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Comment by ilitirit | Hacker News Reader

I like Rust, and I'm probably missing something, but do I feel that some things should be easier. At times the language (or standard library, I suppose) feels a bit unwieldy. For example, trying to coerce a string from certain types:

    let path = Path::new(&app_path)
    println!("file name: {}", path.file_name().unwrap().to_str().unwrap());

Hopefully this is just a case of unfamiliarity, but surely something like should be easier?

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If you were willing to allow for potential escape characters and acknowledge (to the user) the fact that the path might not have a file name, you could write:

    println!("file name: {:?}", path.file_name());

or just for escape characters, but only if the file name existed:

    if let Some(name) = path.file_name() {
        println!("file name: {:?}", name);
    }

Note that the unwrap() isn't boilerplate, in either case: if you are using `unwrap()` you are implicitly saying that you know for a fact that the file name is present (the first time) and a valid UTF-8 string (the second time), which isn't guaranteed in the general case on all platforms. If you wanted to write this more robustly, and these weren't guaranteed, you would need to explicitly take care of those cases.

I/O has lots of tricky error cases, and Rust usually requires you to acknowledge that. You can avoid dealing with those cases with unwrap(), or you can explicitly handle them. Personally, I much prefer this to being surprised only when the weird error case unexpectedly turns up because I never considered the situation in question, and that's generally how Rust handles things, but it depends on the task. It may make Rust less appropriate for quick scripting tasks, for example, where the domain is well-defined and the script is not likely to be reused (but I find that those scripts have an annoying way of getting reused anyway).

However, rust would be relatively well-suited for a "dirty quick scripting task". You could make a more traditional wrapper for the operations you like to use that assumes these rare edge cases don't happen, and panic otherwise.

This is sort of rust's philosophy; being explicit. Rust has no problem with failing, it just has a problem with failing without you telling it to. If you write a wrapper that implies you are willing to fail on these cases, then you're being explicit by choosing to use these functions instead :)

Yes, it's true. Code like this is going to be more verbose. Often, it's due to Rust being a systems language and exposing low-level details to you, combined with the type system ensuring safety. In this instance, `Path` is actually fairly complex. From the RFC: https://github.com/rust-lang/rfcs/blob/master/text/0474-path...

        > The design of a path abstraction is surprisingly hard. Paths work
        > radically differently on different platforms, so providing a
        > cross-platform abstraction is challenging. On some platforms, paths
        > are not required to be in Unicode, posing ergonomic and semantic
        > difficulties for a Rust API. These difficulties are compounded if one
        > also tries to provide efficient path manipulation that does not, for
        > example, require extraneous copying. And, of course, the API should
        > be easy and pleasant to use.

So, in this case, you have to call `file_name` to get the filename, but that returns an `Option`, since it might not be a file, it could be a directory. The `unwrap` says "crash if it's not a filename." You then have an `OsStr`, which is a string type that handles the kinds of cross-platform issues alluded to in the above paragraph. `to_str` will then attempt to turn it into a `&str`, but because `&str` must be unicode, if the conversion fails, it _also_ returns an Option, which is the last unwrap.

In systems programming, details matter. As such, our stdlib APIs try to expose those details. I expect some people will write higher-level abstractions that hide all this if you don't care as much.

Tradeoffs tradeoffs tradeoffs...

"Often, it's due to Rust being a systems language and exposing low-level details to you, combined with the type system ensuring safety."

I'm not convinced it's being a "systems language" (in the sense I think you mean). The last three languages I've learned to fluency have all, in various ways, been very concerned about errors: Erlang, Haskell, and Go. Yes, they have widely varying opinions about what to do with them, but they all agree that you have to be fairly constantly worried about them. You can't sweep them under the rug; you need a strategy, and you need to deal with the resulting issues head on.

Despite their differences, what I feel like I've learned from them is that historically speaking, the vast majority of popular languages and their communities have been downright cavalier about errors. It's often in all the little details rather than a consistent, stated design goal, but the presence of various nils and nulls, APIs that will happily return empty strings for paths instead of errors, and all kinds of APIs and libraries that seem to be subconsciously written with the belief that returning an error is a failure on the part of the library and will go to what now seem to me to be absurd lengths to somehow keep klunking along in the presence of already-manifested failure. Even for languages that think they're being really good with exceptions, I find that when I go back to them, they end up using the exception support to "permit" themselves to be sloppy and hope the exceptions will pick up the pieces... and they often don't. Not because of some theoretical weakness, but because allowing developers to think they don't have to think about the error cases means precisely that they don't!

People are used to errors not poking them in the face until deploy time. It seems to me there's a strong, broad motion towards developing languages that will in one way or another force you to deal with them at compile time. It's a culture shock for people, but, I'll tell you this, get really good at Rust or any of these other languages and go back to any of those other languages, and you will find your code changes in those other languages to suddenly look a lot more like your Rust does anyhow, which means this putative disadvantage of Rust evaporates anyhow. Once you see how sloppy you were being, it's hard to go back in good conscience.

> The last three languages I've learned to fluency have all, in various ways, been very concerned about errors

Yeah, maybe I should revise my statement a bit: systems languages, overall, tend to expose more details. I meant this in a broader context than errors. As a simple example, manual memory management exposes more details than GC. There are many languages that care dearly about errors, as you've rightfully pointed out.

I broadly agree with the rest of your post. Error handling is hard and tedious, and programmers don't like doing it. I myself had an embarrassing bug in `rustbook` where I did `let _ =`. I only intended to start off that way! Programming is hard. But using Haskell, Rust, and other languages like this have certainly changed my code in other languages.

Great comment. I have run into this problem exactly in inherited codebases at my last couple of jobs:

> libraries that seem to be subconsciously written with the belief that returning an error is a failure on the part of the library and will go to what now seem to me to be absurd lengths to somehow keep klunking along in the presence of already-manifested failure.

It's a big problem with hairy repercussions if this idea is allowed to take root.

> surely something like should be easier?

That would be nice, but as others hinted that wouldn't actually work:

* there are paths without a file_name (`/`) so it returns an Option to make that evident

* to_str attempts to convert the OS-dependent thing to a Rust string. Rust strings are valid unicode, but many filesystems don't have such strict requirements[0]. Again Rust returns an Option to makes that evident although there's a "to_string_lossy" method which will simply replace invalid sequences with U+FFFD if you don't really care

The Rust philosophy is generally to expose these issues upfront and force the developer to consider them (and either handle or explicitly ignore them). It is indubitably less convenient than e.g. Python 3's `PurePath(app_path).name` returning an str, but you'll have had to decide what happens in case of failure.

Paths are really a good example of something which looks simple, should be simple, and is actually anything but simple for historical and technical reasons, especially in a cross-platform context.

[0]

* Unix paths are the most troublesome: they're arbitrary bags of (non-NUL) bytes, they don't have to be in a known encoding, they don't have to be in any encoding.

* Windows paths are not guaranteed to be Unicode either: they're UTF-16 code units aka UCS2 + surrogates, and the surrogates may be unpaired, leading to invalid Unicode.

* OSX paths are guaranteed to be valid Unicode.

Also, OSX applies unicode normalization to the filenames, which can cause some interoperability problems with unix, e.g. you try to copy files forth and back between the systems and end up with two copies of a file, with identical-looking filenames that differ in their unicode normal form.

To add to the other comments, Rust prefers to make error cases explicit. We have memory safety. You can opt out of it, but you have to opt out explicitly. We have exhaustive match. You can use a wildcard, but you need to explicitly do so. We have nullables and Results. You need to explicitly acknowledge the error/null case, or use `unwrap()` to say that you don't mind a panic at this point.

This leads to verbosity at times, but usually its a good verbosity.

I'm sure I'm not the first (or the hundredth) to say this, but `unwrap` has always struck me as a horribly innocuous spelling of "yes, feel free to crash here".

I presume there is (or could be) a standard lint to prevent this getting into production code, but it still feels kind of icky.

We almost changed it to `assert` but that made some people _really_ upset.

Dammit, I kept wondering why it wasn't named unwrap_or_panic/unwrap_or_assert but it could be worse at least unwrap is a less friendly name then scala's get.

`.expect()` is slightly better (you can give it a string that will become a part of the panic message)

Just my 2cs, but the complexity of this code is here to remind you there are a lot of cases you have to take into account :

    let path = Path::new(&app_path);
    if let Some(ostr) = path.file_name() {
       if let Some(str) = ostr.to_str() {
           println!("file name : {}", str);
       } else {
          println!("WARNING : file name is not a valid unicode sequence ! (file name : {:?}", ostr);
       }
    } else {
      println!("Path is either a root directory or a dot entry, it has no filename");
    }

I would say it is familiarity, but in another sense. You can use something like

    path.file_name().and_then(|f| f.to_str()).unwrap()

but that really doesn't buy you much.

Where familiarity comes in I believe is in what that line communicates to me: There might not be a file name (like having '/' as path), and that getting a string slice out of it might fail (the kind of string the platform uses for paths might not be UTF-8 compatible).

Plus, the unwrap()s tell me that whoever wrote the code considered it an assertion failure if one of those is happening.

I read a lot more Rust than I write, and it really helps that the code communicates a lot. And with knowing about OsString/OsStr beforehand, I only had to lookup when file_name() returns None, the rest is rather implicit by convention, or familiarity.

I wish `and_then` were named `flat_map` so badly.

We have `flat_map` for iterators. (`and_then` works for Options, which are basically nullables)

You can convert an Option to a zero-cost iterator and then use `flat_map` though.

I think unwrap calls are mainly meant for quick and dirty stuff and if you find yourself doing it it may be the wrong solution.

That would be:

    let path = Path::new(&app_path);
    println!("file name: {}", path.display());

No, I just want the file name.

The 'String', '~str' dichotomy is really unfortunate.

There is no ~str, but when there was it was largely identical to what String is now.

Presumably you mean String/&str? (~str hasn't existed for a long time.)

On the contrary, the lack of the string/string-view distinction is widely considered a flaw in the C++ standard libraries. C++14 introduces std::string_view to correct it. Rust's String/str distinction is just the same as C++14's std::string/std::string_view.

Apart from the Rust's usual rule to invent some abbreviations, but only apply them half the time.

Then "types start with an uppercase letter" except when the language creators break their own rule.

Then the fun "sometimes we use () and sometimes we use [] to do practically the same thing".

Then the various insanities with ::.

Then Generics. How many examples of languages which tried to use <> for Generics do we actually need before people learn the lesson?

I really wished some people took Rust, and wrote a new frontend to get rid of all the pointless inconsistencies and other "C did, so it has to be good"-isms (like_all_this_underscore_stuff or the ridiculous abbreviations), because many ideas of Rust are not only good but brilliant and deserve to be put into languages which don't belong to the purely-academic category.

I really wish Rust to succeed, but can we stop this approach of "3 steps forward and 2 steps backward" to language design?

> Apart from the Rust's usual rule to invent some abbreviations, but only apply them half the time.

Do you have an example?

> Then "types start with an uppercase letter" except when the language creators break their own rule.

They always start with a capital letter, except for built-in types or FFI bindings to libraries such as libc where the original types used a lowercase letter. This convention is exactly the same as Java.

> Then the fun "sometimes we use () and sometimes we use [] to do practically the same thing".

I've never heard this before. Do you have an example? () is used for function calls, grouping, and tuples, while [] is used for array literals and array indexing.

> Then the various insanities with ::.

The double-colon is very useful so that you can tell the difference between module lookup and field projection/method calling. Early Rust used . for module lookup, and it was confusing to tell at a glance whether a function was being called or a method was being invoked.

> Then Generics. How many examples of languages which tried to use <> for Generics do we actually need before people learn the lesson?

Using square brackets like Scala wouldn't actually help with the ambiguity, because it would be ambiguous with array indexing. The only syntax I've seen that actually solves the ambiguity is D's `!()`, which was deemed too unfamiliar. Angle brackets were chosen because they are familiar and aren't really any worse than the other mainstream options.

> I really wished some people took Rust, and wrote a new frontend to get rid of all the pointless inconsistencies and other "C did, so it has to be good"-isms (like_all_this_underscore_stuff or the ridiculous abbreviations)

The underscore naming convention was actually taken from Python's PEP 8. Rust doesn't really have any more abbreviations than most other languages at this point.

> > Then the fun "sometimes we use () and sometimes we use [] to do practically the same thing".

> I've never heard this before. Do you have an example? () is used for function calls, grouping, and tuples, while [] is used for array literals and array indexing.

Maybe referring to the ability to use either type of brace with macros?

Yeah, you can use any type of delimiter with macros, but I think that's an important feature, especially since macros can expand to declarations. Early versions of Rust required parentheses for all macro invocations, and that looked really ugly for anything that looked like a block. Delimiter interchangeability is a feature from Racket that works really well when it comes to macros and I'd hate to give it up.

Agreed. They allow you to naturally create vectors with the bracket delimiter `vec![1, 2, 3]`, create spawn-like macros with the brace delimiter `spawn { code }`, and in the normal case just use parentheses `println!("testing")`

What about '{}'? It's a nice pair of opening/closing characters.

> () is used for function calls, grouping, and tuples, while [] is used for array literals and array indexing.

Array indexing is a function call, because an array is a function from its index set to the set of the values the array stores, or -- equivalent -- a partial function from integers to the value set.

Scala gets this right.

> Scala gets this right.

I don't know, I never liked how all FunctionNs are sort of "unofficial" partial functions (because they may throw), and PartialFunctions are then made a subtype of FunctionN, because they have an extra method where you can ask whether they are defined at given points. Especially since throwing seems to be falling out of favor in Scala, being replaced by Try[] return types.

This is an orthogonal issue. Arrays are clearly partial functions. The Try[.]-vs-exceptions issue is to do with the question whether failures should be reflected in types (the Try[.]-option) or not (exceptions). There are good arguments for and against both options, but both options go well with the understanding that arrays are functions and indexing is function application.

I don't think anything's much orthogonal to partiality of functions in languages with strong type systems, unfortunately :)

You wrote that array indexing is "a partial function from integers to the value set". If we're to take the suggestion seriously, then the practicality of it depends on how partial functions are represented in the language. Which itself is certainly not orthogonal to the Try-vs-exceptions issue.

I'm not sure I can agree. I didn't say that the type of the array function is independent of the choice of error propagation mechanism. If you think an array is a partial function from integers to some value domain, say A, then in the exceptional POV the array type is int -> A, and in the Try[.] POV the array type is int -> Try [ A ].

I was talking about the mechanism for array indexing. In both cases, indexing is function application. So the choice of error representation is orthogonal. This is even more evidence that we should not use a different operator for array indexing as Rust does alas.

OK I can agree with that, I think I was taking what you wrote as a wider endorsement of Scala's approach than what you intended.

In this example the dichotomy is between String (which is guaranteed by the type system to be valid UTF-8) and OsStr (which might be in an unknown encoding or otherwise not decodable to valid Unicode).

This is exactly when you want a systems language to require explicit conversions, rather than converting things silently and possibly losing or corrupting data.

rather than converting things silently and possibly losing or corrupting data

Exactly. Python3 went down the "silently converting" route, and it's not pretty[1]. I would go so far as to call it harmful.

http://lucumr.pocoo.org/2014/5/12/everything-about-unicode/

I understand the difficulty in this space; much of it is caused by forcing the Windows unicode filesystem API onto python as its world-view, rather than sticking to the traditional Unix bytes world-view. I'm unixy, so I'm completely biased, but I think adopting the Windows approach is fundamentally broken.

The problem there is overblown - it's basically all due to the idea that sys.stdin or sys.stdout might get replaced with streams that don't have a binary buffer. The simple solution is just not to do that (and it's pretty easy; instead of replacing with a StringIO, replace it with a wrapped binary buffer). Then the code is quite simple

    import sys
    import shutil

    for filename in sys.argv[1:]:
        if filename != '-':
            try:
                f = open(filename, 'rb')
            except IOError as err:
                msg = 'cat.py: {}: {}\n'.format(filename, err)
                sys.stderr.buffer.write(msg.encode('utf8', 'surrogateescape'))
                continue
        else:
            f = sys.stdin.buffer

        with f:
            shutil.copyfileobj(f, sys.stdout.buffer)

Python's surrogateescape'd strings aren't the best solution, but I personally believe that treating unicode output streams as binary ones is even worse.

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