4 ms·

In the vector example, why not just `collect<...>()` and debug print that, or am I missing the point of what they tried to illustrate? Also, I do agree that C+

In the vector example, why not just `collect<...>()` and debug print that, or am I missing the point of what they tried to illustrate? Also, I do agree that C++'s approach to formatting the date was more user friendly/ergonomic, but on the other hand it's not something I've struggled with. I also feel like there is a more succinct version (I don't know if it would have the same performance characteristics and how much that mattered when writing the post). They also later mentioned overall consistency and coherence in the Rust ecosystem, and I value that more over a couple of hand crafted formatters in application code

tialaramex 4y ago
Arguably we might not be able to store the resulting collection. For example suppose we've got 64MB of RAM, and we've got the iterator 0..i32::MAX If we try to collect() that, we're going to have a bad time, but while that's a lot of numbers, we certainly could write it to a file, we probably do have gigabytes of disk space, or we could send it over the network - the Internet certainly has storage for these numbers.
- estebank 4y ago
  But even then the point still stands: you wouldn't want a non-custom debug representation of the iterator precisely because of the cost. You could create a new-type that wraps your iterator and implements a custom `Debug` impl with the contents you desire. Here the issue at play seems to be a disagreement on what is more useful in a Default implementation: a preview of the results of the iteration or a representation of the type structure. I believe that which of the two is more useful depends entirely on what you're using the Debug info for (with the caveat that going from the later to the former in your head is possible, but the other way around isn't).
  - Arnavion 4y ago
    You can't preview an arbitrary Iterator anyway because iterating is destructive. The elements you iterated will be gone forever. Besides, the typesystem won't let you do it anyway, since collect() requires either `Self` or `&mut Self`, and `fmt` only receives `&Self`. So at best you could only do this with Clone'able Iterators, but even then it might not be ideal for formatting to create "wasteful" clones. So maybe you'd only do it for individual Iterator types like Range or std::slice::Iter for which cloning is cheap. But even .map()ping such a trivially-Cloneable Iterator would yield a non-Cloneable or non-trivially-Cloneable Iterator, so at that point you have to wonder if it's worth making a small subset of Iterators have these "useful" Debug impls.
    - spoiler 4y ago
      But even in Cpp, you need to eventually iterate over the underlying iterator to get the results of the map. So, if you want to debug print it, you still need to iterate over it regardless. Regarding the mutability and collect issues you raised, I think I just explained poorly. What I meant is that the author could've done something like this https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=e58b99ff86a5f26e98c023ed39f596b8 https://play.rust-lang.org/?version=stable&mode=debug&editio... However, debug printing "on the fly" (without collection first) would definitely be more involved with interior mutability and require a wrapper type, for the reasons you outlined, but not impossible.
      - Arnavion 4y ago
        Your playground is what the article author did, except they used `itertools::Itertools::format` so that the entire collection or formatted string doesn't need to be built up-front but can instead be fed to the `std::fmt::Formatter` element-by-element.
        spoiler 4y ago
        Ah, I understand that part, but I assumed they were looking for the most ergonomic/succinct and idiomatic approach (ie not needing to reach for a crate). But it doesn't have the same performance characteristics as using itertools (which is better here)
- shakow 4y ago
  If the vector is 64MB long, you probably don't want to print it either.

tialaramex 4y ago

Arguably we might not be able to store the resulting collection. For example suppose we've got 64MB of RAM, and we've got the iterator 0..i32::MAX If we try to collect() that, we're going to have a bad time, but while that's a lot of numbers, we certainly could write it to a file, we probably do have gigabytes of disk space, or we could send it over the network - the Internet certainly has storage for these numbers.

estebank 4y ago
But even then the point still stands: you wouldn't want a non-custom debug representation of the iterator precisely because of the cost. You could create a new-type that wraps your iterator and implements a custom `Debug` impl with the contents you desire. Here the issue at play seems to be a disagreement on what is more useful in a Default implementation: a preview of the results of the iteration or a representation of the type structure. I believe that which of the two is more useful depends entirely on what you're using the Debug info for (with the caveat that going from the later to the former in your head is possible, but the other way around isn't).
- Arnavion 4y ago
  You can't preview an arbitrary Iterator anyway because iterating is destructive. The elements you iterated will be gone forever. Besides, the typesystem won't let you do it anyway, since collect() requires either `Self` or `&mut Self`, and `fmt` only receives `&Self`. So at best you could only do this with Clone'able Iterators, but even then it might not be ideal for formatting to create "wasteful" clones. So maybe you'd only do it for individual Iterator types like Range or std::slice::Iter for which cloning is cheap. But even .map()ping such a trivially-Cloneable Iterator would yield a non-Cloneable or non-trivially-Cloneable Iterator, so at that point you have to wonder if it's worth making a small subset of Iterators have these "useful" Debug impls.
  - spoiler 4y ago
    But even in Cpp, you need to eventually iterate over the underlying iterator to get the results of the map. So, if you want to debug print it, you still need to iterate over it regardless. Regarding the mutability and collect issues you raised, I think I just explained poorly. What I meant is that the author could've done something like this https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=e58b99ff86a5f26e98c023ed39f596b8 https://play.rust-lang.org/?version=stable&mode=debug&editio... However, debug printing "on the fly" (without collection first) would definitely be more involved with interior mutability and require a wrapper type, for the reasons you outlined, but not impossible.
    - Arnavion 4y ago
      Your playground is what the article author did, except they used `itertools::Itertools::format` so that the entire collection or formatted string doesn't need to be built up-front but can instead be fed to the `std::fmt::Formatter` element-by-element.
      - spoiler 4y ago
        Ah, I understand that part, but I assumed they were looking for the most ergonomic/succinct and idiomatic approach (ie not needing to reach for a crate). But it doesn't have the same performance characteristics as using itertools (which is better here)
shakow 4y ago
If the vector is 64MB long, you probably don't want to print it either.