File System Interfaces for Go — Draft Design

Russ Cox
Rob Pike
July 2020

This is a Draft Design, not a formal Go proposal, because it describes a potential large change, with integration changes needed in multiple packages in the standard library as well potentially in third-party packages. The goal of circulating this draft design is to collect feedback to shape an intended eventual proposal.

We are using this change to experiment with new ways to scale discussions about large changes. For this change, we will use a Go Reddit thread to manage Q&A, since Reddit's threading support can easily match questions with answers and keep separate lines of discussion separate.

There is a video presentation of this draft design.

The prototype code is available for trying out.

See also the related embedded files draft design, which builds on this design.


We present a possible design for a new Go standard library package io/fs that defines an interface for read-only file trees. We also present changes to integrate the new package into the standard library.

This package is motivated in part by wanting to add support for embedded files to the go command. See the draft design for embedded files.


A hierarchical tree of named files serves as a convenient, useful abstraction for a wide variety of resources, as demonstrated by Unix, Plan 9, and the HTTP REST idiom. Even when limited to abstracting disk blocks, file trees come in many forms: local operating-system files, files stored on other computers, files in memory, files in other files like ZIP archives.

Go benefits from good abstractions for the data in a single file, such as the io.Reader, io.Writer, and related interfaces. These have been widely implemented and used in the Go ecosystem. A particular Reader or Writer might be an operating system file, a network connection, an in-memory buffer, a file in a ZIP archive, an HTTP response body, a file stored on a cloud server, or many other things. The common, agreed-upon interfaces enable the creation of useful, general operations like compression, encryption, hashing, merging, splitting, and duplication that apply to all these different resources.

Go would also benefit from a good abstraction for a file system tree. Common, agreed-upon interfaces would help connect the many different resources that might be presented as file systems with the many useful generic operations that could be implemented atop the abstraction.

We started exploring the idea of a file system abstraction years ago, with an internal abstraction used in godoc. That code was later extracted as and inspired a handful of similar packages. That interface and its successors seemed too complex to be the right common abstraction, but they helped us learn more about what a design might look like. In the intervening years we've also learned more about how to use interfaces to model more complex resources.

There have been past discussions about file system interfaces on issue 5636 and issue 14106.

This draft design presents a possible official abstraction for a file system tree.


The core of this design is a new package io/fs defining a file system abstraction. Although the initial interface is limited to read-only file systems, the design can be extended to support write operations later, even from third-party packages.

This design also contemplates minor adjustments to the archive/zip, html/template, net/http, os, and text/template packages to better implement or consume the file system abstractions.

The FS interface

The new package io/fs defines an FS type representing a file system:

type FS interface {
	Open(name string) (File, error)

The FS interface defines the minimum requirement for an implementation: just an Open method. As we will see, an FS implementation may also provide other methods to optimize operations or add new functionality, but only Open is required.

(Because the package name is fs, we need to establish a different typical variable name for a generic file system. The prototype code uses fsys, as do the examples in this draft design. The need for such a generic name only arises in code manipulating arbitrary file systems; most client code will use a meaningful name based on what the file system contains, such as styles for a file system containing CSS files.)

File name syntax

All FS implementations use the same name syntax: paths are unrooted, slash-separated sequences of path elements, like Unix paths without the leading slash, or like URLs without the leading http://host/. Also like in URLs, the separator is a forward slash on all systems, even Windows. These names can be manipulated using the path package. FS path names never contain a ‘.’ or ‘..’ element except for the special case that the root directory of a given FS file tree is named ‘.’. Paths may be case-sensitive or not, depending on the implementation, so clients should typically not depend on one behavior or the other.

The use of unrooted names—x/y/z.jpg instead of /x/y/z.jpg—is meant to make clear that the name is only meaningful when interpreted relative to a particular file system root, which is not specified in the name. Put another way, the lack of a leading slash makes clear these are not host file system paths, nor identifiers in some other global name space.

The File interface

The io/fs package also defines a File interface representing an open file:

type File interface {
	Stat() (os.FileInfo, error)
	Read([]byte) (int, error)
	Close() error

The File interface defines the minimum requirements for an implementation. For File, those requirements are Stat, Read, and Close, with the same meanings as for an *os.File. A File implementation may also provide other methods to optimize operations or add new functionality—for example, an *os.File is a valid File implementation—but only these three are required.

If a File represents a directory, then just like an *os.File, the FileInfo returned by Stat will return true from IsDir() (and from Mode().IsDir()). In this case, the File must also implement the ReadDirFile interface, which adds a ReadDir method. The ReadDir method has the same semantics as the *os.File Readdir method, and (later) this design adds ReadDir with a capital D to *os.File.)

// A ReadDirFile is a File that implements the ReadDir method for directory reading.
type ReadDirFile interface {
	ReadDir(n int) ([]os.FileInfo, error)

Extension interfaces and the extension pattern

This ReadDirFile interface is an example of an old Go pattern that we’ve never named before but that we suggest calling an extension interface. An extension interface embeds a base interface and adds one or more extra methods, as a way of specifying optional functionality that may be provided by an instance of the base interface.

An extension interface is named by prefixing the base interface name with the new method: a File with ReadDir is a ReadDirFile. Note that this convention can be viewed as a generalization of existing names like io.ReadWriter and io.ReadWriteCloser. That is, an io.ReadWriter is an io.Writer that also has a Read method, just like a ReadDirFile is a File that also has a ReadDir method.

The io/fs package does not define extensions like ReadAtFile, ReadSeekFile, and so on, to avoid duplication with the io package. Clients are expected to use the io interfaces directly for such operations.

An extension interface can provide access to new functionality not available in a base interface, or an extension interface can also provide access to a more efficient implementation of functionality already available, using additional method calls, using the base interface. Either way, it can be helpful to pair an extension interface with a helper function that uses the optimized implementation if available and falls back to what is possible in the base interface otherwise.

An early example of this extension pattern—an extension interface paired with a helper function—is the io.StringWriter interface and the io.WriteString helper function, which have been present since Go 1:

package io

// StringWriter is the interface that wraps the WriteString method.
type StringWriter interface {
	WriteString(s string) (n int, err error)

// WriteString writes the contents of the string s to w, which accepts a slice of bytes.
// If w implements StringWriter, its WriteString method is invoked directly.
// Otherwise, w.Write is called exactly once.
func WriteString(w Writer, s string) (n int, err error) {
	if sw, ok := w.(StringWriter); ok {
		return sw.WriteString(s)
	return w.Write([]byte(s))

This example deviates from the discussion above in that StringWriter is not quite an extension interface: it does not embed io.Writer. For a single-method interface where the extension method replaces the original one, not repeating the original method can make sense, as here. But in general we do embed the original interface, so that code that tests for the new interface can access the original and new methods using a single variable. (In this case, StringWriter not embedding io.Writer means that WriteString cannot call sw.Write. That's fine in this case, but consider instead if io.ReadSeeker did not exist: code would have to test for io.Seeker and use separate variables for the Read and Seek operations.)

Extensions to FS

File had just one extension interface, in part to avoid duplication with the existing interfaces in io. But FS has a handful.


One common operation is reading an entire file, as ioutil.ReadFile does for operating system files. The io/fs package provides this functionality using the extension pattern, defining a ReadFile helper function supported by an optional ReadFileFS interface:

func ReadFile(fsys FS, name string) ([]byte, error)

The general implementation of ReadFile can call fs.Open to obtain a file of type File, followed by calls to file.Read and a final call to file.Close. But if an FS implementation can provide file contents more efficiently in a single call, it can implement the ReadFileFS interface:

type ReadFileFS interface {
	ReadFile(name string) ([]byte, error)

The top-level func ReadFile first checks to see if its argument fs implements ReadFileFS. If so, func ReadFile calls fs.ReadFile. Otherwise it falls back to the Open, Read, Close sequence.

For concreteness, here is a complete implementation of func ReadFile:

func ReadFile(fsys FS, name string) ([]byte, error) {
	if fsys, ok := fsys.(ReadFileFS); ok {
		return fsys.ReadFile(name)

	file, err := fsys.Open(name)
	if err != nil {
		return nil, err
	defer file.Close()
	return io.ReadAll(file)

(This assumes io.ReadAll exists; see issue 40025.)


We can use the extension pattern again for Stat (analogous to os.Stat):

type StatFS interface {
	Stat(name string) (os.FileInfo, error)

func Stat(fsys FS, name string) (os.FileInfo, error) {
	if fsys, ok := fsys.(StatFS); ok {
		return fsys.Stat(name)

	file, err := fsys.Open(name)
	if err != nil {
		return nil, err
	defer file.Close()
	return file.Stat()


And we can use the extension pattern again for ReadDir (analogous to ioutil.ReadDir):

type ReadDirFS interface {
	ReadDir(name string) ([]os.FileInfo, error)

func ReadDir(fsys FS, name string) ([]os.FileInfo, error)

The implementation follows the pattern, but the fallback case is slightly more complex: it must handle the case where the named file does not implement ReadDirFile by creating an appropriate error to return.


The io/fs package provides a top-level func Walk (analogous to filepath.Walk) built using func ReadDir, but there is not an analogous extension interface.

The semantics of Walk are such that the only significant optimization would be to have access to a fast ReadDir function. An FS implementation can provide that by implementing ReadDirFS. The semantics of Walk are also quite subtle: it is better to have a single correct implementation than buggy custom ones, especially if a custom one cannot provide any significant optimization.

This can still be seen as a kind of extension pattern, but without the one-to-one match: instead of Walk using WalkFS, we have Walk reusing ReadDirFS.


Another convenience function is Glob, analogous to filepath.Glob:

type GlobFS interface {
	Glob(pattern string) ([]string, error)

func Glob(fsys FS, pattern string) ([]string, error)

The fallback case here is not a trivial single call but instead most of a copy of filepath.Glob: it must decide which directories to read, read them, and look for matches.

Although Glob is like Walk in that its implementation is a non-trivial amount of somewhat subtle code, Glob differs from Walk in that a custom implementation can deliver a significant speedup. For example, suppose the pattern is */gopher.jpg. The general implementation has to call ReadDir(".") and then Stat(dir+"/gopher.jpg") for every directory in the list returned by ReadDir. If the FS is being accessed over a network and * matches many directories, this sequence requires many round trips. In this case, the FS could implement a Glob method that answered the call in a single round trip, sending only the pattern and receiving only the matches, avoiding all the directories that don't contain gopher.jpg.

Possible future or third-party extensions

This design is limited to the above operations, which provide basic, convenient, read-only access to a file system. However, the extension pattern can be applied to add any new operations we might want in the future. Even third-party packages can use it; not every possible file system operation needs to be contemplated in io/fs.

For example, the FS in this design provides no support for renaming files. But it could be added easily, using code like:

type RenameFS interface {
	Rename(oldpath, newpath string) error

func Rename(fsys FS, oldpath, newpath string) error {
	if fsys, ok := fsys.(RenameFS); ok {
		return fsys.Rename(oldpath, newpath)

	return fmt.Errorf("rename %s %s: operation not supported", oldpath, newpath)

Note that this code does nothing that requires being in the io/fs package. A third-party package can define its own FS helpers and extension interfaces.

The FS in this design also provides no way to open a file for writing. Again, this could be done with the extension pattern, even from a different package. If done from a different package, the code might look like:

type OpenFileFS interface {
	OpenFile(name string, flag int, perm os.FileMode) (fs.File, error)

func OpenFile(fsys FS, name string, flag int, perm os.FileMode) (fs.File, error) {
	if fsys, ok := fsys.(OpenFileFS); ok {
		return fsys.OpenFile(name, flag, perm)

	if flag == os.O_RDONLY {
		return fs.Open(name)
	return fmt.Errorf("open %s: operation not supported", name)

Note that even if this pattern were implemented in multiple other packages, they would still all interoperate (provided the method signatures matched, which is likely, since package os has already defined the canonical names and signatures). The interoperation results from the implementations all agreeing on the shared file system type and file type: fs.FS and fs.File.

The extension pattern can be applied to any missing operation: Chmod, Chtimes, Mkdir, MkdirAll, Sync, and so on. Instead of putting them all in io/fs, the design starts small, with read-only operations.

Adjustments to os

As presented above, the io/fs package needs to import os for the os.FileInfo interface and the os.FileMode type. These types do not really belong in os, but we had no better home for them when they were introduced. Now, io/fs is a better home, and they should move there.

This design moves os.FileInfo and os.FileMode into io/fs, redefining the names in os as aliases for the definitions in io/fs. The FileMode constants, such as ModeDir, would move as well, redefining the names in os as constants copying the io/fs values. No user code will need updating, but the move will make it possible to implement an fs.FS by importing only io/fs, not os. This is analogous to io not depending on os. (For more about why io should not depend on os, see “Codebase Refactoring (with help from Go)”, especially section 3.)

For the same reason, the type os.PathError should move to io/fs, with a forwarding type alias left behind.

The general file system errors ErrInvalid, ErrPermission, ErrExist, ErrNotExist, and ErrClosed should also move to io/fs. In this case, those are variables, not types, so no aliases are needed. The definitions left behind in package os would be:

package os

import "io/fs"

var (
	ErrInvalid    = fs.ErrInvalid
	ErrPermission = fs.ErrPermission

To match fs.ReadDirFile and fix casing, the design adds new os.File methods ReadDir and ReadDirNames, equivalent to the existing Readdir and Readdirnames. The old casings should have been corrected long ago; correcting them now in os.File is better than requiring all implementations of fs.File to use the wrong names. (Adding ReadDirNames is not strictly necessary, but we might as well fix them both at the same time.)

Finally, as code starts to be written that expects an fs.FS interface, it will be natural to want an fs.FS backed by an operating system directory. This design adds a new function os.DirFS:

package os

// DirFS returns an fs.FS implementation that
// presents the files in the subtree rooted at dir.
func DirFS(dir string) fs.FS

Note that this function can only be written once the FileInfo type moves into io/fs, so that os can import io/fs instead of the other way around.

Adjustments to html/template and text/template

The html/template and text/template packages each provide a pair of methods reading from the operating system's file system:

func (t *Template) ParseFiles(filenames ...string) (*Template, error)
func (t *Template) ParseGlob(pattern string) (*Template, error)

The design adds one new method:

func (t *template) ParseFS(fsys fs.FS, patterns ...string) (*Template, error)

Nearly all file names are glob patterns matching only themselves, so a single call should suffice instead of having to introduce both ParseFilesFS and ParseGlobFS.

TODO mention top-level calls

Adjustments to net/http

The net/http package defines its own FileSystem and File types, used by http.FileServer:

type FileSystem interface {
	Open(name string) (File, error)

type File interface {
	Readdir(count int) ([]os.FileInfo, error)
	Stat() (os.FileInfo, error)

func FileServer(root FileSystem) Handler

If io/fs had come before net/http, this code could use io/fs directly, removing the need to define those interfaces. Since they already exist, they must be left for compatibility.

The design adds an equivalent to FileServer but for an fs.FS:

func HandlerFS(fsys fs.FS) Handler

The HandlerFS requires of its file system that the opened files support Seek. This is an additional requirement made by HTTP, to support range requests. Not all file systems need to implement Seek.

Adjustments to archive/zip

Any Go type that represents a tree of files should implement fs.FS.

The current zip.Reader has no Open method, so this design adds one, with the signature needed to implement fs.FS. Note that the opened files are streams of bytes decompressed on the fly. They can be read, but not seeked. This means a zip.Reader now implements fs.FS and therefore can be used as a source of templates passed to html/template. While the same zip.Reader can also be passed to net/http using http.HandlerFS—that is, such a program would type-check—the HTTP server would not be able to serve range requests on those files, for lack of a Seek method.

On the other hand, for a small set of files, it might make sense to define file system middleware that cached copies of the underlying files in memory, providing seekability and perhaps increased performance, in exchange for higher memory usage. Such middleware—some kind of CachingFS—could be provided in a third-party package and then used to connect the zip.Reader to an http.HandlerFS. Indeed, enabling that kind of middleware is a key goal for this draft design. Another example might be transparent decryption of the underlying files.

Adjustments to archive/tar (none)

The design does not include changes to archive/tar, because that format cannot easily support random access: the first call to Open would have to read the entire archive to find all its files, caching the list for future calls. And that‘s only even possible if the underlying io.Reader supports Seek or ReadAt. That’s a lot of work for an implementation that would be fairly inefficient; adding it to the standard library would be setting a performance trap. If needed, the functionality could be provided by a third-party package instead.


Why now?

The rationale for the specific design decisions is given along with those decisions above. But there have been discussions about a file system interface for many years, with no progress. Why now?

Two things have changed since those early discussions.

First, we have a direct need for the functionality in the standard library, and necessity remains the mother of invention. The embedded files draft design aims to add direct support for embedded files to the go command, which raises the question of how to integrate them with the rest of the standard library. For example, a common use for embedded files is to parse them as templates or serve them directly over HTTP. Without this design, we'd need to define specific methods in those packages for accepting embedded files. Defining a file system interface lets us instead add general new methods that will apply not just to embedded files but also ZIP files and any other kind of resource presented as an FS implementation.

Second, we have more experience with how to use optional interfaces well. Previous attempts at file system interfaces floundered in the complexity of defining a complete set of operations. The results were unwieldy to implement. This design reduces the necessary implementation to an absolute minimum, with the extension pattern allowing the provision of new functionality, even by third-party packages.

Why not http.FileServer?

The http.FileServer and http.File interfaces are clearly one of the inspirations for the new fs.FS and fs.File, and they have been used beyond HTTP. But they are not quite right: every File need not be required to implement Seek and Readdir. As noted earlier, text/template and html/template are perfectly happy reading from a collection of non-seekable files (for example, a ZIP archive). It doesn‘t make sense to impose HTTP’s requirements on all file systems.

If we are to encourage use of a general interface well beyond HTTP, it is worth getting right; the cost is only minimal adaptation of existing http.FileServer implementations. It should also be easy to write general adapters in both directions.

Why not in

New API sometimes starts in; for example, context was originally That‘s not an option here, because one of the key parts of the design is to define good integrations with the standard library, and those APIs can’t expose references (At that point, the APIs might as well be in the standard library.)


This is all new API. There are no conflicts with the compatibility guidelines.

If we'd had io/fs before Go 1, some API might have been avoided.


A prototype implementation is available.