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I mostly agree with you, but in practice I think using unwrap or panicking in libraries is usually wrong. As an example it’s quite easy to fall astray of takin

I mostly agree with you, but in practice I think using unwrap or panicking in libraries is usually wrong. As an example it’s quite easy to fall astray of taking your perspective to say read a packet off the network, and panic on malformed data. I know you’d agree that would be a inappropriate time to use panic, as that would crash any upstream program with trivial DOS exploits. But this is easy to do if you say have an API that takes a value, translates it to something else and panics on invalid inputs. My advice is generally in line with the GP, pornel—use the tools at hand for avoiding panics where it’s easy to do so, such as ?, map, etc. Only panic/unwrap on bad library API usage (which I think is what you’re suggesting). Feel free to panic anywhere in main. People often translate “ use of unwrap/expect anywhere, so long as its occurrence is never expected” in ways that can create major bugs as the software goes into more use.

burntsushi 7y ago
I understand that perspective to an extent, but I really want to double down here because I think it's important. :-) > but in practice I think using unwrap or panicking in libraries is usually wrong Very strongly disagree. I've created dozens of Rust libraries, and probably all of them have dozens of code paths that can panic. Other core libraries do the same. Just a simple slice access, e.g., `slice[i]`, is a line of code that could panic. (Since it's just a shorter way of writing `slice.get(i).unwrap()`.) I don't think we should be giving advice that runs contrary to how important libraries actually work. A lot of people learn to code by reading others' code, and when we give advice like "don't use unwrap/expect," they get confused when they see that virtually every widely used piece of Rust code violates it. The key here is really that one shouldn't panic unless the panic itself is indicative of a bug somewhere. Blanket advice like "don't use unwrap/panic in libraries" is bad because---as I argued in the Clippy issue I posted in a sibling comment---it's effectively a prohibition against runtime invariants themselves. As a programmer, you insert a panic when you've failed (for any number of reasons) to capture the invariant in the type system. Blanket advice saying that one shouldn't use unwrap/expect in these circumstances leads one toward a path of much more complex APIs with error types that are never constructed unless there is a bug in the code. (It's likely that actually adding all of those error types will be so annoying that it's plausible the programmer will give up on Rust.) > As an example it’s quite easy to fall astray of taking your perspective to say read a packet off the network, and panic on malformed data. I know you’d agree that would be a inappropriate time to use panic, as that would crash any upstream program with trivial DOS exploits. But this is easy to do if you say have an API that takes a value, translates it to something else and panics on invalid inputs. I think this is a reflection of error handling being hard. It's just as easy to code a program with the mistaken assumption that a particular file path will always point to a valid and readable file. It takes a bit of learning to understand which things you can rely on never happening and which you can't. In the mean time, I don't think we should be giving advice that is very easy to misinterpret into a conclusion that just isn't tenable. Personally, I think it's much easier to talk about end user behavior. If there's a panic, then it's a bug that ought to be fixed. If you follow that, it's hard to go wrong. You could rephrase it with, "If there's a reachable code path that panics, then the code path should be removed." You could then talk about what "reachable" means, i.e., code paths that are determine based on data that the program doesn't control vs code paths that are enforced for all inputs to the program.
- bluejekyll 7y ago
  I can’t tell if you and I are agreeing or disagreeing at this point. And yes, I would not disagree with the quality of your code and the libraries you’ve published, they’re some of the best and most widely used in the community. You’ve said don’t panic for code paths that the program doesn’t control. That is nuanced, and often is not always obvious and easy to reason about, but the language makes those exceptional cases obvious, so why not use that to your advantage? edit: and by the way, I didn't say that on indexes, I totally agree with you. Though, sometimes it is valuable to go to the extreme and prevent malicious index values from untrusted sources crashing your program...
  - burntsushi 7y ago
    >I can’t tell if you and I are agreeing or disagreeing at this point. I think we are disagreeing mostly over pedagogy. > That is nuanced, and often is not always obvious and easy to reason about Right. I think this is what I meant by "error handling is hard." But this really boils down to understanding what a runtime invariant is, and that takes time to learn. It's hard to do good error handling without internalizing that. > but the language makes those exceptional cases obvious, so why not use that to your advantage? I think you are, specifically by using unwrap/expect. In many such cases, the unwrap/expect has a comment explaining why it's impossible to panic. > edit: and by the way, I didn't say that on indexes, I totally agree with you. Though, sometimes it is valuable to go to the extreme and prevent malicious index values from untrusted sources crashing your program... Well, `&slice[i]` and `slice.get(i).unwrap()` are equivalent. So if we give blanket advice like "don't use unwrap/expect in libraries" then the latter gets caught up in that advice while the former doesn't, even though they are exactly equivalent. To me, this reveals the problem in that pedagogy because it focuses too much on one particularly common manifestation rather than the thing that actually matters: a panic visible to an end user is always a bug. To be clear, the thing I am disagreeing with is the advice to "not use unwrap/expect; use case analysis instead." I can appreciate a pedagogy that simplifies things upfront with the cost of getting some corner cases wrong. But unwrap/expect are used too much in too many valid cases IMO for that type of strategy to be effective.
- Tainnor 7y ago

burntsushi 7y ago

I understand that perspective to an extent, but I really want to double down here because I think it's important. :-) > but in practice I think using unwrap or panicking in libraries is usually wrong Very strongly disagree. I've created dozens of Rust libraries, and probably all of them have dozens of code paths that can panic. Other core libraries do the same. Just a simple slice access, e.g., `slice[i]`, is a line of code that could panic. (Since it's just a shorter way of writing `slice.get(i).unwrap()`.) I don't think we should be giving advice that runs contrary to how important libraries actually work. A lot of people learn to code by reading others' code, and when we give advice like "don't use unwrap/expect," they get confused when they see that virtually every widely used piece of Rust code violates it. The key here is really that one shouldn't panic unless the panic itself is indicative of a bug somewhere. Blanket advice like "don't use unwrap/panic in libraries" is bad because---as I argued in the Clippy issue I posted in a sibling comment---it's effectively a prohibition against runtime invariants themselves. As a programmer, you insert a panic when you've failed (for any number of reasons) to capture the invariant in the type system. Blanket advice saying that one shouldn't use unwrap/expect in these circumstances leads one toward a path of much more complex APIs with error types that are never constructed unless there is a bug in the code. (It's likely that actually adding all of those error types will be so annoying that it's plausible the programmer will give up on Rust.) > As an example it’s quite easy to fall astray of taking your perspective to say read a packet off the network, and panic on malformed data. I know you’d agree that would be a inappropriate time to use panic, as that would crash any upstream program with trivial DOS exploits. But this is easy to do if you say have an API that takes a value, translates it to something else and panics on invalid inputs. I think this is a reflection of error handling being hard. It's just as easy to code a program with the mistaken assumption that a particular file path will always point to a valid and readable file. It takes a bit of learning to understand which things you can rely on never happening and which you can't. In the mean time, I don't think we should be giving advice that is very easy to misinterpret into a conclusion that just isn't tenable. Personally, I think it's much easier to talk about end user behavior. If there's a panic, then it's a bug that ought to be fixed. If you follow that, it's hard to go wrong. You could rephrase it with, "If there's a reachable code path that panics, then the code path should be removed." You could then talk about what "reachable" means, i.e., code paths that are determine based on data that the program doesn't control vs code paths that are enforced for all inputs to the program.

bluejekyll 7y ago
I can’t tell if you and I are agreeing or disagreeing at this point. And yes, I would not disagree with the quality of your code and the libraries you’ve published, they’re some of the best and most widely used in the community. You’ve said don’t panic for code paths that the program doesn’t control. That is nuanced, and often is not always obvious and easy to reason about, but the language makes those exceptional cases obvious, so why not use that to your advantage? edit: and by the way, I didn't say that on indexes, I totally agree with you. Though, sometimes it is valuable to go to the extreme and prevent malicious index values from untrusted sources crashing your program...
- burntsushi 7y ago
  >I can’t tell if you and I are agreeing or disagreeing at this point. I think we are disagreeing mostly over pedagogy. > That is nuanced, and often is not always obvious and easy to reason about Right. I think this is what I meant by "error handling is hard." But this really boils down to understanding what a runtime invariant is, and that takes time to learn. It's hard to do good error handling without internalizing that. > but the language makes those exceptional cases obvious, so why not use that to your advantage? I think you are, specifically by using unwrap/expect. In many such cases, the unwrap/expect has a comment explaining why it's impossible to panic. > edit: and by the way, I didn't say that on indexes, I totally agree with you. Though, sometimes it is valuable to go to the extreme and prevent malicious index values from untrusted sources crashing your program... Well, `&slice[i]` and `slice.get(i).unwrap()` are equivalent. So if we give blanket advice like "don't use unwrap/expect in libraries" then the latter gets caught up in that advice while the former doesn't, even though they are exactly equivalent. To me, this reveals the problem in that pedagogy because it focuses too much on one particularly common manifestation rather than the thing that actually matters: a panic visible to an end user is always a bug. To be clear, the thing I am disagreeing with is the advice to "not use unwrap/expect; use case analysis instead." I can appreciate a pedagogy that simplifies things upfront with the cost of getting some corner cases wrong. But unwrap/expect are used too much in too many valid cases IMO for that type of strategy to be effective.
Tainnor 7y ago