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// Copyright 2010 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.
// This file contains printing support for ASTs.
package ast
import (
"fmt"
"go/token"
"io"
"os"
"reflect"
)
// A FieldFilter may be provided to Fprint to control the output.
type FieldFilter func(name string, value reflect.Value) bool
// NotNilFilter returns true for field values that are not nil;
// it returns false otherwise.
func NotNilFilter(_ string, v reflect.Value) bool {
switch v.Kind() {
case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice:
return !v.IsNil()
}
return true
}
// Fprint prints the (sub-)tree starting at AST node x to w.
// If fset != nil, position information is interpreted relative
// to that file set. Otherwise positions are printed as integer
// values (file set specific offsets).
//
// A non-nil FieldFilter f may be provided to control the output:
// struct fields for which f(fieldname, fieldvalue) is true are
// printed; all others are filtered from the output. Unexported
// struct fields are never printed.
func Fprint(w io.Writer, fset *token.FileSet, x interface{}, f FieldFilter) error {
return fprint(w, fset, x, f)
}
func fprint(w io.Writer, fset *token.FileSet, x interface{}, f FieldFilter) (err error) {
// setup printer
p := printer{
output: w,
fset: fset,
filter: f,
ptrmap: make(map[interface{}]int),
last: '\n', // force printing of line number on first line
}
// install error handler
defer func() {
if e := recover(); e != nil {
err = e.(localError).err // re-panics if it's not a localError
}
}()
// print x
if x == nil {
p.printf("nil\n")
return
}
p.print(reflect.ValueOf(x))
p.printf("\n")
return
}
// Print prints x to standard output, skipping nil fields.
// Print(fset, x) is the same as Fprint(os.Stdout, fset, x, NotNilFilter).
func Print(fset *token.FileSet, x interface{}) error {
return Fprint(os.Stdout, fset, x, NotNilFilter)
}
type printer struct {
output io.Writer
fset *token.FileSet
filter FieldFilter
ptrmap map[interface{}]int // *T -> line number
indent int // current indentation level
last byte // the last byte processed by Write
line int // current line number
}
var indent = []byte(". ")
func (p *printer) Write(data []byte) (n int, err error) {
var m int
for i, b := range data {
// invariant: data[0:n] has been written
if b == '\n' {
m, err = p.output.Write(data[n : i+1])
n += m
if err != nil {
return
}
p.line++
} else if p.last == '\n' {
_, err = fmt.Fprintf(p.output, "%6d ", p.line)
if err != nil {
return
}
for j := p.indent; j > 0; j-- {
_, err = p.output.Write(indent)
if err != nil {
return
}
}
}
p.last = b
}
if len(data) > n {
m, err = p.output.Write(data[n:])
n += m
}
return
}
// localError wraps locally caught errors so we can distinguish
// them from genuine panics which we don't want to return as errors.
type localError struct {
err error
}
// printf is a convenience wrapper that takes care of print errors.
func (p *printer) printf(format string, args ...interface{}) {
if _, err := fmt.Fprintf(p, format, args...); err != nil {
panic(localError{err})
}
}
// Implementation note: Print is written for AST nodes but could be
// used to print arbitrary data structures; such a version should
// probably be in a different package.
//
// Note: This code detects (some) cycles created via pointers but
// not cycles that are created via slices or maps containing the
// same slice or map. Code for general data structures probably
// should catch those as well.
func (p *printer) print(x reflect.Value) {
if !NotNilFilter("", x) {
p.printf("nil")
return
}
switch x.Kind() {
case reflect.Interface:
p.print(x.Elem())
case reflect.Map:
p.printf("%s (len = %d) {", x.Type(), x.Len())
if x.Len() > 0 {
p.indent++
p.printf("\n")
for _, key := range x.MapKeys() {
p.print(key)
p.printf(": ")
p.print(x.MapIndex(key))
p.printf("\n")
}
p.indent--
}
p.printf("}")
case reflect.Ptr:
p.printf("*")
// type-checked ASTs may contain cycles - use ptrmap
// to keep track of objects that have been printed
// already and print the respective line number instead
ptr := x.Interface()
if line, exists := p.ptrmap[ptr]; exists {
p.printf("(obj @ %d)", line)
} else {
p.ptrmap[ptr] = p.line
p.print(x.Elem())
}
case reflect.Array:
p.printf("%s {", x.Type())
if x.Len() > 0 {
p.indent++
p.printf("\n")
for i, n := 0, x.Len(); i < n; i++ {
p.printf("%d: ", i)
p.print(x.Index(i))
p.printf("\n")
}
p.indent--
}
p.printf("}")
case reflect.Slice:
if s, ok := x.Interface().([]byte); ok {
p.printf("%#q", s)
return
}
p.printf("%s (len = %d) {", x.Type(), x.Len())
if x.Len() > 0 {
p.indent++
p.printf("\n")
for i, n := 0, x.Len(); i < n; i++ {
p.printf("%d: ", i)
p.print(x.Index(i))
p.printf("\n")
}
p.indent--
}
p.printf("}")
case reflect.Struct:
t := x.Type()
p.printf("%s {", t)
p.indent++
first := true
for i, n := 0, t.NumField(); i < n; i++ {
// exclude non-exported fields because their
// values cannot be accessed via reflection
if name := t.Field(i).Name; IsExported(name) {
value := x.Field(i)
if p.filter == nil || p.filter(name, value) {
if first {
p.printf("\n")
first = false
}
p.printf("%s: ", name)
p.print(value)
p.printf("\n")
}
}
}
p.indent--
p.printf("}")
default:
v := x.Interface()
switch v := v.(type) {
case string:
// print strings in quotes
p.printf("%q", v)
return
case token.Pos:
// position values can be printed nicely if we have a file set
if p.fset != nil {
p.printf("%s", p.fset.Position(v))
return
}
}
// default
p.printf("%v", v)
}
}
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