go/printer: example for Fprint

R=golang-dev, dsymonds, rsc
CC=golang-dev
https://golang.org/cl/5785057

src/pkg/go/printer/example_test.go

+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package printer_test
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/printer"
+	"go/token"
+	"strings"
+	"testing"
+)
+
+// Dummy test function so that godoc does not use the entire file as example.
+func Test(*testing.T) {}
+
+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 ExampleFprint() {
+	// 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)
+}