printer

Documentation

Overview

Package printer implements printing of AST nodes.

Index

• func Fprint(output io.Writer, fset *token.FileSet, node any) error
• type CommentedNode
• type Config
  • func (cfg *Config) Fprint(output io.Writer, fset *token.FileSet, node any) error
• type Mode

Examples

• Fprint

Functions

func Fprint

func Fprint(output io.Writer, fset *token.FileSet, node any) error

Fprint "pretty-prints" an AST node to output. It calls Config.Fprint with default settings. Note that gofmt uses tabs for indentation but spaces for alignment; use format.Node (package go/format) for output that matches gofmt.

Example

package main

import (
	"bytes"
	"fmt"
	"go/ast"
	"go/parser"
	"go/printer"
	"go/token"
	"strings"
)

func parseFunc(filename, functionname string) (fun *ast.FuncDecl, fset *token.FileSet) {
	fset = token.NewFileSet()
	if file, err := parser.ParseFile(fset, filename, nil, 0); err == nil {
		for _, d := range file.Decls {
			if f, ok := d.(*ast.FuncDecl); ok && f.Name.Name == functionname {
				fun = f
				return
			}
		}
	}
	panic("function not found")
}

func printSelf() {
	// Parse source file and extract the AST without comments for
	// this function, with position information referring to the
	// file set fset.
	funcAST, fset := parseFunc("example_test.go", "printSelf")

	// Print the function body into buffer buf.
	// The file set is provided to the printer so that it knows
	// about the original source formatting and can add additional
	// line breaks where they were present in the source.
	var buf bytes.Buffer
	printer.Fprint(&buf, fset, funcAST.Body)

	// Remove braces {} enclosing the function body, unindent,
	// and trim leading and trailing white space.
	s := buf.String()
	s = s[1 : len(s)-1]
	s = strings.TrimSpace(strings.ReplaceAll(s, "\n\t", "\n"))

	// Print the cleaned-up body text to stdout.
	fmt.Println(s)
}

func main() {
	printSelf()

}

Output:

funcAST, fset := parseFunc("example_test.go", "printSelf")

var buf bytes.Buffer
printer.Fprint(&buf, fset, funcAST.Body)

s := buf.String()
s = s[1 : len(s)-1]
s = strings.TrimSpace(strings.ReplaceAll(s, "\n\t", "\n"))

fmt.Println(s)

Types

type CommentedNode

type CommentedNode struct {
	Node     any // *ast.File, or ast.Expr, ast.Decl, ast.Spec, or ast.Stmt
	Comments []*ast.CommentGroup
}

A CommentedNode bundles an AST node and corresponding comments. It may be provided as argument to any of the Fprint functions.

type Config

type Config struct {
	Mode     Mode // default: 0
	Tabwidth int  // default: 8
	Indent   int  // default: 0 (all code is indented at least by this much)
}

A Config node controls the output of Fprint.

func (*Config) Fprint

func (cfg *Config) Fprint(output io.Writer, fset *token.FileSet, node any) error

Fprint "pretty-prints" an AST node to output for a given configuration cfg. Position information is interpreted relative to the file set fset. The node type must be *ast.File, *CommentedNode, []ast.Decl, []ast.Stmt, or assignment-compatible to ast.Expr, ast.Decl, ast.Spec, or ast.Stmt.

type Mode

type Mode uint

A Mode value is a set of flags (or 0). They control printing.

const (
	RawFormat Mode = 1 << iota // do not use a tabwriter; if set, UseSpaces is ignored
	TabIndent                  // use tabs for indentation independent of UseSpaces
	UseSpaces                  // use spaces instead of tabs for alignment
	SourcePos                  // emit //line directives to preserve original source positions
)