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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 interface{}) error

Fprint "pretty-prints" an AST node to output. It calls Config.Fprint with default settings.

▾ Example

Code:

// 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", "ExampleFprint")

// 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.Replace(s, "\n\t", "\n", -1))

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

Output:

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

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

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

fmt.Println(s)

type CommentedNode

type CommentedNode struct {
    Node     interface{} // *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 interface{}) 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 comments to preserve original source positions
)