?
    - frankmcsherry 10y ago
      > In computer science, functional programming is a programming paradigm—a style of building the structure and elements of computer programs—that treats computation as the evaluation of mathematical functions and avoids changing-state and mutable data. from https://en.wikipedia.org/wiki/Functional_programming https://en.wikipedia.org/wiki/Functional_programming I totally agree that neither iterators nor 2/3s of closures are "functional programming". That's fine, though, because I really enjoy the sane way that it all was done. If you check out https://doc.rust-lang.org/std/iter/trait.Iterator.html https://doc.rust-lang.org/std/iter/trait.Iterator.html and search for `FnMut`, you see that most of the iterator methods are side-effectful. That is cool and fun and very useful, but it isn't functional programming. Edit. The term you are probably looking for (guessing) is "higher-order programming": https://en.wikipedia.org/wiki/Higher-order_programming https://en.wikipedia.org/wiki/Higher-order_programming

bigger_cheese 10y ago

That was really well written. As I was reading through the example code in the section about closures let add_one = |x| x + 1; let five = add_one(4); I thought to myself "hmm I wonder what happens if I were to call the code with a float rather than an int?" Answered my question a few paragraphs down. Kudos. Is it possible to write a generic closure? i.e something that would add 1 to any numeric type.

steveklabnik 10y ago
Thanks! To answer your last question, not yet. There was an RFC for this, but it was recently postponed; closures rely heavily on traits and we are in the process of re-doing the internals of the trait system. If I remember the reason correctly.
- eddyb 10y ago
  Indeed, the way you would be generic over generic closures would be "type HRTB" (e.g. F: for<T: Clone> Fn(&T) -> T), which depends on the trait system overhaul, just like ATC.
Manishearth 10y ago
> Answered my question a few paragraphs down. Kudos. I absolutely love it when a blog post, talk, or docs like these do that. Especially during talks. Often the speaker will say something and it will immediately pop up questions in my mind which I really really want to ask but of course I should wait till the end. And then they answer it in the next slide. Perfect :)
TheCycoONE 10y ago
The whole new Rust Book is quite well written. It was my first resource when I read it a few months ago and I found it easy to understand everything it covered - even lifetimes which I completely failed to get from the original book or other online resources. I look forward to the rest of the chapters being filled in.
- deleted 10y ago

Dowwie 10y ago

very well written

wahern 10y ago

How would one explicitly allocate space for a closure? (That is, in such a way that one could handle allocation failure.) I can't find any examples nor any hints about what the syntax might look like.

steveklabnik 10y ago
Closures are stack allocated unless you wrap them in something like a Box, so it's up to you. They're no different than any other value.

curun1r 10y ago

Under the "Improving our I/O Project" section, there's this little paragraph: It would be nice if we could use ? on the Option returned from next, but ? only works with Result values currently. Even if we could use ? on Option like we can on Result, the value we would get would be borrowed, and we want to move the String from the iterator into Config. It seems remiss to not mention that Option/Result have combinators/adapters too, no? let search = match args.next() { Some(arg) => arg, None => return Err("Didn't get a search string"), }; becomes: let search = args.next().ok_or("Didn't get a search string")?; I'm not the most fluent in Rust, so please tell me whether the above two snippets are not equivalent in some way. It might actually be a nice segue into a page on the combinators of Option and Result, since functional-style Rust seems to benefit from liberal use of them.

steveklabnik 10y ago
This is a great suggestion! I'm on mobile right now, but I'll file an issue when I get home, or if you feel like it, please do!
pitaj 10y ago
Please don't quote with code blocks.
- curun1r 10y ago
  Umm...If I don't quote the entire code snippet, I'll end up with just the indented lines being quoted...the joys of HN formatting.
  - pitaj 10y ago
    I was speaking with reference to "It would be nice..." which you quoted using a pre/code block (prepending four spaces). Please only use that for actual code, and use `>` at the start of the line to signify a quote, like so: > Umm...If I don't quote the entire code snippet, I'll end up with just the indented lines being quoted...the joys of HN formatting. This allows for responsive formatting on different screen sizes, and is especially beneficial for mobile users.

bmh100 10y ago

This is beautiful. Higher order functions and closures are a big reason why I enjoy functional programming languages. To see these applied with method chaining is very exciting in a systems programming language.