## Introduction to Go 1

For a full explanation of the motivation and design of Go 1, see XXX. Here follows a summary.

Go 1 is intended to be a stable language and core library set that will form a reliable foundation for people and organizations that want to make a long-term commitment to developing in the Go programming language. Go will continue to develop, but in a way that guarantees code written to the Go 1 specification will continue to work. For instance, Go 1 will be a supported platform on Google App Engine for the next few years. Incompatible changes to the environment, should they arise, will be done in a distinct version.

This document describes the changes in the language and libraries in Go 1, relative to the previous release, r60 (at the time of writing, tagged as r60.3). It also explains how to update code at r60 to compile and run under Go 1. Finally, it outlines the new `go` command for building Go programs and the new binary release process being introduced. Most of these topics have more thorough presentations elsewhere; such documents are linked below.

## Changes to the language

### Append

The `append` built-in function is variadic, so one can append to a byte slice using the `...` syntax in the call.

        greeting := []byte{}
        greeting = append(greeting, []byte("hello ")...)

By analogy with the similar property of `copy`, Go 1 permits a string to be appended (byte-wise) directly to a byte slice; the conversion is no longer necessary:

        greeting = append(greeting, "world"...)

Updating: This is a new feature, so existing code needs no changes.

### Close

The `close` built-in function lets a sender tell a receiver that no more data will be transmitted on the channel. In Go 1 the type system enforces the directionality when possible: it is illegal to call `close` on a receive-only channel:

        var c chan int
        var csend chan<- int = c
        var crecv <-chan int = c
        close(c)     // legal
        close(csend) // legal
        close(crecv) // illegal

Updating: Existing code that attempts to close a receive-only channel was erroneous even before Go 1 and should be fixed. The compiler will now reject such code.

### Composite literals

In Go 1, a composite literal of array, slice, or map type can elide the type specification for the elements' initializers if they are of pointer type. All four of the initializations in this example are legal; the last one was illegal before Go 1.

        type Date struct {
            month string
            day   int
        }
        // Struct values, fully qualified; always legal.
        holiday1 := []Date{
            Date{"Feb", 14},
            Date{"Nov", 11},
            Date{"Dec", 25},
        }
        // Struct values, type name elided; always legal.
        holiday2 := []Date{
            {"Feb", 14},
            {"Nov", 11},
            {"Dec", 25},
        }
        // Pointers, fully qualified, always legal.
        holiday3 := []*Date{
            &Date{"Feb", 14},
            &Date{"Nov", 11},
            &Date{"Dec", 25},
        }
        // Pointers, type name elided; legal in Go 1.
        holiday4 := []*Date{
            {"Feb", 14},
            {"Nov", 11},
            {"Dec", 25},
        }

Updating: This change has no effect on existing code, but the command `gofmt` `-s` applied to existing source will, among other things, elide explicit element types wherever permitted.

### Goroutines during init

Go 1 allows goroutines to be created and run during initialization. (They used to be created but were not run until after initialization completed.) Code that uses goroutines can now be called from `init` routines and global initialization expressions without introducing a deadlock.

    var PackageGlobal int

    func init() {
        c := make(chan int)
        go initializationFunction(c)
        PackageGlobal = <-c
    }

Updating: This is a new feature, so existing code needs no changes, although it's possible that code that depends on goroutines not starting before `main` will break. There was no such code in the standard repository.

### The rune type

Go 1 introduces a new basic type, `rune`, to be used to represent individual Unicode code points. It is an alias for `int32`, analogous to `byte` as an alias for `uint8`.

Character literals such as `'a'`, `'語'`, and `'\u0345'` now have default type `rune`, analogous to `1.0` having default type `float64`. A variable initialized to a character constant will therefore have type `rune` unless otherwise specified.

Libraries have been updated to use `rune` rather than `int` when appropriate. For instance, the functions `unicode.ToLower` and relatives now take and return a `rune`.

        delta := 'δ' // delta has type rune.
        var DELTA rune
        DELTA = unicode.ToUpper(delta)
        epsilon := unicode.ToLower(DELTA + 1)
        if epsilon != 'δ'+1 {
            log.Fatal("inconsistent casing for Greek")
        }

Updating: Most source code will be unaffected by this because the type inference from `:=` initializers introduces the new type silently, and it propagates from there. Some code may get type errors that a trivial conversion will resolve.

### The error type

Go 1 introduces a new built-in type, `error`, which has the following definition:

        type error interface {
            Error() string
        }

Since the consequences of this type are all in the package library, it is discussed below.

### Deleting from maps

The original syntax for deleting an element in a map was:

        m[k] = ignored, false

In Go 1, that syntax has gone; instead there is a new built-in function, `delete`. The call

        delete(m, k)

will delete the map entry retrieved by the expression `m[k]`. There is no return value. Deleting a non-existent entry is a no-op.

Updating: Running `go fix` will convert expressions of the form `m[k] = ignored, false` into `delete(m, k)` when it is clear that the ignored value can be safely discarded from the program and `false` refers to the predefined boolean constant. The fix tool will flag other uses of the syntax for inspection by the programmer.

### Iterating in maps

In Go 1, the order in which elements are visited when iterating over a map using a `for` `range` statement is defined to be unpredictable, even if the same loop is run multiple times with the same map. Code should not assume that the elements are visited in any particular order.

        m := map[string]int{"Sunday": 0, "Monday": 1}
        for name, value := range m {
            // This loop should not assume Sunday will be visited first.
            f(name, value)
        }

Updating: This is one change where tools cannot help. Most existing code will be unaffected, but some programs may break or misbehave; we recommend manual checking of all range statements over maps to verify they do not depend on iteration order. There were a few such examples in the standard repository; they have been fixed. Note that it was already incorrect to depend on the iteration order, which was unspecified. This change codifies the unpredictability.

### Multiple assignment

Go 1 fully specifies the evaluation order in multiple assignment statements. In particular, if the left-hand side of the assignment statement contains expressions that require evaluation, such as function calls or array indexing operations, these will all be done using the usual left-to-right rule before any variables are assigned their value. Once everything is evaluated, the actual assignments proceed in left-to-right order.

These examples illustrate the behavior.

        sa := []int{1, 2, 3}
        i := 0
        i, sa[i] = 1, 2 // sets i = 1, sa[0] = 2

        sb := []int{1, 2, 3}
        j := 0
        sb[j], j = 2, 1 // sets sb[0] = 2, j = 1

        sc := []int{1, 2, 3}
        sc[0], sc[0] = 1, 2 // sets sc[0] = 1, then sc[0] = 2 (so sc[0] = 2 at end)

Updating: This is one change where tools cannot help, but breakage is unlikely. No code in the standard repository was broken by this change, and code that depended on the previous unspecified behavior was already incorrect.

### Returns and shadowed variables

A shadowed variable is one that has the same name as another variable in an inner scope. In functions with named return values, the Go 1 compilers disallow return statements without arguments if any of the named return values is shadowed at the point of the return statement. (It isn't part of the specification, because this is one area we are still exploring; the situation is analogous to the compilers rejecting functions that do not end with an explicit return statement.)

This function implicitly returns a shadowed return value and will be rejected by the compiler:

        func Bug() (i, j, k int) {
            for i = 0; i < 5; i++ {
                for j := 0; j < 5; j++ { // Redeclares j.
                    k += i*j
                    if k > 100 {
                        return // Rejected: j is shadowed here.
                    }
                }
            }
            return // OK: j is not shadowed here.
        }

Updating: Code that shadows return values in this way will be rejected by the compiler and will need to be fixed by hand. The few cases that arose in the standard repository were mostly bugs.

### Copying structs with unexported fields

Go 1 relaxes the rules about accessing structs with unexported (lower-case) fields, permitting a client package to assign (and therefore copy) such a struct. Of course, the client package still cannot access such fields individually.

As an example, if package `p` includes the definitions,

        type Struct struct {
            Public int
            secret int
        }
        func NewStruct(a int) Struct {  // Note: not a pointer.
            return Struct{a, f(a)}
        }
        func (s Struct) String() string {
            return fmt.Sprintf("{%d (secret %d)}", s.Public, s.secret)
        }

a package that imports `p` can assign and copy values of type `p.Struct` at will. Behind the scenes the unexported fields will be assigned and copied just as if they were exported, but the client code will never be aware of them. The code

        import "p"

        myStruct := p.NewStruct(23)
        copyOfMyStruct := myStruct
        fmt.Println(myStruct, copyOfMyStruct)

will show that the secret field of the struct has been copied to the new value.

Updating: This is a new feature, so existing code needs no changes.

### Equality of structs and arrays

Go 1 defines equality and inequality (`==` and `!=`) for struct and array values, respectively, provided the elements of the data structures can themselves be compared. That is, if equality is defined for all the fields of a struct (or an array element), then it is defined for the struct (or array).

As a result, structs and arrays can now be used as map keys:

        type Day struct {
            long  string
            short string
        }
        Christmas := Day{"Christmas", "XMas"}
        Thanksgiving := Day{"Thanksgiving", "Turkey"}
        holiday := map[Day]bool{
            Christmas:    true,
            Thanksgiving: true,
        }
        fmt.Printf("Christmas is a holiday: %t\n", holiday[Christmas])

Note that equality is still undefined for slices, for which the calculation is in general infeasible. Also note that the ordered comparison operators (`<` `<=` `>` `>=`) are still undefined for structs and arrays.

Updating: This is a new feature, so existing code needs no changes.

### Function and map equality

Go 1 disallows checking for equality of functions and maps, respectively, except to compare them directly to `nil`.

Updating: Existing code that depends on function or map equality will be rejected by the compiler and will need to be fixed by hand. Few programs will be affected, but the fix may require some redesign.

## The package hierarchy

This section describes how the packages have been rearranged in Go 1. Some have moved, some have been renamed, some have been deleted. New packages are described in later sections.

### The package hierarchy

Go 1 has a rearranged package hierarchy that groups related items into subdirectories. For instance, `utf8` and `utf16` now occupy subdirectories of `unicode`. Also, some packages have moved into subrepositories of `code.google.com/p/go` while others have been deleted outright.

Old path | New path
--- | ---

asn1 | encoding/asn1
csv | encoding/csv
gob | encoding/gob
json | encoding/json
xml | encoding/xml

exp/template/html | html/template

big | math/big
cmath | math/cmplx
rand | math/rand

http | net/http
http/cgi | net/http/cgi
http/fcgi | net/http/fcgi
http/httptest | net/http/httptest
http/pprof | net/http/pprof
mail | net/mail
rpc | net/rpc
rpc/jsonrpc | net/rpc/jsonrpc
smtp | net/smtp
url | net/url

exec | os/exec

scanner | text/scanner
tabwriter | text/tabwriter
template | text/template
template/parse | text/template/parse

utf8 | unicode/utf8
utf16 | unicode/utf16

Note that the package names for the old `cmath` and `exp/template/html` packages have changed to `cmplx` and `template`.

Updating: Running `go fix` will update all imports and package renames for packages that remain inside the standard repository. Programs that import packages that are no longer in the standard repository will need to be edited by hand.

### The package tree exp

Because they are not standardized, the packages under the `exp` directory will not be available in the standard Go 1 release distributions, although they will be available in source code form in the repository for developers who wish to use them.

Several packages have moved under `exp` at the time of Go 1's release:

- `ebnf`
- `go/types`
- `os/signal`

All these packages are available under the same names, with the prefix `exp/`: `exp/ebnf` etc.

Also, the `utf8.String` type has been moved to its own package, `exp/utf8string`.

Finally, the `gotype` command now resides in `exp/gotype`, while `ebnflint` is now in `exp/ebnflint`. If they are installed, they now reside in `$GOROOT/bin/tool`.

Updating: Code that uses packages in `exp` will need to be updated by hand, or else compiled from an installation that has `exp` available. The `go fix` tool or the compiler will complain about such uses.

### The package tree old

Because they are deprecated, the packages under the `old` directory will not be available in the standard Go 1 release distributions, although they will be available in source code form for developers who wish to use them.

The packages in their new locations are:

- `old/netchan`
- `old/regexp`
- `old/template`

Updating: Code that uses packages now in `old` will need to be updated by hand, or else compiled from an installation that has `old` available. The `go fix` tool will warn about such uses.

### Deleted packages

Go 1 deletes several packages outright:

- `container/vector`
- `exp/datafmt`
- `go/typechecker`
- `try`

and also the command `gotry`.

Updating: Code that uses `container/vector` should be updated to use slices directly. See the Go Language Community Wiki for some suggestions. Code that uses the other packages (there should be almost zero) will need to be rethought.

### Packages moving to subrepositories

Go 1 has moved a number of packages into sub-repositories of the main Go repository. This table lists the old and new import paths:

Old | New
--- | ---

crypto/bcrypt | code.google.com/p/go.crypto/bcrypt
crypto/blowfish | code.google.com/p/go.crypto/blowfish
crypto/cast5 | code.google.com/p/go.crypto/cast5
crypto/md4 | code.google.com/p/go.crypto/md4
crypto/ocsp | code.google.com/p/go.crypto/ocsp
crypto/openpgp | code.google.com/p/go.crypto/openpgp
crypto/openpgp/armor | code.google.com/p/go.crypto/openpgp/armor
crypto/openpgp/elgamal | code.google.com/p/go.crypto/openpgp/elgamal
crypto/openpgp/errors | code.google.com/p/go.crypto/openpgp/errors
crypto/openpgp/packet | code.google.com/p/go.crypto/openpgp/packet
crypto/openpgp/s2k | code.google.com/p/go.crypto/openpgp/s2k
crypto/ripemd160 | code.google.com/p/go.crypto/ripemd160
crypto/twofish | code.google.com/p/go.crypto/twofish
crypto/xtea | code.google.com/p/go.crypto/xtea
exp/ssh | code.google.com/p/go.crypto/ssh

image/bmp | code.google.com/p/go.image/bmp
image/tiff | code.google.com/p/go.image/tiff

net/dict | code.google.com/p/go.net/dict
net/websocket | code.google.com/p/go.net/websocket
exp/spdy | code.google.com/p/go.net/spdy

encoding/git85 | code.google.com/p/go.codereview/git85
patch | code.google.com/p/go.codereview/patch

Updating: Running `go fix` will update imports of these packages to use the new import paths. Installations that depend on these packages will need to install them using a `go install` command.

## Major changes to the library

This section describes significant changes to the core libraries, the ones that affect the most programs.

### The error type and errors package

As mentioned above, Go 1 introduces a new built-in interface type called `error`. Its intent is to replace the old `os.Error` type with a more central concept. So the widely-used `String` method does not cause accidental satisfaction of the `error` interface, the `error` interface uses instead the name `Error` for that method:

        type error interface {
            Error() string
        }

The `fmt` library automatically invokes `Error`, as it already does for `String`, for easy printing of error values.

    type SyntaxError struct {
        File    string
        Line    int
        Message string
    }

    func (se *SyntaxError) Error() string {
        return fmt.Sprintf("%s:%d: %s", se.File, se.Line, se.Message)
    }

All standard packages have been updated to use the new interface; the old `os.Error` is gone.

A new package, `errors`, contains the function

    func New(text string) error

to turn a string into an error. It replaces the old `os.NewError`.

        var ErrSyntax = errors.New("syntax error")

Updating: Running `go fix` will update almost all code affected by the change. Code that defines error types with a `String` method will need to be updated by hand to rename the methods to `Error`.

### System call errors

In Go 1, the `syscall` package returns an `error` for system call errors, rather than plain integer `errno` values. On Unix, the implementation is done by a `syscall.Errno` type that satisfies `error` and replaces the old `os.Errno`.

Updating: Running `go fix` will update almost all code affected by the change. Regardless, most code should use the `os` package rather than `syscall` and so will be unaffected.

### Time

One of the most sweeping changes in the Go 1 library is the complete redesign of the `time` package. Instead of an integer number of nanoseconds as an `int64`, and a separate `*time.Time` type to deal with human units such as hours and years, there are now two fundamental types: `time.Time` (a value, so the `*` is gone), which represents a moment in time; and `time.Duration`, which represents an interval. Both have nanosecond resolution. A `Time` can represent any time into the ancient past and remote future, while a `Duration` can span plus or minus only about 290 years. There are methods on these types, plus a number of helpful predefined constant durations such as `time.Second`.

Among the new methods are things like `Time.Add`, which adds a `Duration` to a `Time`, and `Time.Sub`, which subtracts two `Times` to yield a `Duration`.

The most important semantic change is that the Unix epoch (Jan 1, 1970) is now relevant only for those functions and methods that mention Unix: `time.Unix` and the `Unix` and `UnixNano` methods of the `Time` type. In particular, `time.Now` returns a `time.Time` value rather than, in the old API, an integer nanosecond count since the Unix epoch.

    // sleepUntil sleeps until the specified time. It returns immediately if it's too late.
    func sleepUntil(wakeup time.Time) {
        now := time.Now() // A Time.
        if !wakeup.After(now) {
            return
        }
        delta := wakeup.Sub(now) // A Duration.
        log.Printf("Sleeping for %.3fs", delta.Seconds())
        time.Sleep(delta)
    }

The new types, methods, and constants have been propagated through all the standard packages that use time, such as `os` and its representation of file time stamps.

Updating: The `go fix` tool will update many uses of the old `time` package to use the new types and methods, although it does not replace values such as `1e9` representing nanoseconds per second. Also, because of type changes in some of the values that arise, some of the expressions rewritten by the fix tool may require further hand editing; in such cases the rewrite will include the correct function or method for the old functionality, but may have the wrong type or require further analysis.

## Minor changes to the library

This section describes smaller changes, such as those to less commonly used packages or that affect few programs beyond the need to run `go fix`. This category includes packages that are new in Go 1.

### The archive/zip package

In Go 1, `*zip.Writer` no longer has a `Write` method. Its presence was a mistake.

Updating: What little code is affected will be caught by the compiler and must be updated by hand.

### The bufio package

In Go 1, `bufio.NewReaderSize` and `bufio.NewWriterSize` functions no longer return an error for invalid sizes. If the argument size is too small or invalid, it is adjusted.

Updating: Running `go fix` will update calls that assign the error to _. Calls that aren't fixed will be caught by the compiler and must be updated by hand.

### The compress/flate, compress/gzip and compress/zlib packages

In Go 1, the `NewWriterXxx` functions in `compress/flate`, `compress/gzip` and `compress/zlib` all return `(*Writer, error)` if they take a compression level, and `*Writer` otherwise. Package `gzip`'s `Compressor` and `Decompressor` types have been renamed to `Writer` and `Reader`. Package `flate`'s `WrongValueError` type has been removed.

Updating Running `go fix` will update old names and calls that assign the error to _. Calls that aren't fixed will be caught by the compiler and must be updated by hand.

### The crypto/aes and crypto/des packages

In Go 1, the `Reset` method has been removed. Go does not guarantee that memory is not copied and therefore this method was misleading.

The cipher-specific types `*aes.Cipher`, `*des.Cipher`, and `*des.TripleDESCipher` have been removed in favor of `cipher.Block`.

Updating: Remove the calls to Reset. Replace uses of the specific cipher types with cipher.Block.

### The crypto/elliptic package

In Go 1, `elliptic.Curve` has been made an interface to permit alternative implementations. The curve parameters have been moved to the `elliptic.CurveParams` structure.

Updating: Existing users of `*elliptic.Curve` will need to change to simply `elliptic.Curve`. Calls to `Marshal`, `Unmarshal` and `GenerateKey` are now functions in `crypto/elliptic` that take an `elliptic.Curve` as their first argument.

### The crypto/hmac package

In Go 1, the hash-specific functions, such as `hmac.NewMD5`, have been removed from `crypto/hmac`. Instead, `hmac.New` takes a function that returns a `hash.Hash`, such as `md5.New`.

Updating: Running `go fix` will perform the needed changes.

### The crypto/x509 package

In Go 1, the `CreateCertificate` and `CreateCRL` functions in `crypto/x509` have been altered to take an `interface{}` where they previously took a `*rsa.PublicKey` or `*rsa.PrivateKey`. This will allow other public key algorithms to be implemented in the future.

Updating: No changes will be needed.

### The encoding/binary package

In Go 1, the `binary.TotalSize` function has been replaced by `Size`, which takes an `interface{}` argument rather than a `reflect.Value`.

Updating: What little code is affected will be caught by the compiler and must be updated by hand.

### The encoding/xml package

In Go 1, the `xml` package has been brought closer in design to the other marshaling packages such as `encoding/gob`.

The old `Parser` type is renamed `Decoder` and has a new `Decode` method. An `Encoder` type was also introducted.

The functions `Marshal` and `Unmarshal` work with `[]byte` values now. To work with streams, use the new `Encoder` and `Decoder` types.

When marshaling or unmarshaling values, the format of supported flags in field tags has changed to be closer to the `json` package (``xml:"name,flag"``). The matching done between field tags, field names, and the XML attribute and element names is now case-sensitive. The `XMLName` field tag, if present, must also match the name of the XML element being marshaled.

Updating: Running `go fix` will update most uses of the package except for some calls to `Unmarshal`. Special care must be taken with field tags, since the fix tool will not update them and if not fixed by hand they will misbehave silently in some cases. For example, the old `"attr"` is now written `",attr"` while plain `"attr"` remains valid but with a different meaning.

### The expvar package

In Go 1, the `RemoveAll` function has been removed. The `Iter` function and Iter method on `*Map` have been replaced by `Do` and `(*Map).Do`.

Updating: Most code using `expvar` will not need changing. The rare code that used `Iter` can be updated to pass a closure to `Do` to achieve the same effect.

### The flag package

In Go 1, the interface `flag.Value` has changed slightly. The `Set` method now returns an `error` instead of a `bool` to indicate success or failure.

There is also a new kind of flag, `Duration`, to support argument values specifying time intervals. Values for such flags must be given units, just as `time.Duration` formats them: `10s`, `1h30m`, etc.

    var timeout = flag.Duration("timeout", 30*time.Second, "how long to wait for completion")

Updating: Programs that implement their own flags will need minor manual fixes to update their `Set` methods. The `Duration` flag is new and affects no existing code.

### The go/* packages

Several packages under `go` have slightly revised APIs.

A concrete `Mode` type was introduced for configuration mode flags in the packages `go/scanner`, `go/parser`, `go/printer`, and `go/doc`.

The modes `AllowIllegalChars` and `InsertSemis` have been removed from the `go/scanner` package. They were mostly useful for scanning text other then Go source files. Instead, the `text/scanner` package should be used for that purpose.

The `ErrorHandler` provided to the scanner's `Init` method is now simply a function rather than an interface. The `ErrorVector` type has been removed in favor of the (existing) `ErrorList` type, and the `ErrorVector` methods have been migrated. Instead of embedding an `ErrorVector` in a client of the scanner, now a client should maintain an `ErrorList`.

The set of parse functions provided by the `go/parser` package has been reduced to the primary parse function `ParseFile`, and a couple of convenience functions `ParseDir` and `ParseExpr`.

The `go/printer` package supports an additional configuration mode `SourcePos`; if set, the printer will emit `//line` comments such that the generated output contains the original source code position information. The new type `CommentedNode` can be used to provide comments associated with an arbitrary `ast.Node` (until now only `ast.File` carried comment information).

The type names of the `go/doc` package have been streamlined by removing the `Doc` suffix: `PackageDoc` is now `Package`, `ValueDoc` is `Value`, etc. Also, all types now consistently have a `Name` field (or `Names`, in the case of type `Value`) and `Type.Factories` has become `Type.Funcs`. Instead of calling `doc.NewPackageDoc(pkg, importpath)`, documentation for a package is created with:

        doc.New(pkg, importpath, mode)

where the new `mode` parameter specifies the operation mode: if set to `AllDecls`, all declarations (not just exported ones) are considered. The function `NewFileDoc` was removed, and the function `CommentText` has become the method `Text` of `ast.CommentGroup`.

In package `go/token`, the `token.FileSet` method `Files` (which originally returned a channel of `*token.File`s) has been replaced with the iterator `Iterate` that accepts a function argument instead.

Updating: Code that uses packages in `go` will have to be updated by hand; the compiler will reject incorrect uses. Templates used in conjuction with any of the `go/doc` types may need manual fixes; the renamed fields will lead to run-time errors.

### The hash package

In Go 1, the definition of `hash.Hash` includes a new method, `BlockSize`. This new method is used primarily in the cryptographic libraries.

The `Sum` method of the `hash.Hash` interface now takes a `[]byte` argument, to which the hash value will be appended. The previous behavior can be recreated by adding a `nil` argument to the call.

Updating: Existing implementations of `hash.Hash` will need to add a `BlockSize` method. Hashes that process the input one byte at a time can implement `BlockSize` to return 1. Running `go fix` will update calls to the `Sum` methods of the various implementations of `hash.Hash`.

Updating: Since the package's functionality is new, no updating is necessary.

### The http package

In Go 1 the `http` package is refactored, putting some of the utilities into a `httputil` subdirectory. These pieces are only rarely needed by HTTP clients. The affected items are:

- ClientConn
- DumpRequest
- DumpRequest
- DumpRequestOut
- DumpResponse
- NewChunkedReader
- NewChunkedWriter
- NewClientConn
- NewProxyClientConn
- NewServerConn
- NewSingleHostReverseProxy
- ReverseProxy
- ServerConn

The `Request.RawURL` field has been removed; it was a historical artifact.

The `Handle` and `HandleFunc` functions, and the similarly-named methods of `ServeMux`, now panic if an attempt is made to register the same pattern twice.

Updating: Running `go fix` will update the few programs that are affected except for uses of `RawURL`, which must be fixed by hand.

### The image package

The `image` package has had a number of minor changes, rearrangements and renamings.

Most of the color handling code has been moved into its own package, `image/color`. For the elements that moved, a symmetry arises; for instance, each pixel of an `image.RGBA` is a `color.RGBA`.

The old `image/ycbcr` package has been folded, with some renamings, into the `image` and `image/color` packages.

The old `image.ColorImage` type is still in the `image` package but has been renamed `image.Uniform`, while `image.Tiled` has been removed.

This table lists the renamings.

Old | New
--- | ---

image.Color | color.Color
image.ColorModel | color.Model
image.ColorModelFunc | color.ModelFunc
image.PalettedColorModel | color.Palette

image.RGBAColor | color.RGBA
image.RGBA64Color | color.RGBA64
image.NRGBAColor | color.NRGBA
image.NRGBA64Color | color.NRGBA64
image.AlphaColor | color.Alpha
image.Alpha16Color | color.Alpha16
image.GrayColor | color.Gray
image.Gray16Color | color.Gray16

image.RGBAColorModel | color.RGBAModel
image.RGBA64ColorModel | color.RGBA64Model
image.NRGBAColorModel | color.NRGBAModel
image.NRGBA64ColorModel | color.NRGBA64Model
image.AlphaColorModel | color.AlphaModel
image.Alpha16ColorModel | color.Alpha16Model
image.GrayColorModel | color.GrayModel
image.Gray16ColorModel | color.Gray16Model

ycbcr.RGBToYCbCr | color.RGBToYCbCr
ycbcr.YCbCrToRGB | color.YCbCrToRGB
ycbcr.YCbCrColorModel | color.YCbCrModel
ycbcr.YCbCrColor | color.YCbCr
ycbcr.YCbCr | image.YCbCr

ycbcr.SubsampleRatio444 | image.YCbCrSubsampleRatio444
ycbcr.SubsampleRatio422 | image.YCbCrSubsampleRatio422
ycbcr.SubsampleRatio420 | image.YCbCrSubsampleRatio420

image.ColorImage | image.Uniform

The image package's `New` functions (`NewRGBA`, `NewRGBA64`, etc.) take an `image.Rectangle` as an argument instead of four integers.

Finally, there are new predefined `color.Color` variables `color.Black`, `color.White`, `color.Opaque` and `color.Transparent`.

Updating: Running `go fix` will update almost all code affected by the change.

### The log/syslog package

In Go 1, the `syslog.NewLogger` function returns an error as well as a `log.Logger`.

Updating: What little code is affected will be caught by the compiler and must be updated by hand.

### The mime package

In Go 1, the `FormatMediaType` function of the `mime` package has been simplified to make it consistent with `ParseMediaType`. It now takes `"text/html"` rather than `"text"` and `"html"`.

Updating: What little code is affected will be caught by the compiler and must be updated by hand.

### The net package

In Go 1, the various `SetTimeout`, `SetReadTimeout`, and `SetWriteTimeout` methods have been replaced with `SetDeadline`, `SetReadDeadline`, and `SetWriteDeadline`, respectively. Rather than taking a timeout value in nanoseconds that apply to any activity on the connection, the new methods set an absolute deadline (as a `time.Time` value) after which reads and writes will time out and no longer block.

There are also new functions `net.DialTimeout` to simplify timing out dialing a network address and `net.ListenMulticastUDP` to allow multicast UDP to listen concurrently across multiple listeners. The `net.ListenMulticastUDP` function replaces the old `JoinGroup` and `LeaveGroup` methods.

Updating: Code that uses the old methods will fail to compile and must be updated by hand. The semantic change makes it difficult for the fix tool to update automatically.

### The os package

The `Time` function has been removed; callers should use the `Time` type from the `time` package.

The `Exec` function has been removed; callers should use `Exec` from the `syscall` package, where available.

The `ShellExpand` function has been renamed to `ExpandEnv`.

The `NewFile` function now takes a `uintptr` fd, instead of an `int`. The `Fd` method on files now also returns a `uintptr`.

Updating: What little code is affected will be caught by the compiler and must be updated by hand.

#### The os.FileInfo type

Go 1 redefines the `os.FileInfo` type, changing it from a struct to an interface:

        type FileInfo interface {
            Name() string       // base name of the file
            Size() int64        // length in bytes
            Mode() FileMode     // file mode bits
            ModTime() time.Time // modification time
            IsDir() bool        // abbreviation for Mode().IsDir()
            Sys() interface{}   // underlying data source (can return nil)
        }

The file mode information has been moved into a subtype called `os.FileMode`, a simple integer type with `IsDir`, `Perm`, and `String` methods.

The system-specific details of file modes and properties such as (on Unix) i-number have been removed from `FileInfo` altogether. Instead, each operating system's `os` package provides an implementation of the `FileInfo` interface, `*os.FileStat`, which has a `Sys` method that returns the system-specific representation of file metadata. For instance, to discover the i-number of a file on a Unix system, unpack the `FileInfo` like this:

        fi, err := os.Stat("hello.go")
        if err != nil {
            log.Fatal(err)
        }
        // Check that it's a Unix file.
        unixStat, ok := fi.Sys().(*syscall.Stat_t)
        if !ok {
            log.Fatal("hello.go: not a Unix file")
        }
        fmt.Printf("file i-number: %d\n", unixStat.Ino)

Assuming (which is unwise) that `"hello.go"` is a Unix file, the i-number expression could be contracted to

        fi.Sys().(*syscall.Stat_t).Ino

The vast majority of uses of `FileInfo` need only the methods of the standard interface.

Updating: Running `go fix` will update code that uses the old equivalent of the current `os.FileInfo` and `os.FileMode` API. Code that needs system-specific file details will need to be updated by hand.

### The path/filepath package

In Go 1, the `Walk` function of the `path/filepath` package has been changed to take a function value of type `WalkFunc` instead of a `Visitor` interface value. `WalkFunc` unifies the handling of both files and directories.

        type WalkFunc func(path string, info *os.FileInfo, err os.Error) os.Error

The `WalkFunc` function will be called even for files or directories that could not be opened; in such cases the error argument will describe the failure. If a directory's contents are to be skipped, the function should return the value `SkipDir`.

TODO: add an example?

Updating: The change simplifies most code but has subtle consequences, so affected programs will need to be updated by hand. The compiler will catch code using the old interface.

### The os/signal package

The `os/signal` package in Go 1 replaces the `Incoming` function, which returned a channel that received all incoming signals, with the selective `Notify` function, which asks for delivery of specific signals on an existing channel.

Updating: Code must be updated by hand. A literal translation of

    c := signal.Incoming()

is

    c := make(chan os.Signal)
    signal.Notify(c) // ask for all signals

but most code should list the specific signals it wants to handle instead:

    c := make(chan os.Signal)
    signal.Notify(c, syscall.SIGHUP, syscall.SIGQUIT)

### The runtime package

The `runtime` package in Go 1 includes a new niladic function, `runtime.NumCPU`, that returns the number of CPUs available for parallel execution, as reported by the operating system kernel. Its value can inform the setting of `GOMAXPROCS`.

Updating: No existing code is affected.

### The strconv package

In Go 1, the `strconv` package has been significantly reworked to make it more Go-like and less C-like, although `Atoi` lives on (it's similar to `int(ParseInt(x, 10, 0))`, as does `Itoa(x)` (`FormatInt(int64(x), 10)`). There are also new variants of some of the functions that append to byte slices rather than return strings, to allow control over allocation.

This table summarizes the renamings; see the package documentation for full details.

Old call | New call
--- | ---

Atob(x) | ParseBool(x)

Atof32(x) | ParseFloat(x, 32)§
Atof64(x) | ParseFloat(x, 64)
AtofN(x, n) | ParseFloat(x, n)

Atoi(x) | Atoi(x)
Atoi(x) | ParseInt(x, 10, 0)§
Atoi64(x) | ParseInt(x, 10, 64)

Atoui(x) | ParseUint(x, 10, 0)§
Atoi64(x) | ParseInt(x, 10, 64)

Btoi64(x, b) | ParseInt(x, b, 64)
Btoui64(x, b) | ParseUint(x, b, 64)

Btoa(x) | FormatBool(x)

Ftoa32(x, f, p) | FormatFloat(x, float64(f), p, 32)
Ftoa64(x, f, p) | FormatFloat(x, f, p, 64)
FtoaN(x, f, p, n) | FormatFloat(x, f, p, n)

Itoa(x) | Itoa(x)
Itoa(x) | FormatInt(int64(x), 10)
Itoa64(x) | FormatInt(x, 10)

Itob(x, b) | FormatInt(int64(x), b)
Itob64(x, b) | FormatInt(x, b)

Uitoa(x) | FormatUint(uint64(x), 10)
Uitoa64(x) | FormatUint(x, 10)

Uitob(x, b) | FormatUint(uint64(x), b)
Uitob64(x, b) | FormatUint(x, b)

Updating: Running `go fix` will update almost all code affected by the change.
§ `Atoi` persists but `Atoui` and `Atof32` do not, so they may require a cast that must be added by hand; the `go fix` tool will warn about it.

### The testing package

The testing package has a type, `B`, passed as an argument to benchmark functions. In Go 1, `B` has new methods, analogous to those of `T`, enabling logging and failure reporting.

    func BenchmarkSprintf(b *testing.B) {
        // Verify correctness before running benchmark.
        b.StopTimer()
        got := fmt.Sprintf("%x", 23)
        const expect = "17"
        if expect != got {
            b.Fatalf("expected %q; got %q", expect, got)
        }
        b.StartTimer()
        for i := 0; i < b.N; i++ {
            fmt.Sprintf("%x", 23)
        }
    }

Updating: Existing code is unaffected, although benchmarks that use `println` or `panic` should be updated to use the new methods.

### The testing/script package

The testing/script package has been deleted. It was a dreg.

Updating: No code is likely to be affected.

### The url package

In Go 1 several fields from the `url.URL` type were removed or replaced.

The `String` method now predictably rebuilds an encoded URL string using all of `URL`'s fields as necessary. The resulting string will also no longer have passwords escaped.

The `Raw` field has been removed. In most cases the `String` method may be used in its place.

The old `RawUserinfo` field is replaced by the `User` field, of type `*net.Userinfo`. Values of this type may be created using the new `net.User` and `net.UserPassword` functions. The `EscapeUserinfo` and `UnescapeUserinfo` functions are also gone.

The `RawAuthority` field has been removed. The same information is available in the `Host` and `User` fields.

The `RawPath` field and the `EncodedPath` method have been removed. The path information in rooted URLs (with a slash following the schema) is now available only in decoded form in the `Path` field. Occasionally, the encoded data may be required to obtain information that was lost in the decoding process. These cases must be handled by accessing the data the URL was built from.

URLs with non-rooted paths, such as `"mailto:dev@golang.org?subject=Hi"`, are also handled differently. The `OpaquePath` boolean field has been removed and a new `Opaque` string field introduced to hold the encoded path for such URLs. In Go 1, the cited URL parses as:

        URL{
            Scheme: "mailto",
            Opaque: "dev@golang.org",
            RawQuery: "subject=Hi",
        }

A new `RequestURI` method was added to `URL`.

Updating: Code that uses the old fields will fail to compile and must be updated by hand. The semantic changes make it difficult for the fix tool to update automatically.

## The go command

## Packaged releases