# Constructs a styled segment, a building block for styled texts.
#
# - If `$object` is a string, constructs a styled segment with `$object` as the
#   content, and the properties specified by the options.
#
# - If `$object` is a styled segment, constructs a styled segment that is a
#   copy of `$object`, with the properties specified by the options overridden.
#
# The properties of styled segments can be inspected by indexing into it. Valid
# keys are the same as the options to `styled-segment`, plus `text` for the
# string content:
#
# ```elvish-transcript
# ~> var s = (styled-segment abc &bold)
# ~> put $s[text]
# ▶ abc
# ~> put $s[fg-color]
# ▶ default
# ~> put $s[bold]
# ▶ $true
# ```
#
# Prefer the high-level [`styled`]() command to build and transform styled
# texts. Styled segments are a low-level construct, and you only have to deal
# with it when building custom style transformers.
#
# In the following example, a custom transformer sets the `inverse` property
# for every bold segment:
#
# ```elvish
# styled foo(styled bar bold) {|x| styled-segment $x &inverse=$x[bold] }
# # transforms "foo" + bold "bar" into "foo" + bold and inverse "bar"
# ```
fn styled-segment {|object &fg-color=default &bg-color=default &bold=$false &dim=$false &italic=$false &underlined=$false &blink=$false &inverse=$false| }

# Constructs a **styled text** by applying the supplied transformers to the
# supplied `$object`, which may be a string, a [styled
# segment](#styled-segment), or an existing styled text.
#
# Each `$style-transformer` can be one of the following:
#
# - A boolean attribute name:
#
#   - One of `bold`, `dim`, `italic`, `underlined`, `blink` and `inverse` for
#     setting the corresponding attribute.
#
#   - An attribute name prefixed by `no-` for unsetting the attribute.
#
#   - An attribute name prefixed by `toggle-` for toggling the attribute
#     between set and unset.
#
# - A color name for setting the text color, which may be one of the
#   following:
#
#   - One of the 8 basic ANSI colors: `black`, `red`, `green`, `yellow`,
#     `blue`, `magenta`, `cyan` and `white`.
#
#   - The bright variant of the 8 basic ANSI colors, with a `bright-` prefix.
#
#   - Any color from the xterm 256-color palette, as `colorX` (such as
#     `color12`).
#
#   - A 24-bit RGB color written as `#RRGGBB` (such as `'#778899'`).
#
#     **Note**: You need to quote such values, since an unquoted `#` introduces
#     a comment (e.g. use `'bg-#778899'` instead of `bg-#778899`).
#
# - A color name prefixed by `fg-` to set the foreground color. This has
#   the same effect as specifying the color name without the `fg-` prefix.
#
# - A color name prefixed by `bg-` to set the background color.
#
# - A function that receives a styled segment as the only argument and outputs
#   a single styled segment, which will be applied to all the segments.
#
# When a styled text is converted to a string the corresponding
# [ANSI SGR code](https://en.wikipedia.org/wiki/ANSI_escape_code#SGR_.28Select_Graphic_Rendition.29_parameters)
# is built to render the style.
#
# If the [`NO_COLOR`](https://no-color.org) environment variable is set and
# non-empty when Elvish starts, color output is suppressed. Modifications to
# `NO_COLOR` within Elvish (including from `rc.elv`) do not affect the current
# process, but will affect child Elvish processes.
#
# Examples:
#
# ```elvish
# echo (styled foo red bold) # prints red bold "foo"
# echo (styled (styled foo red bold) green) # prints green bold "foo"
# ```
#
# A styled text can contain multiple [segments](#styled-segment) with different
# styles. Such styled texts can be constructed by concatenating multiple styled
# texts with the [compounding](language.html#compounding) syntax. Strings and
# styled segments are automatically "promoted" to styled texts when
# concatenating. Examples:
#
# ```elvish
# echo foo(styled bar red) # prints "foo" + red "bar"
# echo (styled foo bold)(styled bar red) # prints bold "foo" + red "bar"
# ```
#
# The individual segments in a styled text can be extracted by indexing:
#
# ```elvish
# var s = (styled abc red)(styled def green)
# put $s[0] $s[1]
# ```
#
# When printed to the terminal, a styled text is not affected by any existing
# SGR styles in effect, and it will always reset the SGR style afterwards. For
# example:
#
# ```elvish
# print "\e[1m"
# echo (styled foo red)
# echo bar
# # "foo" will be printed as red, but not bold
# # "bar" will be printed without any style
# ```
fn styled {|object @style-transformer| }