vendor/cmd/go/internal/modload/help.go

// Copyright 2018 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package modload

import "cmd/go/internal/base"

// TODO(rsc): The links out to research.swtch.com here should all be
// replaced eventually with links to proper documentation.

var HelpModules = &base.Command{
	UsageLine: "modules",
	Short:     "modules, module versions, and more",
	Long: `
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/mux@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.
	`,
}