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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