Yes, we can handle Go errors without language changes; that’s true. But how we should annotate errors? The presented version of CopyFile leads to following 'stuttering': cannot copy '/tmpfs/play' to './tmp33/not-exist.txt': cannot create destination: open ./tmp33/not-exist.txt: no such file or directory
You can get compact but informative error messages just by following K&D annotation rules: 1. Annotate the function you are implementing 2. Let the functions that you are calling do their own job copy file: open ./tmp33/not-exist.txt: no such file or directory All of this is just basic error propagation, which can be automated to avoid annoying human errors. You can see three different error annotation scenarios of the FileCopy implementations in this playground <https://go.dev/play/p/W9ym0JkLsVF>. And the function itself is compact as well — and fully automated (tip: change the name of the function): func copyFile(src, dst string) (err error) { defer err2.Handle(&err) r := try.To1(os.Open(src)) defer r.Close() w := try.To1(os.Create(dst)) defer err2.Handle(&err, func() { os.Remove(dst) }) defer w.Close() try.To1(io.Copy(w, r)) return nil } On Friday, August 4, 2023 at 6:07:52 AM UTC+3 Miguel Angel Rivera Notararigo wrote: > func CopyFile(src, dst string) error { > r, err := os.Open(src) > if err != nil { > return fmt.Errorf("copy %s %s: %v", src, dst, err) > } > defer r.Close() > > w, err := os.Create(dst) > if err != nil { > return fmt.Errorf("copy %s %s: %v", src, dst, err) > } > > if _, err := io.Copy(w, r); err != nil { > w.Close() > os.Remove(dst) > return fmt.Errorf("copy %s %s: %v", src, dst, err) > } > > if err := w.Close(); err != nil { > os.Remove(dst) > return fmt.Errorf("copy %s %s: %v", src, dst, err) > } > } > > I think it is a bad example, how do you know where CopyFile failed? > > The "copy ..." part shouldn't be in there, you should add valuable context > to your errors, if CopyFile fails, the caller already knows it was a copy > error because the function has a big "Copy" on his name right? you should > do this instead: > > func CopyFile(dst, src string) error { > r, errOS := os.Open(src) // Avoid shadowing errors, don't use err > if errOS != nil { > return fmt.Errorf("cannot open source: %v", errOS) > } > > defer r.Close() > > w, errCD := os.Create(dst) > if errCD != nil { > return fmt.Errorf("cannot create destination: %v", errCD) > } > > defer w.Close() > > if _, err := io.Copy(w, r); err != nil { // Local scope error, so err is > fine > os.Remove(dst) > return fmt.Errorf("cannot copy data from source: %v", err) > > } > > if err := w.Close(); err != nil { > os.Remove(dst) > return fmt.Errorf("cannot close destination", err) > } > } > > // Caller should do this. > if err := CopyFile("dst.txt", "src.txt"); err != nil { > // Here is where you should add 'copy' to the error message. > return fmt.Errorf("cannot copy '%s' to '%s': %v", src, dst, err) > } > > People complaining about Go's error handling regularly don't handle > errors, they just throw them like exceptions. > > If you really hate Go's error handling, just use: > > func catch(err error) { > if err != nil { > panic(err) > } > > // And use recover somewhere > } > > Which is a bad practice, but at least we (the people who like how Go > handle errors) can still handle our errors without any language change. > > On Tue, Aug 1, 2023, 13:06 DrGo <salah....@gmail.com> wrote: > >> Thanks. >> The keystroke saving is not the motivation. The aim is to reduce the code >> reader’s mental load. My approach allows for clearer code where the main >> program logic is not dwarfed by the error handling code while maintaining >> the explicitly of error handling and the possible error-induced >> interruption in program flow. It avoids creating new if scope when one is >> not desired and offers opportunities for further deduplication of error >> handling code although each error is still handled individually. Compare >> the following; which one would you prefer to read a lot of? >> >> - current approach; error handling to program logic ratio: 13:5 >> >> func CopyFile(src, dst string) error { >> r, err := os.Open(src) >> if err != nil { >> return fmt.Errorf("copy %s %s: %v", src, dst, err) >> } >> defer r.Close() >> >> w, err := os.Create(dst) >> if err != nil { >> return fmt.Errorf("copy %s %s: %v", src, dst, err) >> } >> >> if _, err := io.Copy(w, r); err != nil { >> w.Close() >> os.Remove(dst) >> return fmt.Errorf("copy %s %s: %v", src, dst, err) >> } >> >> if err := w.Close(); err != nil { >> os.Remove(dst) >> return fmt.Errorf("copy %s %s: %v", src, dst, err) >> } >> } >> >> - new approach ratio 5:5 >> func CopyFile(src, dst string) error { >> r, err := os.Open(src) *orelse* return fmt.Errorf("copy %s %s: %v", src, >> dst, err) >> defer r.Close() >> >> w, err := os.Create(dst); *orelse* return fmt.Errorf("copy %s %s: %v", >> src, dst, err) >> err := io.Copy(w, r) *orelse* { >> w.Close() >> os.Remove(dst) >> return fmt.Errorf("copy %s %s: %v", src, dst, err) >> } >> >> err := w.Close() *orelse* { >> os.Remove(dst) >> return fmt.Errorf("copy %s %s: %v", src, dst, err) >> } >> } >> >> On Sunday, July 30, 2023 at 9:27:27 PM UTC-6 Marcello H wrote: >> >>> I think the current error handling is just fine. >>> For the extra typing, they invented keyboard snippets and such. >>> >>> But for this proposal, I would like to see how a return with multiple >>> values would look to get a better understanding. >>> ``` >>> // translate this in the proposed solution? >>> func myFirstFunction() (string, err) { >>> result, err := myFunction() >>> if err != nill { >>> return rest, err >>> } >>> } >>> ``` >>> >>> Op maandag 31 juli 2023 om 04:32:01 UTC+2 schreef DrGo: >>> >>>> Another possibility Jeremy is that the orelse block is executed if any >>>> of the returned error values is not nil. >>>> >>>> On Sunday, July 30, 2023 at 8:14:58 PM UTC-6 DrGo wrote: >>>> >>>>> Thanks... >>>>> yes indeed. Too many requirements but I think this solution comes >>>>> close to meeting them. If a rare function returns more than one error >>>>> value >>>>> (yet to see one in the wild) then the compiler should reject orelse use >>>>> and >>>>> the user can fallback on the (the if err!= nil) approach. >>>>> >>>>> On Sunday, July 30, 2023 at 6:02:57 PM UTC-6 Jeremy French wrote: >>>>> >>>>>> Also, errors are values, which means - although uncommon - a function >>>>>> could return two or more error values. Which would orelse evaluate? >>>>>> Even >>>>>> if you arbitrarily chose one, that would violate the explicit vs >>>>>> implicit >>>>>> code flow principle. >>>>>> >>>>>> My sympathies, OP. I too hate the "if err!= nil" boilerplate, and >>>>>> have suggested my own idea for fixing it, which was similarly dismantled >>>>>> for good reasons by those more knowledgeable than me. The truth is, >>>>>> this >>>>>> problem/issue has so many restrictions placed on it (currently idiomatic >>>>>> principles, backwards compatibility promise, explicit vs implicit, etc) >>>>>> that the set of possible solutions is VERY narrow, possibly infinitely >>>>>> so. >>>>>> >>>>>> On Sunday, July 30, 2023 at 3:51:49 PM UTC-4 Brian Candler wrote: >>>>>> >>>>>> err := io.Copy(w, r) *orelse* { >>>>>> w.Close() >>>>>> os.Remove(dst) >>>>>> return fmt.Errorf("copy %s %s: %v", src, dst, err) >>>>>> } >>>>>> >>>>>> My question still stands. Semantically, what value exactly does the >>>>>> "orelse" condition test is not equal to nil? >>>>>> >>>>>> - does it test the value from the preceding assignment? If so, is >>>>>> "orelse" only valid immediately following an assignment expression? The >>>>>> original posting didn't say this. And if it *is* linked to an >>>>>> assignment >>>>>> expression which assigns multiple values, does it only look at the last >>>>>> value? (Again, that was not specified) >>>>>> >>>>>> - does it always test a variable called "err"? The original posting >>>>>> said it was equivalent to "if err!=nil" but a later post contradicted >>>>>> this >>>>>> >>>>>> - does it test the value from the 'return' expression at the end of >>>>>> the block following orelse? Except in this case, it can't because it's >>>>>> buried inside fmt.Errorf >>>>>> >>>>>> On Sunday, 30 July 2023 at 17:55:34 UTC+1 DrGo wrote: >>>>>> >>>>>> Good point Harri, >>>>>> >>>>>> This is what the correct version will look like using this proposal >>>>>> >>>>>> func CopyFile(src, dst string) error { >>>>>> r, err := os.Open(src) *orelse* return fmt.Errorf("copy %s %s: %v", >>>>>> src, dst, err) >>>>>> defer r.Close() >>>>>> >>>>>> w, err := os.Create(dst); *orelse* return fmt.Errorf("copy %s %s: %v", >>>>>> src, dst, err) >>>>>> err := io.Copy(w, r) *orelse* { >>>>>> w.Close() >>>>>> os.Remove(dst) >>>>>> return fmt.Errorf("copy %s %s: %v", src, dst, err) >>>>>> } >>>>>> >>>>>> err := w.Close() *orelse* { >>>>>> os.Remove(dst) >>>>>> return fmt.Errorf("copy %s %s: %v", src, dst, err) >>>>>> } >>>>>> } >>>>>> >>>>>> In a more complex func, the error formatting/handling code can be >>>>>> further deduplicated by extracting it into a closure. >>>>>> e.g., >>>>>> >>>>>> func CopyFile(src, dst string) error { >>>>>> copyErr:= func(err error) { >>>>>> return fmt.Errorf("copy %s %s: %v", src, dst, err) >>>>>> } >>>>>> r, err := os.Open(src) *orelse* return copyErr(err) >>>>>> defer r.Close() >>>>>> >>>>>> w, err := os.Create(dst); *orelse* return copyErr(err) >>>>>> err := io.Copy(w, r) *orelse* { >>>>>> w.Close() >>>>>> os.Remove(dst) >>>>>> return copyErr(err) >>>>>> } >>>>>> >>>>>> err := w.Close() *orelse* { >>>>>> os.Remove(dst) >>>>>> return copyErr(err) >>>>>> } >>>>>> } >>>>>> >>>>>> On Sunday, July 30, 2023 at 8:17:31 AM UTC-6 Harri L wrote: >>>>>> >>>>>> IMHO, you have used the irrelevant example (== 2nd code block) from >>>>>> Russ Cox's paper. The paper says: >>>>>> > This code is not nice, not clean, not elegant, *and still wrong:* >>>>>> like the previous version, it does not remove dst when io.Copy or >>>>>> w.Close fails. >>>>>> >>>>>> I want to compare your proposal with the third example from the >>>>>> paper, which does (proper) error annotation and cleanup. Thanks. >>>>>> On Sunday, July 30, 2023 at 8:57:15 AM UTC+3 DrGo wrote: >>>>>> >>>>>> I looked at the long list of proposals to improve error handling in >>>>>> go but I have not seen the one I am describing below. If I missed a >>>>>> similar >>>>>> , can you pls direct me to where I can find it. If not what do you think >>>>>> of >>>>>> this approach. >>>>>> >>>>>> This involves introducing a new keyword "orelse" that is a syntactic >>>>>> sugar for an "if err!=nil" block. >>>>>> >>>>>> The example code in Russ Cox's paper[1] will look something like this: >>>>>> >>>>>> func CopyFile(src, dst string) error { >>>>>> >>>>>> r, err := os.Open(src) orelse return err >>>>>> >>>>>> defer r.Close() >>>>>> >>>>>> w, err := os.Create(dst) orelse return err >>>>>> >>>>>> defer w.Close() >>>>>> >>>>>> err = io.Copy(w, r) orelse return err >>>>>> >>>>>> err = w.Close() orelse return err >>>>>> >>>>>> } >>>>>> >>>>>> It is an error to not return an error from an orelse block. >>>>>> >>>>>> In my eyes, this has the same explicitness and flexibility of the >>>>>> current style but is significantly less verbose. It permits ignoring the >>>>>> error, returning it as is or wrapping it. Because orelse is not used for >>>>>> any other purpose, it would be easy for reviewers and linters to spot >>>>>> lack >>>>>> of error handling. >>>>>> >>>>>> It also works well with named returns. e.g., >>>>>> >>>>>> func returnsObjorErro() (obj Obj, err error) { >>>>>> >>>>>> obj, err := createObj() orelse return //returns nil and err >>>>>> >>>>>> } >>>>>> >>>>>> otherwise orelse is like "else" so e.g., it can be followed by a >>>>>> block if additional cleanup or error formatting etc is needed before >>>>>> returning, eg >>>>>> w, err := os.Create(dst) orelse { >>>>>> .... >>>>>> return err >>>>>> } >>>>>> >>>>>> Similarity to "else" hopefully means that it is easy to learn. It is >>>>>> obviously backward compatible >>>>>> >>>>>> What do you think? >>>>>> >>>>>> [1] >>>>>> https://go.googlesource.com/proposal/+/master/design/go2draft-error-handling-overview.md >>>>>> >>>>>> -- >> > You received this message because you are subscribed to the Google Groups >> "golang-nuts" group. >> > To unsubscribe from this group and stop receiving emails from it, send an >> email to golang-nuts...@googlegroups.com. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/golang-nuts/36110a8d-a26f-48be-83fd-73af755e88f4n%40googlegroups.com >> >> <https://groups.google.com/d/msgid/golang-nuts/36110a8d-a26f-48be-83fd-73af755e88f4n%40googlegroups.com?utm_medium=email&utm_source=footer> >> . >> > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/546820c2-bd0a-4943-8eae-ce6b09d9cf3dn%40googlegroups.com.
