From 076a45acd5b8d2ce08a2dbe898dc9228554db92d Mon Sep 17 00:00:00 2001 From: Bryan Boreham Date: Mon, 12 Oct 2020 14:29:28 +0000 Subject: [PATCH] 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: 6571b499f211a2266812af66dd3b88dff602cabf GitHub-Pull-Request: golang/go#39860 Reviewed-on: https://go-review.googlesource.com/c/go/+/240000 Reviewed-by: Rob Pike Trust: Rob Pike Trust: Ian Lance Taylor --- src/fmt/doc.go | 3 ++- src/fmt/print.go | 8 ++++---- 2 files changed, 6 insertions(+), 5 deletions(-) diff --git a/src/fmt/doc.go b/src/fmt/doc.go index a7115809d3..d05ee519c3 100644 --- a/src/fmt/doc.go +++ b/src/fmt/doc.go @@ -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. diff --git a/src/fmt/print.go b/src/fmt/print.go index 778b5b0938..8bc225f548 100644 --- a/src/fmt/print.go +++ b/src/fmt/print.go @@ -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, -- 2.50.0