Field Modifiers¶
Field modifiers are flags which modify the way content emitted for particular output styles:
M Name Description a argument The content appears as a ‘const char *’ argument c colon A colon (“:”) is appended after the label d display Only emit field for display styles (text/HTML) e encoding Only emit for encoding styles (XML/JSON) g gettext Call gettext on field’s render content h humanize (hn) Format large numbers in human-readable style hn-space Humanize: Place space between numeric and unit hn-decimal Humanize: Add a decimal digit, if number < 10 hn-1000 Humanize: Use 1000 as divisor instead of 1024 k key Field is a key, suitable for XPath predicates l leaf-list Field is a leaf-list n no-quotes Do not quote the field when using JSON style p plural Gettext: Use comma-separated plural form q quotes Quote the field when using JSON style t trim Trim leading and trailing whitespace w white A blank (” “) is appended after the label
Roles and modifiers can also use more verbose names, when preceded by a comma. For example, the modifier string “Lwc” (or “L,white,colon”) means the field has a label role (text that describes the next field) and should be followed by a colon (‘c’) and a space (‘w’). The modifier string “Vkq” (or “:key,quote”) means the field has a value role (the default role), that it is a key for the current instance, and that the value should be quoted when encoded for JSON.
The Argument Modifier ({a:})¶
The argument modifier indicates that the content of the field descriptor will be placed as a UTF-8 string (const char *) argument within the xo_emit parameters:
EXAMPLE:
xo_emit("{La:} {a:}\n", "Label text", "label", "value");
TEXT:
Label text value
JSON:
"label": "value"
XML:
<label>value</label>
The argument modifier allows field names for value fields to be passed on the stack, avoiding the need to build a field descriptor using snprintf. For many field roles, the argument modifier is not needed, since those roles have specific mechanisms for arguments, such as “{C:fg-%s}”.
The Colon Modifier ({c:})¶
The colon modifier appends a single colon to the data value:
EXAMPLE:
xo_emit("{Lc:Name}{:name}\n", "phil");
TEXT:
Name:phil
The colon modifier is only used for the TEXT and HTML output styles. It is commonly combined with the space modifier (‘{w:}’). It is purely a convenience feature.
The Display Modifier ({d:})¶
The display modifier indicated the field should only be generated for the display output styles, TEXT and HTML:
EXAMPLE:
xo_emit("{Lcw:Name}{d:name} {:id/%d}\n", "phil", 1);
TEXT:
Name: phil 1
XML:
<id>1</id>
The display modifier is the opposite of the encoding modifier, and they are often used to give to distinct views of the underlying data.
The Encoding Modifier ({e:})¶
The display modifier indicated the field should only be generated for the display output styles, TEXT and HTML:
EXAMPLE:
xo_emit("{Lcw:Name}{:name} {e:id/%d}\n", "phil", 1);
TEXT:
Name: phil
XML:
<name>phil</name><id>1</id>
The encoding modifier is the opposite of the display modifier, and they are often used to give to distinct views of the underlying data.
The Gettext Modifier ({g:})¶
The gettext modifier is used to translate individual fields using the
gettext domain (typically set using the “{G:}” role) and current
language settings. Once libxo renders the field value, it is passed
to gettext(3), where it is used as a key to find the native language
translation.
In the following example, the strings “State” and “full” are passed to gettext() to find locale-based translated strings:
xo_emit("{Lgwc:State}{g:state}\n", "full");
See The Gettext Role ({G:}), The Plural Modifier ({p:}), and Howto: Internationalization (i18n) for additional details.
The Humanize Modifier ({h:})¶
The humanize modifier is used to render large numbers as in a human-readable format. While numbers like “44470272” are completely readable to computers and savants, humans will generally find “44M” more meaningful.
“hn” can be used as an alias for “humanize”.
The humanize modifier only affects display styles (TEXT and HMTL).
The “no-humanize” option (See Command-line Arguments) will block
the function of the humanize modifier.
There are a number of modifiers that affect details of humanization.
These are only available in as full names, not single characters. The
“hn-space” modifier places a space between the number and any
multiplier symbol, such as “M” or “K” (ex: “44 K”). The
“hn-decimal” modifier will add a decimal point and a single tenths
digit when the number is less than 10 (ex: “4.4K”). The “hn-1000”
modifier will use 1000 as divisor instead of 1024, following the
JEDEC-standard instead of the more natural binary powers-of-two
tradition:
EXAMPLE:
xo_emit("{h:input/%u}, {h,hn-space:output/%u}, "
"{h,hn-decimal:errors/%u}, {h,hn-1000:capacity/%u}, "
"{h,hn-decimal:remaining/%u}\n",
input, output, errors, capacity, remaining);
TEXT:
21, 57 K, 96M, 44M, 1.2G
In the HTML style, the original numeric value is rendered in the “data-number” attribute on the <div> element:
<div class="data" data-tag="errors"
data-number="100663296">96M</div>
The Key Modifier ({k:})¶
The key modifier is used to indicate that a particular field helps uniquely identify an instance of list data:
EXAMPLE:
xo_open_list("user");
for (i = 0; i < num_users; i++) {
xo_open_instance("user");
xo_emit("User {k:name} has {:count} tickets\n",
user[i].u_name, user[i].u_tickets);
xo_close_instance("user");
}
xo_close_list("user");
Currently the key modifier is only used when generating XPath value for the HTML output style when XOF_XPATH is set, but other uses are likely in the near future.
The Leaf-List Modifier ({l:})¶
The leaf-list modifier is used to distinguish lists where each instance consists of only a single value. In XML, these are rendered as single elements, where JSON renders them as arrays:
EXAMPLE:
for (i = 0; i < num_users; i++) {
xo_emit("Member {l:user}\n", user[i].u_name);
}
XML:
<user>phil</user>
<user>pallavi</user>
JSON:
"user": [ "phil", "pallavi" ]
The name of the field must match the name of the leaf list.
The No-Quotes Modifier ({n:})¶
The no-quotes modifier (and its twin, the ‘quotes’ modifier) affect the quoting of values in the JSON output style. JSON uses quotes for string value, but no quotes for numeric, boolean, and null data. xo_emit applies a simple heuristic to determine whether quotes are needed, but often this needs to be controlled by the caller:
EXAMPLE:
const char *bool = is_true ? "true" : "false";
xo_emit("{n:fancy/%s}", bool);
JSON:
"fancy": true
The Plural Modifier ({p:})¶
The plural modifier selects the appropriate plural form of an expression based on the most recent number emitted and the current language settings. The contents of the field should be the singular and plural English values, separated by a comma:
xo_emit("{:bytes} {Ngp:byte,bytes}\n", bytes);
The plural modifier is meant to work with the gettext modifier ({g:}) but can work independently. See The Gettext Modifier ({g:}).
When used without the gettext modifier or when the message does not appear in the message catalog, the first token is chosen when the last numeric value is equal to 1; otherwise the second value is used, mimicking the simple pluralization rules of English.
When used with the gettext modifier, the ngettext(3) function is called to handle the heavy lifting, using the message catalog to convert the singular and plural forms into the native language.
The Quotes Modifier ({q:})¶
The quotes modifier (and its twin, the ‘no-quotes’ modifier) affect the quoting of values in the JSON output style. JSON uses quotes for string value, but no quotes for numeric, boolean, and null data. xo_emit applies a simple heuristic to determine whether quotes are needed, but often this needs to be controlled by the caller:
EXAMPLE:
xo_emit("{q:time/%d}", 2014);
JSON:
"year": "2014"
The heuristic is based on the format; if the format uses any of the following conversion specifiers, then no quotes are used:
d i o u x X D O U e E f F g G a A c C p
The Trim Modifier ({t:})¶
The trim modifier removes any leading or trailing whitespace from the value:
EXAMPLE:
xo_emit("{t:description}", " some input ");
JSON:
"description": "some input"
The White Space Modifier ({w:})¶
The white space modifier appends a single space to the data value:
EXAMPLE:
xo_emit("{Lw:Name}{:name}\n", "phil");
TEXT:
Name phil
The white space modifier is only used for the TEXT and HTML output styles. It is commonly combined with the colon modifier (‘{c:}’). It is purely a convenience feature.
Note that the sense of the ‘w’ modifier is reversed for the units role ({Uw:}); a blank is added before the contents, rather than after it.