Add zsh-completion reference guide

complete/_rg

@@ -6,8 +6,8 @@
 # Run ci/test_complete.sh after building to ensure that the options supported by
 # this function stay in synch with the `rg` binary.
 #
-# @see http://zsh.sourceforge.net/Doc/Release/Completion-System.html
-# @see https://github.com/zsh-users/zsh/blob/master/Etc/completion-style-guide
+# For convenience, a completion reference guide is included at the bottom of
+# this file.
 #
 # Originally based on code from the zsh-users project — see copyright notice
 # below.
@@ -362,6 +362,157 @@ _rg_types() {
 
 _rg "$@"
 
+################################################################################
+# ZSH COMPLETION REFERENCE
+#
+# For the convenience of developers who aren't especially familiar with zsh
+# completion functions, a brief reference guide follows. This is in no way
+# comprehensive; it covers just enough of the basic structure, syntax, and
+# conventions to help someone make simple changes like adding new options. For
+# more complete documentation regarding zsh completion functions, please see the
+# following:
+#
+# * http://zsh.sourceforge.net/Doc/Release/Completion-System.html
+# * https://github.com/zsh-users/zsh/blob/master/Etc/completion-style-guide
+#
+# OVERVIEW
+#
+# Most zsh completion functions are defined in terms of `_arguments`, which is a
+# shell function that takes a series of argument specifications. The specs for
+# `rg` are stored in an array, which is common for more complex functions; the
+# elements of the array are passed to `_arguments` on invocation.
+#
+# ARGUMENT-SPECIFICATION SYNTAX
+#
+# The following is a contrived example of the argument specs for a simple tool:
+#
+#   '(: * -)'{-h,--help}'[display help information]'
+#   '(-q -v --quiet --verbose)'{-q,--quiet}'[decrease output verbosity]'
+#   '!(-q -v --quiet --verbose)--silent'
+#   '(-q -v --quiet --verbose)'{-v,--verbose}'[increase output verbosity]'
+#   '--color=[specify when to use colors]:when:(always never auto)'
+#   '*:example file:_files'
+#
+# Although there may appear to be six specs here, there are actually nine; we
+# use brace expansion to combine specs for options that go by multiple names,
+# like `-q` and `--quiet`. This is customary, and ties in with the fact that zsh
+# merges completion possibilities together when they have the same description.
+#
+# The first line defines the option `-h`/`--help`. With most tools, it isn't
+# useful to complete anything after `--help` because it effectively overrides
+# all others; the `(: * -)` at the beginning of the spec tells zsh not to
+# complete any other operands (`:` and `*`) or options (`-`) after this one has
+# been used. The `[...]` at the end associates a description with `-h`/`--help`;
+# as mentioned, zsh will see the identical descriptions and merge these options
+# together when offering completion possibilities.
+#
+# The next line defines `-q`/`--quiet`. Here we don't want to suppress further
+# completions entirely, but we don't want to offer `-q` if `--quiet` has been
+# given (since they do the same thing), nor do we want to offer `-v` (since it
+# doesn't make sense to be quiet and verbose at the same time). We don't need to
+# tell zsh not to offer `--quiet` a second time, since that's the default
+# behaviour, but since this line expands to two specs describing `-q` *and*
+# `--quiet` we do need to explicitly list all of them here.
+#
+# The next line defines a hidden option `--silent` — maybe it's a deprecated
+# synonym for `--quiet`. The leading `!` indicates that zsh shouldn't offer this
+# option during completion. The benefit of providing a spec for an option that
+# shouldn't be completed is that, if someone *does* use it, we can correctly
+# suppress completion of other options afterwards.
+#
+# The next line defines `-v`/`--verbose`; this works just like `-q`/`--quiet`.
+#
+# The next line defines `--color`. In this example, `--color` doesn't have a
+# corresponding short option, so we don't need to use brace expansion. Further,
+# there are no other options it's exclusive with (just itself), so we don't need
+# to define those at the beginning. However, it does take a mandatory argument.
+# The `=` at the end of `--color=` indicates that the argument may appear either
+# like `--color always` or like `--color=always`; this is how most GNU-style
+# command-line tools work. The corresponding short option would normally use `+`
+# — for example, `-c+` would allow either `-c always` or `-calways`. For this
+# option, the arguments are known ahead of time, so we can simply list them in
+# parentheses at the end (`when` is used as the description for the argument).
+#
+# The last line defines an operand (a non-option argument). In this example, the
+# operand can be used any number of times (the leading `*`), and it should be a
+# file path, so we tell zsh to call the `_files` function to complete it. The
+# `example file` in the middle is the description to use for this operand; we
+# could use a space instead to accept the default provided by `_files`.
+#
+# GROUPING ARGUMENT SPECIFICATIONS
+#
+# Newer versions of zsh support grouping argument specs together. All specs
+# following a `+` and then a group name are considered to be members of the
+# named group. Grouping is useful mostly for organisational purposes; it makes
+# the relationship between different options more obvious, and makes it easier
+# to specify exclusions.
+#
+# We could rewrite our example above using grouping as follows:
+#
+#   '(: * -)'{-h,--help}'[display help information]'
+#   '--color=[specify when to use colors]:when:(always never auto)'
+#   '*:example file:_files'
+#   + '(verbosity)'
+#   {-q,--quiet}'[decrease output verbosity]'
+#   '!--silent'
+#   {-v,--verbose}'[increase output verbosity]'
+#
+# Here we take advantage of a useful feature of spec grouping — when the group
+# name is surrounded by parentheses, as in `(verbosity)`, it tells zsh that all
+# of the options in that group are exclusive with each other. As a result, we
+# don't need to manually list out the exclusions at the beginning of each
+# option.
+#
+# Groups can also be referred to by name in other argument specs; for example:
+#
+#   '(xyz)--aaa' '*: :_files'
+#   + xyz --xxx --yyy --zzz
+#
+# Here we use the group name `xyz` to tell zsh that `--xxx`, `--yyy`, and
+# `--zzz` are not to be completed after `--aaa`. This makes the exclusion list
+# much more compact and reusable.
+#
+# CONVENTIONS
+#
+# zsh completion functions generally adhere to the following conventions:
+#
+# * Use two spaces for indentation
+# * Combine specs for options with different names using brace expansion
+# * In combined specs, list the short option first (as in `{-a,--text}`)
+# * Use `+` or `=` as described above for options that take arguments
+# * Provide a description for all options, option-arguments, and operands
+# * Capitalise/punctuate argument descriptions as phrases, not complete
+#   sentences — 'display help information', never 'Display help information.'
+#   (but still capitalise acronyms and proper names)
+# * Write argument descriptions as verb phrases — 'display x', 'enable y',
+#   'use z'
+# * Word descriptions to make it clear when an option expects an argument;
+#   usually this is done with the word 'specify', as in 'specify x' or
+#   'use specified x')
+# * Write argument descriptions as tersely as possible — for example, articles
+#   like 'a' and 'the' should be omitted unless it would be confusing
+#
+# Other conventions currently used by this function:
+#
+# * Order argument specs alphabetically by group name, then option name
+# * Group options that are directly related, mutually exclusive, or frequently
+#   referenced by other argument specs
+# * Use only characters in the set [a-z0-9_-] in group names
+# * Order exclusion lists as follows: short options, long options, groups
+# * Use American English in descriptions
+# * Use 'don't' in descriptions instead of 'do not'
+# * Word descriptions for related options as similarly as possible. For example,
+#   `--foo[enable foo]` and `--no-foo[disable foo]`, or `--foo[use foo]` and
+#   `--no-foo[don't use foo]`
+# * Word descriptions to make it clear when an option only makes sense with
+#   another option, usually by adding '(with -x)' to the end
+# * Don't quote strings or variables unnecessarily. When quotes are required,
+#   prefer single-quotes to double-quotes
+# * Prefix option specs with `$no` when the option serves only to negate the
+#   behaviour of another option that must be provided explicitly by the user.
+#   This prevents rarely used options from cluttering up the completion menu
+################################################################################
+
 # ------------------------------------------------------------------------------
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