Print the stack trace when formatting with %+v #28
Conversation
| // helper for Format | ||
| type unformatter Err | ||
|
|
||
| func (unformatter) Format() { /* break the fmt.Formatter interface */ } |
There was a problem hiding this comment.
well, in that case it would fulfill the fmt.Formatter interface.
By adding a method with an incompatible type signature we're breaking the interface.
This allows us to do:
fmt.Fprintf(s, "%#v", (*unformatter)(e))
otherwise fmt.Fprintf will do a runtime type assertion e.(fmt.Formatter), which will succeed and cause infinite recursion here.
@mjs, do you know of a better way to preserve the default rendering that the object would have had with %#v?
There was a problem hiding this comment.
There's no need to define a method - the unformatter type has no methods, because a newly defined named type in Go has no methods (other than those from embedded types).
I'd suggest defining the unformatter type inline inside Err.Format.
There was a problem hiding this comment.
yes indeed! thanks!
I believe I confused it with the embedding rules.
|
@howbazaar, @mjs friendly ping |
|
|
|
Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju-errors |
|
Build failed: Tests failed |
| // helper for Format | ||
| type unformatter Err | ||
|
|
||
| func (unformatter) Format() { /* break the fmt.Formatter interface */ } |
There was a problem hiding this comment.
There's no need to define a method - the unformatter type has no methods, because a newly defined named type in Go has no methods (other than those from embedded types).
I'd suggest defining the unformatter type inline inside Err.Format.
| fallthrough | ||
| case 's': | ||
| fmt.Fprintf(s, "%s", e.Error()) | ||
| } |
There was a problem hiding this comment.
Please add a default case so that we get some indication when someone uses the wrong verb.
There was a problem hiding this comment.
also added support for %T
| } | ||
| fallthrough | ||
| case 's': | ||
| fmt.Fprintf(s, "%s", e.Error()) |
There was a problem hiding this comment.
s.Write([]byte(e.Error()))
?
There was a problem hiding this comment.
while we're at it, let's use io.WriteString
| case s.Flag('#'): | ||
| // avoid infinite recursion by wrapping e into a type | ||
| // that doesn't implement Formatter. | ||
| fmt.Fprintf(s, "%#v", (*unformatter)(e)) |
There was a problem hiding this comment.
This isn't going to produce quite what you want, I think - it'll show the "unformatter" type name, which will be quite unexpected.
It depends how useful we think the default %#v output is - personally, I think I'd write errors.Details(e) here, which means that you'd get reasonable default output when %#v is used (for example when gopkg.in/check.v1 prints a value).
There was a problem hiding this comment.
no strong opinions here; my expectations for %#v was to show the actual thing without any smart reformatting, i.e. as the doc says:
%#v a Go-syntax representation of the value
So currently, with gopkg.in/check.v1 you see the guts of juju/errors:
err := fmt.Errorf("foo")
c.Check(err, Equals, 42)
err = errors.Trace(err)
c.Check(err, Equals, 42)
... go test:
foo_test.go:21:
c.Check(err, Equals, 42)
... obtained *errors.errorString = &errors.errorString{s:"foo"} ("foo")
... expected int = 42
foo_test.go:23:
c.Check(err, Equals, 42)
... obtained *errors.Err = &errors.Err{message:"", cause:(*errors.errorString)(0xc42000ef60), previous:(*errors.errorString)(0xc42000ef60), file:"pippo/foo_test.go", line:22} ("foo")
... expected int = 42
With this PR, the latter will become:
&errors.unformatter{message:"", cause:(*errors.errorString)(0xc42000ef60), previous:(*errors.errorString)(0xc42000ef60), file:"pippo/foo_test.go", line:22}
IMHO it's a bit ugly but at least it's not hiding anything.
@rogpeppe: Still prefer errors.Details(e) ?
There was a problem hiding this comment.
@rogpeppe: or what about this: mangle the output so that it outputs &errors.Err{ exactly as before
Yeah, makes sense. I'll add some test |
|
jujubot trigger happy :-) |
Discovered in:
http://dave.cheney.net/2016/06/12/stack-traces-and-the-errors-package
Code based on:
https://github.com/pkg/errors
With a minor but hackish change to still allow %#v to print the structure fields, in order to not break the principle of least surprise. Let me know if there is a better way to strip a type of an interface or invoke the default %#v behaviour on a type that implements fmt.Formatter.
NOTE: this is the continuation of #26. I no longer work for cesanta and thus cannot address comments in the original PR.
#26 received the green light from @mjs, and had an open question by @howbazaar, whom I replied to.