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Package printer

import "go/printer"
Overview
Index
Examples

Overview ▾

Package printer implements printing of AST nodes.

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

Code:

package printer_test

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 ExampleFprint() {
    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)
}

type CommentedNode

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

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

type Config

A Config node controls the output of Fprint. 

type Config struct {
    Mode     Mode // default: 0
    Tabwidth int  // default: 8
    Indent   int  // default: 0 (all code is indented at least by this much); added in Go 1.1
}

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

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

type Mode uint
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
)