go modules have landed

Thanks,


Paul

$ (cd module_path; go install -v -mod vendor ./...)
net
# net
exec: "gcc": executable file not found in $PATH 

As of CL 123580, submitted a few minutes ago, the development copy of the go command, in the main go repository, has support for Go modules. There are various minor known issues, but quite a lot works very well. 

There's still more to be done but this is a key milestone: if you run on the bleeding edge, you can play with modules just by using "go". Work on the module support will now be done exclusively in the main repo. We will automatically export the main repo to the x/vgo repo when we have a snapshot we want to make available to people using "vgo".

The open issues have moved from the vgo milestone to the Go 1.11 milestone, but labeled with the new 'modules' label.

A note on terminology: Go 1.11 is not shipping with "vgo support". It is shipping with "Go module support". Vgo was only ever the prototype/demo of that module support. Just like all the other features we've introduced to the go command over the past ten years, the goal for this one is to fit so well into the rest of the go command fabric that the seams are invisible - module support is just part of "go", the same way that, say, race detector support or GOPATH support is.

Speaking of GOPATH, the go command automatically chooses between using modules and using GOPATH depending on where it is run. If a go command runs outside GOPATH/src and there is a go.mod file in the current directory or one of its parent directories, then modules are enabled (otherwise not):

$ mkdir /tmp/hello

$ cd /tmp/hello

$ echo module hello >go.mod

$ cat >hello.go <<EOF

package main 

import "rsc.io/quote"

func main() {

println(quote.Glass())

}

EOF

$ go run .

I can eat glass and it doesn't hurt me.

$ cat go.mod

module hello

require rsc.io/quote v1.5.2

$ go list -m all

hello

golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c

rsc.io/quote v1.5.2

rsc.io/sampler v1.3.0

$ 

For more information, see 'go help modules', which I've included at the end of this mail.

Many thanks to everyone in the community who has helped us get to this point, but especially to Paul Jolly for really hammering on the module support and filing tons of great bugs, and to Baokun Lee for a steady sequence of helpful CLs. Thanks also to Bryan Mills for helping work out a lot of important details in the past few weeks. 

Thanks again, and keep the bug reports coming.

Best,

Russ

go help modules

A module is a collection of related Go packages.

Modules are the unit of source code interchange and versioning.

The go command has direct support for working with modules,

including recording and resolving dependencies on other modules.

Modules replace the old GOPATH-based approach to specifying

which source files are used in a given build.

Experimental module support

Go 1.11 includes experimental support for Go modules,

including a new module-aware 'go get' command.

We intend to keep revising this support, while preserving compatibility,

until it can be declared official (no longer experimental),

and then at a later point we may remove support for work

in GOPATH and the old 'go get' command.

The quickest way to take advantage of the new Go 1.11 module support

is to check out your repository into a directory outside GOPATH/src,

create a go.mod file (described in the next section) there, and run

go commands from within that file tree.

For more fine-grained control, the module support in Go 1.11 respects

a temporary environment variable, GO111MODULE, which can be set to one

of three string values: off, on, or auto (the default).

If GO111MODULE=off, then the go command never uses the

new module support. Instead it looks in vendor directories and GOPATH

to find dependencies; we now refer to this as "GOPATH mode."

If GO111MODULE=on, then the go command requires the use of modules,

never consulting GOPATH. We refer to this as the command being

module-aware or running in "module-aware mode".

If GO111MODULE=auto or is unset, then the go command enables or

disables module support based on the current directory.

Module support is enabled only when the current directory is outside

GOPATH/src and itself contains a go.mod file or is below a directory

containing a go.mod file.

Defining a module

A module is defined by a tree of Go source files with a go.mod file

in the tree's root directory. The directory containing the go.mod file

is called the module root. Typically the module root will also correspond

to a source code repository root (but in general it need not).

The module is the set of all Go packages in the module root and its

subdirectories, but excluding subtrees with their own go.mod files.

The "module path" is the import path prefix corresponding to the module root.

The go.mod file defines the module path and lists the specific versions

of other modules that should be used when resolving imports during a build,

by giving their module paths and versions.

For example, this go.mod declares that the directory containing it is the root

of the module with path example.com/m, and it also declares that the module

depends on specific versions of golang.org/x/text and gopkg.in/yaml.v2:

module example.com/m

require (

golang.org/x/text v0.3.0

gopkg.in/yaml.v2 v2.1.0

)

The go.mod file can also specify replacements and excluded versions

that only apply when building the module directly; they are ignored

when the module is incorporated into a larger build.

For more about the go.mod file, see https://research.swtch.com/vgo-module.

To start a new module, simply create a go.mod file in the root of the

module's directory tree, containing only a module statement.

The 'go mod' command can be used to do this:

go mod -init -module example.com/m

In a project already using an existing dependency management tool like

godep, glide, or dep, 'go mod -init' will also add require statements

matching the existing configuration.

Once the go.mod file exists, no additional steps are required:

go commands like 'go build', 'go test', or even 'go list' will automatically

add new dependencies as needed to satisfy imports.

The main module and the build list

The "main module" is the module containing the directory where the go command

is run. The go command finds the module root by looking for a go.mod in the

current directory, or else the current directory's parent directory,

or else the parent's parent directory, and so on.

The main module's go.mod file defines the precise set of packages available

for use by the go command, through require, replace, and exclude statements.

Dependency modules, found by following require statements, also contribute

to the definition of that set of packages, but only through their go.mod

files' require statements: any replace and exclude statements in dependency

modules are ignored. The replace and exclude statements therefore allow the

main module complete control over its own build, without also being subject

to complete control by dependencies.

The set of modules providing packages to builds is called the "build list".

The build list initially contains only the main module. Then the go command

adds to the list the exact module versions required by modules already

on the list, recursively, until there is nothing left to add to the list.

If multiple versions of a particular module are added to the list,

then at the end only the latest version (according to semantic version

ordering) is kept for use in the build.

The 'go list' command provides information about the main module

and the build list. For example:

go list -m              # print path of main module

go list -m -f={{.Dir}}  # print root directory of main module

go list -m all          # print build list

Maintaining module requirements

The go.mod file is meant to be readable and editable by both

programmers and tools. The go command itself automatically updates the go.mod file

to maintain a standard formatting and the accuracy of require statements.

Any go command that finds an unfamiliar import will look up the module

containing that import and add the latest version of that module

to go.mod automatically. In most cases, therefore, it suffices to

add an import to source code and run 'go build', 'go test', or even 'go list':

as part of analyzing the package, the go command will discover

and resolve the import and update the go.mod file.

Any go command can determine that a module requirement is

missing and must be added, even when considering only a single

package from the module. On the other hand, determining that a module requirement

is no longer necessary and can be deleted requires a full view of

all packages in the module, across all possible build configurations

(architectures, operating systems, build tags, and so on).

The 'go mod -sync' command builds that view and then

adds any missing module requirements and removes unnecessary ones.

As part of maintaining the require statements in go.mod, the go command

tracks which ones provide packages imported directly by the current module

and which ones provide packages only used indirectly by other module

dependencies. Requirements needed only for indirect uses are marked with a

"// indirect" comment in the go.mod file. Indirect requirements are

automatically removed from the go.mod file once they are implied by other

direct requirements. Indirect requirements only arise when using modules

that fail to state some of their own dependencies or when explicitly

upgrading a module's dependencies ahead of its own stated requirements.

Because of this automatic maintenance, the information in go.mod is an

up-to-date, readable description of the build.

The 'go get' command updates go.mod to change the module versions used in a

build. An upgrade of one module may imply upgrading others, and similarly a

downgrade of one module may imply downgrading others. The 'go get' command

makes these implied changes as well. If go.mod is edited directly, commands

like 'go build' or 'go list' will assume that an upgrade is intended and

automatically make any implied upgrades and update go.mod to reflect them.

The 'go mod' command provides other functionality for use in maintaining

and understanding modules and go.mod files. See 'go help mod'.

Pseudo-versions

The go.mod file and the go command more generally use semantic versions as

the standard form for describing module versions, so that versions can be

compared to determine which should be considered earlier or later than another.

A module version like v1.2.3 is introduced by tagging a revision in the

underlying source repository. Untagged revisions can be referred to

using a "pseudo-version" of the form v0.0.0-yyyymmddhhmmss-abcdefabcdef,

where the time is the commit time in UTC and the final suffix is the prefix

of the commit hash. The time portion ensures that two pseudo-versions can

be compared to determine which happened later, the commit hash identifes

the underlying commit, and the v0.0.0- prefix identifies the pseudo-version

as a pre-release before version v0.0.0, so that the go command prefers any

tagged release over any pseudo-version.

Pseudo-versions never need to be typed by hand: the go command will accept

the plain commit hash and translate it into a pseudo-version (or a tagged

version if available) automatically. This conversion is an example of a

module query.

Module queries

The go command accepts a "module query" in place of a module version

both on the command line and in the main module's go.mod file.

(After evaluating a query found in the main module's go.mod file,

the go command updates the file to replace the query with its result.)

A fully-specified semantic version, such as "v1.2.3",

evaluates to that specific version.

A semantic version prefix, such as "v1" or "v1.2",

evaluates to the latest available tagged version with that prefix.

A semantic version comparison, such as "<v1.2.3" or ">=v1.5.6",

evaluates to the available tagged version nearest to the comparison target

(the latest version for < and <=, the earliest version for > and >=).

The string "latest" matches the latest available tagged version,

or else the underlying source repository's latest untagged revision.

A revision identifier for the underlying source repository,

such as a commit hash prefix, revision tag, or branch name,

selects that specific code revision. If the revision is

also tagged with a semantic version, the query evaluates to

that semantic version. Otherwise the query evaluates to a

pseudo-version for the commit.

All queries prefer release versions to pre-release versions.

For example, "<v1.2.3" will prefer to return "v1.2.2"

instead of "v1.2.3-pre1", even though "v1.2.3-pre1" is nearer

to the comparison target.

Module versions disallowed by exclude statements in the

main module's go.mod are considered unavailable and cannot

be returned by queries.

For example, these commands are all valid:

go get github.com/gorilla/mux@latest    # same (@latest is default for 'go get')

go get github.com/gorilla/m...@v1.6.2    # records v1.6.2

go get github.com/gorilla/mux@e3702bed2 # records v1.6.2

go get github.com/gorilla/mux@c856192   # records v0.0.0-20180517173623-c85619274f5d

go get github.com/gorilla/mux@master    # records current meaning of master

Module compatibility and semantic versioning

The go command requires that modules use semantic versions and expects that

the versions accurately describe compatibility: it assumes that v1.5.4 is a

backwards-compatible replacement for v1.5.3, v1.4.0, and even v1.0.0.

More generally the go command expects that packages follow the

"import compatibility rule", which says:

"If an old package and a new package have the same import path, 

the new package must be backwards compatible with the old package."

Because the go command assumes the import compatibility rule,

a module definition can only set the minimum required version of one 

of its dependencies: it cannot set a maximum or exclude selected versions.

Still, the import compatibility rule is not a guarantee: it may be that

v1.5.4 is buggy and not a backwards-compatible replacement for v1.5.3.

Because of this, the go command never updates from an older version

to a newer version of a module unasked.

In semantic versioning, changing the major version number indicates a lack

of backwards compatibility with earlier versions. To preserve import

compatibility, the go command requires that modules with major version v2

or later use a module path with that major version as the final element.

For example, version v2.0.0 of example.com/m must instead use module path

example.com/m/v2, and packages in that module would use that path as

their import path prefix, as in example.com/m/v2/sub/pkg. Including the

major version number in the module path and import paths in this way is

called "semantic import versioning". Pseudo-versions for modules with major

version v2 and later begin with that major version instead of v0, as in

v2.0.0-20180326061214-4fc5987536ef.

The go command treats modules with different module paths as unrelated:

it makes no connection between example.com/m and example.com/m/v2.

Modules with different major versions can be used together in a build

and are kept separate by the fact that their packages use different

import paths.

In semantic versioning, major version v0 is for initial development,

indicating no expectations of stability or backwards compatibility.

Major version v0 does not appear in the module path, because those

versions are preparation for v1.0.0, and v1 does not appear in the

module path either.

As a special case, for historical reasons, module paths beginning with

gopkg.in/ continue to use the conventions established on that system:

the major version is always present, and it is preceded by a dot 

instead of a slash: gopkg.in/yaml.v1 and gopkg.in/yaml.v2, not

gopkg.in/yaml and gopkg.in/yaml/v2.

See https://research.swtch.com/vgo-import and https://semver.org/

for more information.

Module verification

The go command maintains, in the main module's root directory alongside

go.mod, a file named go.sum containing the expected cryptographic checksums

of the content of specific module versions. Each time a dependency is

used, its checksum is added to go.sum if missing or else required to match

the existing entry in go.sum.

The go command maintains a cache of downloaded packages and computes

and records the cryptographic checksum of each package at download time.

In normal operation, the go command checks these pre-computed checksums

against the main module's go.sum file, instead of recomputing them on

each command invocation. The 'go mod -verify' command checks that

the cached copies of module downloads still match both their recorded

checksums and the entries in go.sum.

Modules and vendoring

When using modules, the go command completely ignores vendor directories.

By default, the go command satisfies dependencies by downloading modules

from their sources and using those downloaded copies (after verification,

as described in the previous section). To allow interoperation with older

versions of Go, or to ensure that all files used for a build are stored

together in a single file tree, 'go mod -vendor' creates a directory named

vendor in the root directory of the main module and stores there all the

packages from dependency modules that are needed to support builds and

tests of packages in the main module.

To build using the main module's top-level vendor directory to satisfy

dependencies (disabling use of the usual network sources and local

caches), use 'go build -getmode=vendor'. Note that only the main module's

top-level vendor directory is used; vendor directories in other locations

are still ignored.