## Introduction to Go 1.1

TODO - overview - link back to Go 1 and also Go 1 Compatibility docs.

## Changes to the language

The Go compatibility document promises that programs written to the Go 1 language specification will continue to operate, and those promises are maintained. In the interest of firming up the specification, though, there are details about some error cases that have been clarified. There are also some new language features.

### Integer division by zero

In Go 1, integer division by a constant zero produced a run-time panic:

    func f(x int) int {
    	return x/0
    }

In Go 1.1, an integer division by constant zero is not a legal program, so it is a compile-time error.

### Surrogates in Unicode literals

The definition of string and rune literals has been refined to exclude surrogate halves from the set of valid Unicode code points. See the Unicode section for more information.

### Method values

Go 1.1 now implements method values, which are functions that have been bound to a specific receiver value. For instance, given a `Writer` value `w`, the expression `w.Write`, a method value, is a function that will always write to `w`; it is equivalent to a function literal closing over `w`:

    func (p []byte) (n int, err error) {
    	return w.Write(n, err)
    }

Method values are distinct from method expressions, which generate functions from methods of a given type; the method expression `(*bufio.Writer).Write` is equivalent to a function with an extra first argument, a receiver of type `(*bufio.Writer)`:

    func (w *bufio.Writer, p []byte) (n int, err error) {
    	return w.Write(n, err)
    }

Updating: No existing code is affected; the change is strictly backward-compatible.

### Return requirements

Before Go 1.1, a function that returned a value needed an explicit "return" or call to `panic` at the end of the function; this was a simple way to make the programmer be explicit about the meaning of the function. But there are many cases where a final "return" is clearly unnecessary, such as a function with only an infinite "for" loop.

In Go 1.1, the rule about final "return" statements is more permissive. It introduces the concept of a terminating statement, a statement that is guaranteed to be the last one a function executes. Examples include "for" loops with no condition and "if-else" statements in which each half ends in a "return". If the final statement of a function can be shown syntactically to be a terminating statement, no final "return" statement is needed.

Note that the rule is purely syntactic: it pays no attention to the values in the code and therefore requires no complex analysis.

Updating: The change is backward-compatible, but existing code with superfluous "return" statements and calls to `panic` may be simplified manually. Such code can be identified by `go vet`.

## Changes to the implementations and tools

### Command-line flag parsing

In the gc tool chain, the compilers and linkers now use the same command-line flag parsing rules as the Go flag package, a departure from the traditional Unix flag parsing. This may affect scripts that invoke the tool directly. For example, `go tool 6c -Fw -Dfoo` must now be written `go tool 6c -F -w -D foo`.

### Size of int on 64-bit platforms

The language allows the implementation to choose whether the `int` type and `uint` types are 32 or 64 bits. Previous Go implementations made `int` and `uint` 32 bits on all systems. Both the gc and gccgo implementations now make `int` and `uint` 64 bits on 64-bit platforms such as AMD64/x86-64. Among other things, this enables the allocation of slices with more than 2 billion elements on 64-bit platforms.

Updating: Most programs will be unaffected by this change. Because Go does not allow implicit conversions between distinct numeric types, no programs will stop compiling due to this change. However, programs that contain implicit assumptions that `int` is only 32 bits may change behavior. For example, this code prints a positive number on 64-bit systems and a negative one on 32-bit systems:

    x := ^uint32(0) // x is 0xffffffff
    i := int(x)     // i is -1 on 32-bit systems, 0xffffffff on 64-bit
    fmt.Println(i)

Portable code intending 32-bit sign extension (yielding `-1` on all systems) would instead say:

    i := int(int32(x))

### Unicode

To make it possible to represent code points greater than 65535 in UTF-16, Unicode defines surrogate halves, a range of code points to be used only in the assembly of large values, and only in UTF-16. The code points in that surrogate range are illegal for any other purpose. In Go 1.1, this constraint is honored by the compiler, libraries, and run-time: a surrogate half is illegal as a rune value, when encoded as UTF-8, or when encoded in isolation as UTF-16. When encountered, for example in converting from a rune to UTF-8, it is treated as an encoding error and will yield the replacement rune, `utf8.RuneError`, U+FFFD.

This program,

    import "fmt"

    func main() {
        fmt.Printf("%+q\n", string(0xD800))
    }

printed `"\ud800"` in Go 1.0, but prints `"\ufffd"` in Go 1.1.

Surrogate-half Unicode values are now illegal in rune and string constants, so constants such as `'\ud800'` and `"\ud800"` are now rejected by the compilers. When written explicitly as UTF-8 encoded bytes, such strings can still be created, as in `"\xed\xa0\x80"`. However, when such a string is decoded as a sequence of runes, as in a range loop, it will yield only `utf8.RuneError` values.

The Unicode byte order marks U+FFFE and U+FEFF, encoded in UTF-8, are now permitted as the first character of a Go source file. Even though their appearance in the byte-order-free UTF-8 encoding is clearly unnecessary, some editors add them as a kind of "magic number" identifying a UTF-8 encoded file.

Updating: Most programs will be unaffected by the surrogate change. Programs that depend on the old behavior should be modified to avoid the issue. The byte-order-mark change is strictly backward-compatible.

### The gc assemblers

Due to the change of the `int` to 64 bits and some other changes, the arrangement of function arguments on the stack has changed in the gc tool chain. Functions written in assembly will need to be revised at least to adjust frame pointer offsets.

Updating: The `go vet` command now checks that functions implemented in assembly match the Go function prototypes they implement.

### Changes to the go command

The `go` command has acquired several changes intended to improve the experience for new Go users.

First, when compiling, testing, or running Go code, the `go` command will now give more detailed error messages, including a list of paths searched, when a package cannot be located.

    $ go build foo/quxx
    can't load package: package foo/quxx: cannot find package "foo/quxx" in any of:
            /home/you/go/src/pkg/foo/quxx (from $GOROOT)
            /home/you/src/foo/quxx (from $GOPATH)

Second, the `go get` command no longer allows `$GOROOT` as the default destination when downloading package source. To use the `go get` command, a valid `$GOPATH` is now required.

    $ GOPATH= go get code.google.com/p/foo/quxx
    package code.google.com/p/foo/quxx: cannot download, $GOPATH not set. For more details see: go help gopath

Finally, as a result of the previous change, the `go get` command will also fail when `$GOPATH` and `$GOROOT` are set to the same value.

    $ GOPATH=$GOROOT go get code.google.com/p/foo/quxx
    warning: GOPATH set to GOROOT (/home/User/go) has no effect
    package code.google.com/p/foo/quxx: cannot download, $GOPATH must not be set to $GOROOT. For more details see: go help gopath

### Changes to the go test command

The `go test` command no longer deletes the binary when run with profiling enabled, to make it easier to analyze the profile. The implementation sets the `-c` flag automatically, so after running,

    $ go test -cpuprofile cpuprof.out mypackage

the file `mypackage.test` will be left in the directory where `go test` was run.

The `go test` command can now generate profiling information that reports where goroutines are blocked, that is, where they tend to stall waiting for an event such as a channel communication. The information is presented as a blocking profile enabled with the `-blockprofile` option of `go test`. Run `go help test` for more information.

### Changes to the go fix command

The `fix` command, usually run as `go fix`, no longer applies fixes to update code from before Go 1 to use Go 1 APIs. To update pre-Go 1 code to Go 1.1, use a Go 1.0 tool chain to convert the code to Go 1.0 first.

## Performance

TODO introduction

- TODO better code generation (inlining, ...?)
- TODO parallel gc
- TODO more precise gc
- TODO networking is more efficient (known to runtime)

## Changes to the standard library

### bufio.Scanner

The various routines to scan textual input in the `bufio` package, `ReadBytes`, `ReadString` and particularly `ReadLine`, are needlessly complex to use for simple purposes. In Go 1.1, a new type, `Scanner`, has been added to make it easier to do simple tasks such as read the input as a sequence of lines or space-delimited words. It simplifies the problem by terminating the scan on problematic input such as pathologically long lines, and having a simple default: line-oriented input, with each line stripped of its terminator. Here is code to reproduce the input a line at a time:

    scanner := bufio.NewScanner(os.Stdin)
    for scanner.Scan() {
        fmt.Println(scanner.Text()) // Println will add back the final '\n'
    }
    if err := scanner.Err(); err != nil {
        fmt.Fprintln(os.Stderr, "reading standard input:", err)
    }

Scanning behavior can be adjusted through a function to control subdividing the input (see the documentation for `SplitFunc`), but for tough problems or the need to continue past errors, the older interface may still be required.

### net

The protocol-specific resolvers in the `net` package were formerly lax about the network name passed in. Although the documentation was clear that the only valid networks for `ResolveTCPAddr` are `"tcp"`, `"tcp4"`, and `"tcp6"`, the Go 1.0 implementation silently accepted any string. The Go 1.1 implementation returns an error if the network is not one of those strings. The same is true of the other protocol-specific resolvers `ResolveIPAddr`, `ResolveUDPAddr`, and `ResolveUnixAddr`.

The previous implementation of `ListenUnixgram` returned a `UDPConn` as a representation of the connection endpoint. The Go 1.1 implementation instead returns a `UnixConn` to allow reading and writing with its `ReadFrom` and `WriteTo` methods.

### reflect

The `reflect` package has several significant additions.

It is now possible to run a "select" statement using the `reflect` package; see the description of `Select` and `SelectCase` for details.

The new method `Value.Convert` (or `Type.ConvertibleTo`) provides functionality to execute a Go conversion or type assertion operation on a `Value` (or test for its possibility).

The new function `MakeFunc` creates a wrapper function to make it easier to call a function with existing `Values`, doing the standard Go conversions among the arguments, for instance to pass an actual `int` to a formal `interface{}`.

Finally, the new functions `ChanOf`, `MapOf` and `SliceOf` construct new `Types` from existing types, for example to construct a the type `[]T` given only `T`.

### time

On FreeBSD, Linux, NetBSD, OS X and OpenBSD, previous versions of the `time` package returned times with microsecond precision. The Go 1.1 implementation on these systems now returns times with nanosecond precision. Programs that write to an external format with microsecond precision and read it back, expecting to recover the original value, will be affected by the loss of precision. There are two new methods of `Time`, `Round` and `Truncate`, that can be used to remove precision from a time before passing it to external storage.

The new method `YearDay` returns the one-indexed integral day number of the year specified by the time value.

The `Timer` type has a new method `Reset` that modifies the timer to expire after a specified duration.

Finally, the new function `ParseInLocation` is like the existing `Parse` but parses the time in the context of a location (time zone), ignoring time zone information in the parsed string. This function addresses a common source of confusion in the time API.

Updating: Code that needs to read and write times using an external format with lower precision should be modified to use the new methods.

### Exp and old subtrees moved to go.exp and go.text subrepositories

To make it easier for binary distributions to access them if desired, the `exp` and `old` source subtrees, which are not included in binary distributions, have been moved to the new `go.exp` subrepository at `code.google.com/p/go.exp`. To access the `ssa` package, for example, run

    $ go get code.google.com/p/go.exp/ssa

and then in Go source,

    import "code.google.com/p/go.exp/ssa"

The old package `exp/norm` has also been moved, but to a new repository `go.text`, where the Unicode APIs and other text-related packages will be developed.

### Minor changes to the library

The following list summarizes a number of minor changes to the library, mostly additions. See the relevant package documentation for more information about each change.

- The `bytes` package has two new functions, `TrimPrefix` and `TrimSuffix`, with self-evident properties. Also, the `Buffer` type has a new method `Grow` that provides some control over memory allocation inside the buffer. Finally, the `Reader` type now has a `WriteTo` method so it implements the `io.WriterTo` interface.
- The `crypto/hmac` package has a new function, `Equal`, to compare two MACs.
- The `crypto/x509` package now supports PEM blocks (see `DecryptPEMBlock` for instance), and a new function `ParseECPrivateKey` to parse elliptic curve private keys.
- The `database/sql/` package has a new `Ping` method for its `DB` type that tests the health of the connection.
- The `database/sql/driver` package has a new `Queryer` interface that a `Conn` may implement to improve performance.
- The `encoding/json` package's `Decoder` has a new method `Reader` to provide access to the remaining data in its buffer, as well as a new method `UseNumber` to unmarshal a value into the new type `Number`, a string, rather than a float64.
- The `endoding/xml` package has a new function, `EscapeText`, which writes escaped XML output, and a method on `Encoder`, `Indent`, to specify indented output.
- In the `go/ast` package, a new type `CommentMap` and associated methods makes it easier to extract and process comments in Go programs.
- In the `go/doc` package, the parser now keeps better track of stylized annotations such as `TODO(joe)` throughout the code, information that the `godoc` command can filter or present according to the value of the `-notes` flag.
- A new package, `go/format`, provides a convenient way for a program to access the formatting capabilities of `gofmt`. It has two functions, `Node` to format a Go parser `Node`, and `Source` to format arbitrary Go source code.
- The undocumented and only partially implemented "noescape" feature of the html/template package has been removed; programs that depend on it will break.
- The io package now exports the `io.ByteWriter` interface to capture the common functionality of writing a byte at a time.
- The `log/syslog` package now provides better support for OS-specific logging features.
- The `math/big` package's `Int` type now has now has methods `MarshalJSON` and `UnmarshalJSON` to convert to and from a JSON representation. Also, `Int` can now convert directly to and from a `uint64` using `Uint64` and `SetUint64`, while `Rat` can do the same with `float64` using `Float64` and `SetFloat64`.
- The `mime/multipart` package has a new method for its `Writer`, `SetBoundary`, to define the boundary separator used to package the output.
- The `net` package's `net/ListenUnixgram` function has changed return types: it now returns a `net/UnixConn` rather than a `net/UDPConn`, which was clearly a mistake in Go 1.0. Since this API change fixes a bug, it is permitted by the Go 1 compatibility rules.
- The new net/http/cookiejar package provides the basics for managing HTTP cookies.
- TODO: `net/http`: ParseTime, CloseNotifier, Request.PostFormValue, ServeMux.Handler, Transport.CancelRequest
- TODO: `net/mail`: ParseAddress, ParseAddressList
- TODO: `net/smtp`: Client.Hello
- The `net/textproto` package has two new functions, `TrimBytes` and `TrimString`, which do ASCII-only trimming of leading and trailing spaces.
- TODO: `net`: DialOption, DialOpt, ListenUnixgram, LookupNS, IPConn.ReadMsgIP, IPConn.WriteMsgIP, UDPConn.ReadMsgUDP, UDPConn.WriteMsgUDP, UnixConn.CloseRead, UnixConn.CloseWrite
- The new method `os.FileMode.IsRegular` makes it easy to ask if a file is a plain file.
- The `image/jpeg` package now reads progressive JPEG files and handles a few more subsampling configurations.
- The `regexp` package now supports Unix-original leftmost-longest matches through the `Regexp.Longest` method, while `Regexp.Split` slices strings into pieces based on separators defined by the regular expression.
- The `runtime/debug` package has three new functions regarding memory usage. The `FreeOSMemory` function triggers a run of the garbage collector and then attempts to return unused memory to the operating system; the `ReadGCStats` function retrieves statistics about the collector; and `SetGCPercent` provides a programmatic way to control how often the collector runs, including disabling it altogether.
- The `sort` package has a new function, `Reverse`. Wrapping the argument of a call to `sort.Sort` with a call to `Reverse` causes the sort order to be reversed.
- The `strings` package has two new functions, `TrimPrefix` and `TrimSuffix` with self-evident properties, and the the new method `Reader.WriteTo` so the `Reader` type now implements the `io.WriterTo` interface.
- The `syscall` package has received many updates to make it more inclusive of constants and system calls for each supported operating system.
- The `testing` package now automates the generation of allocation statistics in benchmarks using the new `AllocsPerRun` function and the `AllocsPerOp` method of `BenchmarkResult`. There is also a new `Verbose` function to test the state of the `-v` command-line flag, and a new `Skip` method of `testing.B` and `testing.T` to simplify skipping an inappropriate test.
- In the `text/template` and `html/template` packages, templates can now use parentheses to group the elements of pipelines, simplifying the construction of complex pipelines. TODO: Link to example. Also, as part of the new parser, the `Node` interface got two new methods to provide better error reporting. Although this violates the Go 1 compatibility rules, no existing code should be affected because this interface is explicitly intended only to be used by the `text/template` and `html/template` packages and there are safeguards to guarantee that.
- In the `unicode/utf8` package, the new function `ValidRune` reports whether the rune is a valid Unicode code point. To be valid, a rune must be in range and not be a surrogate half.
- The implementation of the `unicode` package has been updated to Unicode version 6.2.0.