src/go/ast/print.go

// 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 is a [FieldFilter] that 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.Pointer, 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 any, f FieldFilter) error {
	return fprint(w, fset, x, f)
}

func fprint(w io.Writer, fset *token.FileSet, x any, f FieldFilter) (err error) {
	// setup printer
	p := printer{
		output: w,
		fset:   fset,
		filter: f,
		ptrmap: make(map[any]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 any) error {
	return Fprint(os.Stdout, fset, x, NotNilFilter)
}

type printer struct {
	output io.Writer
	fset   *token.FileSet
	filter FieldFilter
	ptrmap map[any]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 ...any) {
	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.Pointer:
		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)
	}
}