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fmt: explain how Formatter interface affects verbs and flags 

Formatter is mentioned further down, but it's helpful
to add it amongst the verbs and flags.

Background: I spent a while puzzling how "%+v" prints
a stack trace for github.com/pkg/errors when this isn't
documented under 'flags'.

Change-Id: Ic70145902a36780147dedca568b3cf482974fc38
GitHub-Last-Rev: 6571b499f2
GitHub-Pull-Request: golang/go#39860
Reviewed-on: https://go-review.googlesource.com/c/go/+/240000
Reviewed-by: Rob Pike <r@golang.org>
Trust: Rob Pike <r@golang.org>
Trust: Ian Lance Taylor <iant@golang.org>
This commit is contained in:
Bryan Boreham 2020-10-12 14:29:28 +00:00 committed by Ian Lance Taylor
parent e08059f4fc
commit 076a45acd5
2 changed files with 6 additions and 5 deletions
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3
src/fmt/doc.go
View File

@ -150,7 +150,8 @@
concrete value that it holds, and printing continues with the next rule.
2. If an operand implements the Formatter interface, it will
be invoked. Formatter provides fine control of formatting.
be invoked. In this case the interpretation of verbs and flags is
controlled by that implementation.
3. If the %v verb is used with the # flag (%#v) and the operand
implements the GoStringer interface, that will be invoked.

8
src/fmt/print.go
View File

@ -47,11 +47,11 @@ type State interface {
Flag(c int) bool
}
// Formatter is the interface implemented by values with a custom formatter.
// The implementation of Format may call Sprint(f) or Fprint(f) etc.
// to generate its output.
// Formatter is implemented by any value that has a Format method.
// The implementation controls how State and rune are interpreted,
// and may call Sprint(f) or Fprint(f) etc. to generate its output.
type Formatter interface {
Format(f State, c rune)
Format(f State, verb rune)
}
// Stringer is implemented by any value that has a String method,