Introduction to Go 1.2

RED TEXT IS FROM THE 1.1 DOC AND NEEDS TO BE UPDATED. (It is here for formatting and style reference.)

Since the release of Go version 1.1 in April, 2013, the release schedule has been shortened to make the release process more efficient. This release, Go version 1.2 or Go 1.2 for short, arrives roughly six months after 1.1, while 1.1 took over a year to appear after 1.0. Because of the shorter time scale, 1.2 is a smaller delta than the step from 1.0 to 1.1, but it still has some significant developments, including a better scheduler and one new language feature. Of course, Go 1.2 keeps the promise of compatibility. The overwhelming majority of programs built with Go 1.1 (or 1.0 for that matter) will run without any changes whatsoever when moved to 1.2, although the introduction of one restriction to a corner of the language may expose already-incorrect code (see the discussion of the use of nil).

Changes to the language

In the interest of firming up the specification, one corner case has been clarified, with consequences for programs. There is also one new language feature.

Use of nil

The language now specifies that, for safety reasons, certain uses of nil pointers are guaranteed to trigger a run-time panic. For instance, in Go 1.0, given code like 

type T struct {
    X [1<<24]byte
    Field int32
}

func main() {
    var x *T
    ...
}

the nil pointer x could be used to access memory incorrectly: the expression x.Field could access memory at address 1<<24. To prevent such unsafe behavior, in Go 1.2 the compilers now guarantee that any indirection through a nil pointer, such as illustrated here but also in nil pointers to arrays, nil interface values, nil slices, and so on, will either panic or return a correct, safe non-nil value. In short, any expression that explicitly or implicitly requires evaluation of a nil address is an error. The implementation may inject extra tests into the compiled program to enforce this behavior.

Further details are in the design document.

Updating: Most code that depended on the old behavior is erroneous and will fail when run. Such programs will need to be updated by hand.

Three-index slices

Go 1.2 adds the ability to specify the capacity as well as the length when using a slicing operation on an existing array or slice. A slicing operation creates a new slice by describing a contiguous section of an already-created array or slice: 

var array [10]int
slice := array[2:4]

The capacity of the slice is the maximum number of elements that the slice may hold, even after reslicing; it reflects the size of the underlying array. In this example, the capacity of the slice variable is 8.

Go 1.2 adds new syntax to allow a slicing operation to specify the capacity as well as the length. A second colon introduces the capacity value, which must be less than or equal to the capacity of the source slice or array, adjusted for the origin. For instance, 

slice = array[2:4:6]

sets the slice to have the same length as in the earlier example but its capacity is now only 4 elements (6-2). It is impossible to use this new slice value to access the last two elements of the original array.

In this three-index notation, a missing first index ([:i:j]) defaults to zero but the other two indices must always be specified explicitly. It is possible that future releases of Go may introduce default values for these indices.

Further details are in the design document.

Updating: This is a backwards-compatible change that affects no existing programs.

Changes to the implementations and tools

Godoc and vet moved to the go.tools subrepository

Both binaries are still included with the distribution, but the source code for the godoc and vet commands has moved to the go.tools subrepository.

Also, the core of the godoc program has been split into a library, while the command itself is in a separate directory. The move allows the code to be updated easily and the separation into a library and command makes it easier to construct custom binaries for local sites and different deployment methods.

Updating: Since godoc and vet are not part of the library, no client Go code depends on the their source and no updating is required.

The binary distributions available from golang.org include these binaries, so users of these distributions are unaffected.

When building from source, users must use "go get" to install godoc and vet. 

$ go get code.google.com/p/go.tools/cmd/godoc
$ go get code.google.com/p/go.tools/cmd/vet

Status of gccgo

We expect the future GCC 4.9 release to include gccgo with full support for Go 1.2. In the current (4.8.2) release of GCC, gccgo implements Go 1.1.2.

TODO

TODO: write prose

Test coverage

One major new feature of go test is that it can now compute and, with help from a new, separately installed "go tool cover" program, display test coverage results.

The cover tool is part of the go.tools subrepository. It can be installed by running 

$ go get code.google.com/p/go.tools/cmd/cover

The cover tool does two things. First, when "go test" is given the -cover flag, it is run automatically to rewrite the source for the package and insert instrumentation statements. The test is then compiled and run as usual, and basic coverage statistics are reported: 

$ go test -cover fmt
ok  	fmt	0.060s	coverage: 91.4% of statements
$

Second, for more detailed reports, different flags to "go test" can create a coverage profile file, which the cover program, invoked with "go tool cover", can then analyze.

Details on how to generate and analyze coverage statistics can be found by running the commands 

$ go help testflag
$ go tool cover -help

The go doc command is deleted

The "go doc" command is deleted. Note that the godoc tool itself is not deleted, just the wrapping of it by the go command. All it did was show the documents for a package by package path, which godoc itself already does with more flexibility. It has therefore been deleted to reduce the number of documentation tools and, as part of the restructuring of godoc, encourage better options in future.

Updating: For those who still need the precise functionality of running 

$ go doc

in a directory, the behavior is identical to running 

$ godoc .

Changes to the go command

The go get command now has a -t flag that causes it to download the dependencies of the tests run by the package, not just those of the package itself. By default, as before, dependencies of the tests are not downloaded.

Additional platforms

The Go 1.1 tool chain adds experimental support for freebsd/arm, netbsd/386, netbsd/amd64, netbsd/arm, openbsd/386 and openbsd/amd64 platforms.

An ARMv6 or later processor is required for freebsd/arm or netbsd/arm.

Go 1.1 adds experimental support for cgo on linux/arm.

Performance

The performance of code compiled with the Go 1.1 gc tool suite should be noticeably better for most Go programs. Typical improvements relative to Go 1.0 seem to be about 30%-40%, sometimes much more, but occasionally less or even non-existent. There are too many small performance-driven tweaks through the tools and libraries to list them all here, but the following major changes are worth noting:

Changes to the standard library

The archive/tar and archive/zip packages

Breaking change: TODO archive/tar,archive/zip: fix os.FileInfo implementation to provide base name only (CL 13118043).

The new encoding package

encoding: TODO new package defining generic encoding interfaces (CL 12541051).

The fmt package

The fmt package's formatted print routines such as Printf now allow the data items to be printed to be accessed in arbitrary order by using an indexing operation in the formatting specifications. Wherever an argument is to be fetched from the argument list for formatting, either as the value to be formatted or as a width or specification integer, a new optional indexing notation [n] fetches argument n instead. The value of n is 1-indexed. After such an indexing operating, the next argument to be fetched by normal processing will be n+1.

For example, the normal Printf call 

fmt.Sprintf("%c %c %c\n", 'a', 'b', 'c')

would create the string "a b c", but with indexing operations like this, 

fmt.Sprintf("%[3]c %[1]c %c\n", 'a', 'b', 'c')

the result is ""c a b". The [3] index accesses the third formatting argument, whch is 'c', [1] accesses the first, 'a', and then the next fetch accesses the argument following that one, 'b'.

The motivation for this feature is programmable format statements to access the arguments in different order for localization, but it has other uses: 

log.Printf("trace: value %v of type %[1]T\n", expensiveFunction(a.b[c]))

Updating: The change to the syntax of format specifications is strictly backwards compatible, so it affects no working programs.

The text/template and html/template packages

The text/template package has a couple of changes in Go 1.2, both of which are also mirrored in the html/template package.

First, there are new default functions for comparing basic types. The functions are listed in this table, which shows their names and the associated familiar comparison operator.

Name Operator
eq ==
ne !=
lt <
le <=
gt >
ge >=

These functions behave slightly differently from the corresponding Go operators. First, they operate only on basic types (bool, int, float64, string, etc.). (Go allows comparison of arrays and structs as well, under some circumstances.) Second, values can be compared as long as they are the same sort of value: any signed integer value can be compared to any other signed integer value for example. (Go does not permit comparing an int8 and an int16). Finally, the eq function (only) allows comparison of the first argument with one or more following arguments. The template in this example, 

{{"{{"}}if eq .A 1 2 3 {{"}}"}} equal {{"{{"}}else{{"}}"}} not equal {{"{{"}}end{{"}}"}}

reports "equal" if .A is equal to any of 1, 2, or 3.

The second change is that a small addition to the grammar makes "if else if" chains easier to write. Instead of writing, 

{{"{{"}}if eq .A 1{{"}}"}} X {{"{{"}}else{{"}}"}} {{"{{"}}if eq .A 2{{"}}"}} Y {{"{{"}}end{{"}}"}} {{"{{"}}end{{"}}"}}

one can fold the second "if" into the "else" and have only one "end", like this: 

{{"{{"}}if eq .A 1{{"}}"}} X {{"{{"}}else if eq .A 2{{"}}"}} Y {{"{{"}}end{{"}}"}}

The two forms are identical in effect; the difference is just in the syntax.

Finally, the package now correctly diagnoses unmatched right delimiters. They were accepted without complaint before, and templates that had them will now fail to parse.

Updating: Neither the "else if" change nor the comparison functions affect existing programs. Those that already define functions called eq and so on through a function map are unaffected because the associated function map will override the new default function definitions. Templates with unmatched right delimiters will now fail to parse and will need to be fixed by hand.

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.