@Miguel Angel Rivera Notararigo
Thanks for taking the time to write...
In my proposal, people are free to add as much context as they want... but
as a demonstration, I am using the example from
Ross Cox's paper on error handling that is used by all error handling
proposals to show case their approach. I do not think Ross will
take your criticism personally :) I on the other hand take exception to
your generalization re people who complain about error handling in Go.
I am sure you did not make that claim without having some sort of solid
research to support it. But, Go designers themselves admit that this is an
issue and have written
tons on it.
In one or two replies above we were discussing how error handling opinions
can become religions each with its own priests who think they are the only
saved faction, and that their rituals are the only right approach for all
situations.
Best wishes,
On Thursday, August 3, 2023 at 9:07:52 PM UTC-6 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 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
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