Source: print.go in package golang.org/x/mod/modfile

Module file printer.


package modfile

import (
	"bytes"
	"fmt"
	"strings"
)

Format returns a go.mod file as a byte slice, formatted in standard style.

func Format(f *FileSyntax) []byte {
	pr := &printer{}
	pr.file(f)
	return pr.Bytes()
}

A printer collects the state during printing of a file or expression.

type printer struct {
	bytes.Buffer           // output buffer
	comment      []Comment // pending end-of-line comments
	margin       int       // left margin (indent), a number of tabs
}

printf prints to the buffer.

func (p *printer) printf(format string, args ...interface{}) {
	fmt.Fprintf(p, format, args...)
}

indent returns the position on the current line, in bytes, 0-indexed.

func (p *printer) indent() int {
	b := p.Bytes()
	n := 0
	for n < len(b) && b[len(b)-1-n] != '\n' {
		n++
	}
	return n
}

newline ends the current line, flushing end-of-line comments.

func (p *printer) newline() {
	if len(p.comment) > 0 {
		p.printf(" ")
		for i, com := range p.comment {
			if i > 0 {
				p.trim()
				p.printf("\n")
				for i := 0; i < p.margin; i++ {
					p.printf("\t")
				}
			}
			p.printf("%s", strings.TrimSpace(com.Token))
		}
		p.comment = p.comment[:0]
	}

	p.trim()
	p.printf("\n")
	for i := 0; i < p.margin; i++ {
		p.printf("\t")
	}
}

trim removes trailing spaces and tabs from the current line.

Remove trailing spaces and tabs from line we're about to end.

	b := p.Bytes()
	n := len(b)
	for n > 0 && (b[n-1] == '\t' || b[n-1] == ' ') {
		n--
	}
	p.Truncate(n)
}

file formats the given file into the print buffer.

func (p *printer) file(f *FileSyntax) {
	for _, com := range f.Before {
		p.printf("%s", strings.TrimSpace(com.Token))
		p.newline()
	}

	for i, stmt := range f.Stmt {
		switch x := stmt.(type) {

comments already handled

			p.expr(x)

		default:
			p.expr(x)
			p.newline()
		}

		for _, com := range stmt.Comment().After {
			p.printf("%s", strings.TrimSpace(com.Token))
			p.newline()
		}

		if i+1 < len(f.Stmt) {
			p.newline()
		}
	}
}

Emit line-comments preceding this expression.

Want to print a line comment. Line comments must be at the current margin.

		p.trim()

There's other text on the line. Start a new line.

			p.printf("\n")

Re-indent to margin.

		for i := 0; i < p.margin; i++ {
			p.printf("\t")
		}
		for _, com := range before {
			p.printf("%s", strings.TrimSpace(com.Token))
			p.newline()
		}
	}

	switch x := x.(type) {
	default:
		panic(fmt.Errorf("printer: unexpected type %T", x))

done


	case *LParen:
		p.printf("(")
	case *RParen:
		p.printf(")")

	case *Line:
		p.tokens(x.Token)

	case *LineBlock:
		p.tokens(x.Token)
		p.printf(" ")
		p.expr(&x.LParen)
		p.margin++
		for _, l := range x.Line {
			p.newline()
			p.expr(l)
		}
		p.margin--
		p.newline()
		p.expr(&x.RParen)
	}

Queue end-of-line comments for printing when we reach the end of the line.

	p.comment = append(p.comment, x.Comment().Suffix...)
}

func (p *printer) tokens(tokens []string) {
	sep := ""
	for _, t := range tokens {
		if t == "," || t == ")" || t == "]" || t == "}" {
			sep = ""
		}
		p.printf("%s%s", sep, t)
		sep = " "
		if t == "(" || t == "[" || t == "{" {
			sep = ""
		}
	}