I've encountered some go code that appears to use %e for formatting an error for display to the screen. A simplified version would be code like this
err := errors.New("La de da")
fmt.Printf("%e
", err)
outputs
&{%!e(string=La de da)}
However, if I look at the go manual, it says %e is for formatting floating point numbers in scientific notation. That output doesn't look like scientific notation, so I'm wondering
If this is a specific notation, what is it? (i.e. is there a %. formatting option I could use to get that format)
If it's not a specific notation, what weird thing is going on under the hood that leads to an error being rendered in this way?
What silly, obvious thing am I missing that renders most of what I've said in this post wrong?
Read the Go documentation.
Printing
Format errors:
If an invalid argument is given for a verb, such as providing a string to %d, the generated string will contain a description of the problem, as in these examples:
Wrong type or unknown verb: %!verb(type=value) Printf("%d", hi): %!d(string=hi) Too many arguments: %!(EXTRA type=value) Printf("hi", "guys"): hi%!(EXTRA string=guys) Too few arguments: %!verb(MISSING) Printf("hi%d"): hi%!d(MISSING) Non-int for width or precision: %!(BADWIDTH) or %!(BADPREC) Printf("%*s", 4.5, "hi"): %!(BADWIDTH)hi Printf("%.*s", 4.5, "hi"): %!(BADPREC)hi Invalid or invalid use of argument index: %!(BADINDEX) Printf("%*[2]d", 7): %!d(BADINDEX) Printf("%.[2]d", 7): %!d(BADINDEX)All errors begin with the string "%!" followed sometimes by a single character (the verb) and end with a parenthesized description.
For your example,
package main
import (
"errors"
"fmt"
)
func main() {
err := errors.New("La de da")
fmt.Printf("%e
", err)
}
Playground: https://play.golang.org/p/NKC6WWePyxM
Output:
&{%!e(string=La de da)}
Documentation:
All errors begin with the string "%!" followed sometimes by a single character (the verb) and end with a parenthesized description.
Wrong type or unknown verb: %!verb(type=value) Printf("%d", hi): %!d(string=hi)