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|
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta content="width=device-width,initial-scale=1" name="viewport">
<style>
td, th {
border: 2px solid black;
padding: 0.5em;
}
table {
border-collapse: collapse;
}
</style>
<title>POM Language Specification</title>
<link rel="icon" href="data:,"><!--TODO-->
</head>
<body>
<p>
<a href="/index.html">POM homepage</a>
</p>
<h1>POM Language Specification, v. 0.1.0</h1>
<h2>Introduction</h2>
<p>
POM is a “markup” language, primarily intended for software configuration,
and designed to be easy to parse without any third-party libraries
(e.g. for looking up Unicode classes or matching regular expressions),
while still being terse and legible.
The POM specification is quite strict, to avoid cases where dubious files can
be accepted by some parsers while being rejected by others.
</p>
<p>
Every file describes a <i>configuration</i>, which is a mapping from keys to values.
A <i>key</i> is a string consisting of <i>components</i> separated by dots (<code>.</code>).
A <i>value</i> is a string whose interpretation is entirely decided by the application.
Notably there is no distinction in POM’s syntax between, say,
the number <code>5</code> and the string <code>5</code>.
A configuration, such as the one obtained from the POM file
</p>
<pre><code>[ingredients.sugar]
amount = 50 g
type = brown
[ingredients.flour]
amount = 100 g
type = all-purpose
[baking]
temperature = 150 °C
time = 35 min
</code></pre>
<p>
can either be seen as a simple mapping from keys to values:
</p>
<table>
<thead>
<tr><th>Key</th><th>Value</th></tr>
</thead>
<tbody>
<tr><td>ingredients.sugar.amount</td><td>50 g</td></tr>
<tr><td>ingredients.sugar.type</td><td>brown</td></tr>
<tr><td>ingredients.flour.amount</td><td>100 g</td></tr>
<tr><td>ingredients.flour.type</td><td>all-purpose</td></tr>
<tr><td>baking.temperature</td><td>150 °C</td></tr>
<tr><td>baking.time</td><td>35 min</td></tr>
</tbody>
</table>
<p>
or a tree of keys, with a value associated to each leaf node:
</p>
<div style="text-align:center;">
<svg
viewBox="-281 0 500 200"
style="width:80%;max-width:600px;"
xmlns="http://www.w3.org/2000/svg">
<style>
text { text-anchor: middle; }
rect { stroke: black; fill: none; }
line { stroke: black; stroke-width: 1.5; }
</style>
<rect x="-30" y="6" width="60" height="20" />
<text x="0" y="20" font-size="12">(root)</text>
<line x1="-20" y1="26" x2="-118" y2="46" />
<line x1="20" y1="26" x2="118" y2="46" />
<g transform="translate(-118 46)">
<rect x="-45" y="0" width="90" height="20" />
<text x="0" y="14" font-size="12">ingredients</text>
<line x1="-20" y1="20" x2="-75" y2="54" />
<line x1="20" y1="20" x2="75" y2="54" />
<g transform="translate(-75 54)">
<rect x="-25" y="0" width="50" height="20" />
<text x="0" y="14" font-size="12">sugar</text>
<line x1="-10" y1="20" x2="-43" y2="40" />
<line x1="10" y1="20" x2="43" y2="40" />
<g transform="translate(-43 40)">
<rect x="-40" y="0" width="80" height="36" />
<text x="0" y="14" font-size="12">type</text>
<text x="0" y="28" font-size="12">brown</text>
</g>
<g transform="translate(43 40)">
<rect x="-40" y="0" width="80" height="36" />
<text x="0" y="14" font-size="12">amount</text>
<text x="0" y="28" font-size="12">50 g</text>
</g>
</g>
<g transform="translate(75 54)">
<rect x="-25" y="0" width="50" height="20" />
<text x="0" y="14" font-size="12">flour</text>
<line x1="-10" y1="20" x2="-15" y2="40" />
<line x1="10" y1="20" x2="71" y2="40" />
<g transform="translate(-15 40)">
<rect x="-40" y="0" width="80" height="36" />
<text x="0" y="14" font-size="12">type</text>
<text x="0" y="28" font-size="12">all-purpose</text>
</g>
<g transform="translate(71 40)">
<rect x="-40" y="0" width="80" height="36" />
<text x="0" y="14" font-size="12">amount</text>
<text x="0" y="28" font-size="12">100 g</text>
</g>
</g>
</g>
<g transform="translate(118 46)">
<rect x="-45" y="0" width="90" height="20" />
<line x1="-20" y1="20" x2="-50" y2="40" />
<line x1="20" y1="20" x2="50" y2="40" />
<text x="0" y="14" font-size="12">baking</text>
<g transform="translate(-50 40)">
<rect x="-45" y="0" width="90" height="36" />
<text x="0" y="14" font-size="12">temperature</text>
<text x="0" y="30" font-size="12">150 °C</text>
</g>
<g transform="translate(50 40)">
<rect x="-45" y="0" width="90" height="36" />
<text x="0" y="14" font-size="12">time</text>
<text x="0" y="30" font-size="12">35 min</text>
</g>
</g>
</svg>
</div>
<h2>Error handling</h2>
<p>
All error conditions are described in this specification.
A general-purpose POM parser should not reject a file in any other case,
outside of exceptional circumstances such as running out of memory.
When an error occurs, it should be reported, ideally with information
about the file name and line number, and the file must be entirely rejected
(i.e. parsers must not attempt to preserve only the correct parts of an erroneous file).
Warnings may also be issued according to the judgment of the parser author.
</p>
<h2>Text encoding</h2>
<p>
All POM files are encoded using UTF-8. Both LF and CRLF line endings may be used (see below).
If invalid UTF-8 is encountered, including overlong sequences and UTF-16
surrogate halves (U+D800-DFFF), an error occurs.
</p>
<h2>Valid keys/values</h2>
<p>
Keys in a POM file may contain the following characters
</p>
<ul>
<li>
The ASCII characters <code>a</code>–<code>z</code>, <code>A</code>–<code>Z</code>,
<code>0</code>–<code>9</code>, as well as each of
<code>./-*_</code>.
</li>
<li>
Any non-ASCII Unicode scalar value (U+0080–10FFFF, but not U+D800–U+DFFF).
</li>
</ul>
<p>
A non-empty string containing only these characters is a valid key
if and only if it does not start or end with a dot
and does not contain two dots in a row (<code>..</code>).
</p>
<p>
Any string of non-zero Unicode scalar values (U+0001–10FFFF, but not U+D800–U+DFFF)
is a valid value.
</p>
<h2>Parsing</h2>
<p>
If a “byte order mark” of <code>EF BB BF</code> appears at the start of the file,
it is ignored.
Every carriage return character (U+000D) which immediately
precedes a line feed (U+000A) is deleted.
Then, if any control characters in the range U+0000 to U+001F
other than the line feed and horizontal tab (U+0009) are
present in the file, an error occurs.
</p>
<p>
The <i>current-section</i> is a string variable which should be maintained during parsing.
It is initally equal to the empty string.
</p>
<p>
An <i>accepted-space</i> is either a space (U+0020) or horizontal tab (U+0009) character.
</p>
<p>
Parsing now proceeds line-by-line, with lines being delimited by line feed characters.
For each line:
</p>
<ol>
<li>Any accepted-spaces that appear at the start of the line are removed.</li>
<li>
If the line begins with <code>#%disable warnings</code> or <code>#%enable warnings</code>,
warnings should be disabled/enabled if any are implemented.
</li>
<li>
If the line is empty or begins with <code>#</code>,
parsing proceeds to the next line.
</li>
<li>
If the line begins with <code>[</code>, it is interpreted as a <i>section header</i>.
In this case:
<ol>
<li>
If the line does not end with <code>]</code> optionally succeeded
by any number of accepted-spaces, an error occurs.
</li>
<li>
The current-section is set to the text in between
the initial <code>[</code> and final <code>]</code>
(white space after the <code>[</code> and before the
<code>]</code> is <em>not</em> trimmed).
</li>
<li>
If the new current-section is not empty and not a valid key (see above), an error occurs.
</li>
</ol>
</li>
<li>
Otherwise, the line is now interpreted as a key-value assignment. In this case:
<ol>
<li>If the line does not contain an equal sign (<code>=</code>), an error occurs.</li>
<li>
The <i>relative-key</i> is the text preceding the <code>=</code>,
not including any space or horizontal tab characters
immediately before the <code>=</code>.
</li>
<li>
If the relative-key is not a valid key (see above), an error occurs.
</li>
<li>
Let <i>c</i> be the first character after the <code>=</code> and any succeeding accepted-spaces.
</li>
<li>
If <i>c</i> is <code>"</code> (U+0022 QUOTATION MARK) or <code>`</code> (U+0060 GRAVE ACCENT),
the value is <i>quoted</i>, and spans from the first character after <i>c</i>
to the next unescaped instance of <i>c</i> in the file (which may be on a different line).
In this case,
<ol>
<li>
Escape sequences are processed as described below.
</li>
<li>
Following the closing instance of <i>c</i>, there must be no characters other
than accepted-spaces until the next line feed or the end of the file;
otherwise an error occurs.
</li>
</ol>
</li>
<li>
Otherwise, the value is <i>unquoted</i>. In this case,
<ol>
<li>accepted-spaces at the end of the line are removed.</li>
<li>
The value is the exact text starting from <i>c</i> and going
to the end of the line (escape sequences are not processed).
</li>
</ol>
</li>
<li>If the value is not a valid value (i.e. it contains a null character), an error occurs.</li>
<li>
The key is equal to the relative-key if the current-section is empty; otherwise, it is equal to
the concatenation of current-section, a dot, and the relative-key.
</li>
<li>
The key is assigned to the value.
</li>
</ol>
</li>
</ol>
<h2>Escape sequences</h2>
<p>
POM defines the following escape sequences, which may appear in quoted values.
If a backslash character occurs in a quoted value but does not form
a defined escape sequence, an error occurs.
</p>
<table>
<thead>
<tr><th>Escape sequence</th><th>Value</th></tr>
</thead>
<tbody>
<tr><td><code>\n</code></td><td>Line feed (U+000A)</td></tr>
<tr><td><code>\r</code></td><td>Carriage return (U+000D)</td></tr>
<tr><td><code>\t</code></td><td>Horizontal tab (U+0009)</td></tr>
<tr><td><code>\\</code></td><td>Literal <code>\</code> (U+005C)</td></tr>
<tr><td><code>\"</code></td><td>Literal <code>"</code> (U+0022)</td></tr>
<tr><td><code>\'</code></td><td>Literal <code>'</code> (U+0027)</td></tr>
<tr><td><code>\`</code></td><td>Literal <code>`</code> (U+0060)</td></tr>
<tr><td><code>\,</code></td><td>Literal <code>\,</code> (U+005C U+002C)</td></tr>
<tr><td><code>\x</code><i>NM</i></td>
<td>ASCII character with code <i>NM</i>,
<br>interpreted as hexadecimal
<br>(must be in the range 01–7F).</td>
</tr>
<tr><td><code>\u{</code><i>digits</i><code>}</code></td>
<td>Unicode code point <i>digits</i>,<br>
interpreted as hexadecimal.<br>
<i>digits</i> must be 1–6 characters long,<br>
and may contain leading zeros,<br>
but must not be zero and<br>
must not be a UTF-16 surrogate<br>
half D800–DFFF.</td>
</tr>
</tbody>
</table>
<h2>Lists</h2>
<p>
Although POM does not have a way of specially designating a value as being a list,
there is a recommended syntax for encoding them. Specifically, a value can be treated as a list
by first splitting it into comma-delimited parts, treating <code>\,</code> as a literal comma
in a list entry and <code>\\</code> as a literal backslash,
then removing any accepted-spaces surrounding list entries.
</p>
<p>
List entries may be empty, but if the last entry in a list is empty, it is removed
(if there are two or more empty entries at the end of a list, only one is removed).
As a consequence, an empty string is considered to be an empty list.
</p>
<p>
If a list’s order is irrelevant and it might be large or benefit from labelling its entries,
a key prefix should be used instead
(see the <code>ingredients</code> “list” in the opening example).
</p>
<h3>Examples</h3>
<p>
The following lines describe 3-entry lists.
</p>
<table>
<thead>
<tr><th>POM line</th><th>Entry 1</th><th>Entry 2</th><th>Entry 3</th></tr>
</thead>
<tbody>
<tr>
<td><code>fonts = monospace, sans-serif, serif</code></td>
<td><code>monospace</code></td>
<td><code>sans-serif</code></td>
<td><code>serif</code></td>
</tr>
<tr>
<td><code>files = " foo.txt, weird\,name,z "</code></td>
<td><code>foo.txt</code></td>
<td><code>weird,name</code></td>
<td><code>z</code></td>
</tr>
<tr>
<td><code>things = \,,,76</code></td>
<td><code>,</code></td>
<td><code></code></td>
<td><code>76</code></td>
</tr>
<tr>
<td><code>empties = ,,,,</code></td>
<td><code></code></td>
<td><code></code></td>
<td><code></code></td>
</tr>
<tr>
<td><code>escapees = \\,\a,\,</code></td>
<td><code>\</code></td>
<td><code>\a</code></td>
<td><code>,</code></td>
</tr>
</tbody>
</table>
<h2>Merging configurations</h2>
<p>
A configuration <i>B</i> can be <i>merged into</i> another configuration <i>A</i>
by parsing both of them and setting the value associated with a key <i>k</i> to be
</p>
<ol>
<li>The value associated with <i>k</i> in <i>B</i>, if any.</li>
<li>Otherwise, the value associated with <i>k</i> in <i>A</i>, if any.</li>
</ol>
<p>
(Likewise, an ordered series of configurations <i>A<sub>1</sub></i>, …, <i>A<sub>n</sub></i>
can be merged by merging <i>A<sub>2</sub></i> into <i>A<sub>1</sub></i>,
then <i>A<sub>3</sub></i> into the resulting configuration, etc.)
</p>
<p>
This is useful, for example, when you want to have a global configuration for
a piece of software installed on a multi-user machine where individual settings
can be overriden by each user (in this case, the user configuration would
be merged into the global configuration).
</p>
<h2>Schemas</h2>
<p>
A <i>schema</i> is a POM file that describes how other POM files should be formatted
(i.e. what keys they should include, and what values they can be associated with).
A configuration can be said to <i>follow</i> a schema
when it obeys all of the schema’s rules.
</p>
<p>
POM’s schema format is not powerful to enforce all possible restrictions;
some will have to be enforced by the application.
</p>
<p>
Every schema key must be of the form <i>k</i><code>.</code><i>rule</i>, where
<i>k</i> is a valid key, and <i>rule</i> is one of the rule names listed below.
</p>
<p>
For any valid key <i>k</i> the value of the rule <i>rule</i> for <i>k</i> is determined as follows:
</p>
<ul>
<li>Break <i>k</i> down into its <i>n</i> components.</li>
<li>For each schema key <i>s</i>.<i>rule</i> where <i>s</i> has <i>n</i> components:
<ul>
<li>Match up the components of <i>s</i> with the components of <i>k</i>.</li>
<li>
If every component of <i>s</i> is equal to <code>*</code> or the corresponding component
of <i>k</i>, add <i>s</i>.<i>rule</i> to the <i>candidate-list</i>.
</li>
</ul></li>
<li>If the candidate-list is empty, use the default value for the rule.</li>
<li>If the candidate-list has exactly one schema key, use the value of that schema key.</li>
<li>
Otherwise select the schema key in the candidate-list with the
latest first <code>*</code>-component (in terms of component index),
preferring keys with no <code>*</code>,
breaking ties by the latest second <code>*</code>-component,
preferring keys with only one <code>*</code>, etc. Use its value.
</li>
</ul>
<h3>Example</h3>
<p>
Given the following schema:
</p>
<pre><code>
*.*.id.type = Int
vehicle.*.id.type = UInt
*.truck.id.type = Float
vehicle.car.id.type = String
</code></pre>
<p>
The value of the <code>type</code> rule for vehicle.car.id is String,
for vehicle.truck.id is UInt,
for my.truck.id is Float,
for my.car.id is Int,
and for my.nephews.car.id is String (no schema key matches, so the default of String is used).
</p>
<h3><code>type</code> rule</h3>
<p>
Default: <code>String</code>.
</p>
<p>
This describes what values a key is allowed to be associated with.
The following types are defined:
</p>
<ul>
<li><code>String</code> — accepts any value</li>
<li><code>None</code> — accepts an unset value (special—see below)</li>
<li><code>Empty</code> — accepts an empty value</li>
<li><code>Bool</code> — accepts <code>true</code>, <code>on</code>, <code>yes</code>,
<code>false</code>, <code>off</code>, <code>no</code> (case-sensitive).</li>
<li><code>UInt</code> — accepts any unsigned integer that fits in 63 [sic] bits,
written in decimal or <code>0x</code> or <code>0X</code>-prefixed hexadecimal.
A leading <code>+</code> is permitted, but <code>-0</code> is not.
Only 63 bits are allowed to support languages (Java) with no 64-bit unsigned integers.</li>
<li><code>Int</code> — accepts any (two’s complement) signed integer that fits in 64 bits,
written in decimal or <code>0x</code> or <code>0X</code>-prefixed hexadecimal.
A leading <code>+</code> (or, of course, <code>-</code>) is permitted.</li>
<li><code>Float</code> —
A floating-point number, written in ordinary decimal
(e.g. <code>-1.234</code>, <code>7.</code>, <code>265</code>) or in scientific notation
(e.g. <code>3e5</code>, <code>3.E-5</code>, <code>-3.7e+5</code>).
A leading <code>+</code> (or, of course, <code>-</code>) is permitted.
<li>
<code>'</code><i>value</i><code>'</code> —
accepts the literal value <i>value</i>.
<i>value</i> cannot contain a literal apostrophe <code>'</code>.
</li>
<li>
<i>T</i><code> | </code><i>U</i>, where <i>T</i>, <i>U</i> are types
— accepts a value of type <i>T</i> or <i>U</i>.
</li>
<li>
<code>Optional[</code><i>T</i><code>]</code>, where <i>T</i> is a type
— equivalent to <i>T</i><code> | None</code>.
</li>
<li>
<code>List[</code><i>T</i><code>]</code>, where <i>T</i> is a type
— accepts a list of entries of type <i>T</i> (see description
of lists above). Nested lists are not permitted.
</li>
</ul>
<p>
A type may optionally have accepted-spaces on either side; this does not change its meaning.
</p>
<h3><code>allow_unknown</code> rule</h3>
<p>
Default: inherited from parent (i.e. if <i>k</i> = <i>j</i><code>.</code><i>component</i>,
look up the <code>allow_unknown</code> rule for <i>j</i>),
or <code>yes</code> if <i>k</i> has no parent (does not contain a dot).
</p>
<p>
This describes whether or the key <i>k</i> is allowed if it is not described in the schema.
It must be set to either <code>yes</code> or <code>no</code>.
</p>
<p>
If a key is encountered in a configuration
and the value of its <code>allow_unknown</code> rule is <code>no</code>,
and it has no matching <code>type</code>, <code>maxlength</code>, or <code>default</code> rule,
the configuration does not follow the schema.
</p>
<h3><code>min</code>, <code>max</code> rules</h3>
<p>
This schema key’s value sets the minimum/maximum value for the key’s value.
This may only be set if <code>type</code> explicitly allows numeric values
(i.e. it contains a type <code>Int</code>/<code>UInt</code>/<code>Float</code>).
</p>
<h3><code>maxlength</code> rule</h3>
<p>
This schema key’s value must be a non-negative integer no greater than 2<sup>31</sup>−1,
written in decimal with no leading sign.
It specifies that the value of a key can be no longer than that number of UTF-8 bytes.
</p>
<h3><code>default</code> rule</h3>
<p>
Sets the default value for a key.
</p>
<h3>Missing values</h3>
<p>
If there is a schema key <i>k</i><code>.type</code>, where <i>k</i>
does not contain any <code>*</code>-components,
and the type does not allow unset values (<code>None</code>),
and there is no schema key <i>k</i><code>.default</code>,
then a configuration must contain the key <i>k</i> to follow the schema.
</p>
<p>
Additionally, if there is a schema key <i>j</i><code>.*.</code><i>k</i><code>.type</code>
that does not allow unset values and no correspoding <code>default</code> schema key,
where <i>k</i> does not contain any <code>*</code>-components, then a configuration
containing a key <i>x</i> matching <i>j</i><code>.*</code> must also contain
the key <i>x</i><code>.</code><i>k</i>.
</p>
<h2>Extensions</h2>
<p>
If needed for a particular domain, an parser may accept an extended form of the POM syntax.
Ideally, extensions should use lines containing invalid key characters (e.g. <code>!&%</code>)
before the <code>=</code> (if any)
so that there is no ambiguity, and the file cannot be parsed without the extension.
</p>
<h2>API recommendations</h2>
<p>
The following functions are (lightly) recommended
in any general-purpose library for parsing POM files
(their exact names/signatures can be changed to fit the style of the language).
</p>
<ul>
<li>
<code>load(file: File) -> Configuration</code><br>
Load a configuration from a file.
</li>
<li>
<code>load_string(string: String) -> Configuration</code><br>
Load a configuration from a string
(may be overloaded with <code>load</code> if language supports it).
</li>
<li>
<code>load_path(path: String) -> Configuration</code><br>
Convenience function for loading by path.
</li>
<li>
<code>print(conf: Configuration)</code><br>
Print <code>key = value</code> on a separate line for each key in <code>conf</code>,
sorted alphabetically by key. The value format doesn’t need to match POM’s format exactly,
since this function should only be used for debugging anyways.
</li>
<li>
<code>has(conf: Configuration, key: String) -> Bool</code><br>
Returns whether <code>key</code> is associated with any value.
</li>
<li>
<code>keys(conf: Configuration) -> List<String></code><br>
Returns a list of all unique first components of keys in the configuration,
in an arbitrary order.
</li>
<li>
<code>subkeys(conf: Configuration, key: String) -> List<String></code><br>
Equivalent to <code>keys(section(conf, key))</code> but may be implemented more efficiently.
</li>
<li>
<code>location(conf: Configuration, key: String) -> Optional<Location></code><br>
Location of the definition of <code>key</code> in the configuration (file and line number).
Useful for reporting invalid values when
the format of valid values can’t be described by a schema type.
If a key <i>k</i> isn’t given a value in the configuration, but a key of the form
<i>k</i><code>.</code><i>j</i> is, then the
location of the definition of an arbitrary such key
should be considered the location of <i>k</i>.
</li>
<li>
<code>get(conf: Configuration, key: String) -> Optional<String></code><br>
Get value associated with <code>key</code>, if any exists.
</li>
<li>
<code>get_or_default(conf: Configuration, key: String, default: String) -> String</code><br>
Get value associated with <code>key</code>, if any exists, returning <code>default</code> if not.
</li>
<li>
<code>get_int(conf: Configuration, key: String) -> Optional<Int></code><br>
<code>get_int_or_default(conf: Configuration, key: String, default: Int) -> Int</code><br>
Get value associated with <code>key</code>,
if any exists, and parse it as a signed 64-bit integer,
following the <code>Int</code> schema type described above
(returning <code>default</code> if the key doesn’t exist).
Returns an error if the key exists but its value is not a valid signed 64-bit integer.
</li>
<li>
<code>get_uint(conf: Configuration, key: String) -> Optional<UInt></code><br>
<code>get_uint_or_default(conf: Configuration, key: String, default: UInt) -> UInt</code><br>
Get value associated with <code>key</code>, if any exists,
and parse it as an unsigned 63-bit [sic] integer,
following the <code>UInt</code> schema type described above
(returning <code>default</code> if the key doesn’t exist).
Returns an error if the key exists but its value is not a valid unsigned 63-bit integer.
</li>
<li>
<code>get_float(conf: Configuration, key: String) -> Optional<Float></code><br>
<code>get_float_or_default(conf: Configuration, key: String, default: Float) -> Float</code><br>
Get value associated with <code>key</code>, if any exists,
and parse it as a 64-bit IEEE-754 double precision
floating-point number, following the <code>Float</code> schema type described above
(returning <code>default</code> if the key doesn’t exist).
Returns an error if the key exists but its value is not a valid floating-point number.
</li>
<li>
<code>get_bool(conf: Configuration, key: String) -> Optional<Bool></code><br>
<code>get_bool_or_default(conf: Configuration, key: String, default: Bool) -> Bool</code><br>
Get value associated with <code>key</code>, if any exists, and parse it as a boolean,
taking <code>true</code>, <code>on</code>, <code>yes</code> to be <code>true</code>, and
<code>false</code>, <code>off</code>, <code>no</code> to be <code>false</code> (case-sensitive).
Returns an error if the key exists but is not one of those values.
</li>
<li>
<code>get_list(conf: Configuration, key: String) -> Optional<List<String>></code><br>
<code>get_list_or_default(conf: Configuration, key: String, default: List<String>) -> List<String></code><br>
Get value associated with <code>key</code>, if any exists, and parse it as a list
(returning <code>default</code> if the key isn’t present).
</li>
<li>
<code>section(conf: Configuration, key: String) -> Configuration</code><br>
Get the sub-configuration consisting of the descendants of <code>key</code>
(i.e. keys starting with <code>key.</code>),
with the initial <code>key.</code> removed, and their corresponding values.
Returns an empty configuration if there are no descendants of <code>key</code> defined.
</li>
<li>
<code>check_against_schema(conf: Configuration, schema: Configuration) -> Configuration</code> (<i>optional</i>)<br>
Check that <code>schema</code>
is a valid schema and that <code>conf</code> follows it,
returning an error with detailed information
if not. If successful, returns a configuration with default values filled out.
</li>
<li>
<code>merge(conf_a: Configuration, conf_b: Configuration) -> Configuration</code><br>
Returns the result of merging <code>conf_b</code> into <code>conf_a</code>.
</li>
</ul>
<h2>Examples</h2>
<p>
This section lists some examples of POM files. For more examples, see the <code>tests/</code>
directory in the main POM repository.
</p>
<h3>All syntax</h3>
This is a configuration which demonstrates almost all of the syntactic forms of POM.
<pre><code>
title = 'Crème brûlée'
0-*/_description_/*-0 =`A 'beautiful' crème br\u{FB}l\u{0000e9}e recipe
that\'s sure to delight your friends!`
author == `Jean\0\\"P." D'Martingale
[ingredients.flour]
quantity= "100 g"
type="all-purpose"
[ingredients.sugar]
quantity = 50 g
type = "br\x6f\u{77}n"
[ingrédients]
œufs.quantité=3
œufs.type = "extra large\,farm fresh\\,free-range"
[]
DIRECTIONS.en_CA.version.5 = "
1. Separate the egg yolks from the \"whites\".
2. Mix the yolks in a bowl with the sugar.
…
59. Enjoy!
"
</code></pre>
<p>
This configuration has the following mapping of keys to values:
</p>
<table>
<thead>
<tr><th>Key</th><th>Value</th></tr>
</thead>
<tbody>
<tr><td>title</td><td>'Crème brûlée'</td></tr>
<tr><td>0-*/_description_/*-0</td><td>A 'beautiful' crème brûlée recipe<br>
that's sure to delight your friends!</td></tr>
<tr><td>author</td><td>= `Jean\0\\"P." D'Martingale</td></tr>
<tr><td>ingredients.flour.quantity</td><td>100 g</td></tr>
<tr><td>ingredients.flour.type</td><td>all-purpose</td></tr>
<tr><td>ingredients.sugar.quantity</td><td>50 g</td></tr>
<tr><td>ingredients.sugar.type</td><td>brown</td></tr>
<tr><td>ingrédients.œufs.quantité</td><td>3</td></tr>
<tr><td>ingrédients.œufs.type</td><td>extra large\,farm fresh\,free-range</td></tr>
<tr><td>DIRECTIONS.en_CA.version.5</td><td><br>1. Separate the egg yolks from the "whites".<br>
2. Mix the yolks in a bowl with the sugar.<br>
…<br>
59. Enjoy!<br><br></td></tr>
</tbody>
</table>
<h3>Configuration for a text editor</h3>
<pre><code>
indentation-type = tabs
show-line-numbers = yes
tab-size = 4
font-size = "18"
[file-extensions]
C = .c
Cpp = .cpp, .h, .hpp
[plug-in.edit-over-ssh]
path = ~/misc/edit-over-ssh.so
enabled = yes
[plug-in.edit-over-ssh.settings]
favourite-host = my-web-server
[plug-in.edit-over-ssh.settings.hosts.my-web-server]
address = example.org
port = 22
ssh-key = ~/.ssh/id_ed25519
</code></pre>
<p>
This configuration has the following mapping of keys to values:
</p>
<table>
<thead>
<tr><th>Key</th><th>Value</th></tr>
</thead>
<tbody>
<tr><td>indentation-type</td><td>tabs</td></tr>
<tr><td>show-line-numbers</td><td>yes</td></tr>
<tr><td>tab-size</td><td>4</td></tr>
<tr><td>font-size</td><td>18</td></tr>
<tr><td>file-extensions.C</td><td>.c</td></tr>
<tr><td>file-extensions.Cpp</td><td>.cpp, .h, .hpp</td></tr>
<tr><td>plug-in.edit-over-ssh.path</td><td>~/misc/edit-over-ssh.so</td></tr>
<tr><td>plug-in.edit-over-ssh.enabled</td><td>yes</td></tr>
<tr><td>plug-in.edit-over-ssh.settings.favourite-host</td><td>my-web-server</td></tr>
<tr><td>plug-in.edit-over-ssh.settings.hosts.my-web-server.address</td><td>example.org</td></tr>
<tr><td>plug-in.edit-over-ssh.settings.hosts.my-web-server.port</td><td>22</td></tr>
<tr><td>plug-in.edit-over-ssh.settings.hosts.my-web-server.ssh-key</td><td>~/.ssh/id_ed25519</td></tr>
</tbody>
</table>
<h3>Schema for text editor’s configuration</h3>
<p>
Here is a schema which a text editor might use.
The example text editor configuration above follows it.
</p>
<pre><code>
# don't allow unknown keys by default
*.allow_unknown = no
# must put this in your config! I can't make the decision for you!
indentation-type.type = 'spaces' | 'tabs'
show-line-numbers.type = Bool
show-line-numbers.default = on
[tab-size]
type = UInt
min = 1
default = 4
[font-size]
# allow fractional font sizes; why not!
type = Float
min = 0.5
max = 100
default = 14
[plug-in.*]
enabled.type = Bool
enabled.default = yes
path.type = String
# everyone be nice to the Microsoft Windows®
path.maxlength = 260
# allow arbitrary keys in plug-ins' settings
settings.allow_unknown = yes
[file-extensions]
*.type = List[String]
C.default = .c, .h
Cpp.default = .cpp, .hpp, .cc, .hh
C-sharp.default = .cs
</code></pre>
<h3>Errors</h3>
<p>
This section lists some erroneous lines that might appear in a POM file:
</p>
<pre><code>
# Invalid key character '!'
cool-key! = 23
# Invalid key character ' '
fun times = yes
# Missing equals
music is on
# No closing ]
[my.section
# Invalid key character ' '
[ my.section ]
# Invalid escape sequence "\?"
no_trigraph = "a?\?=b"
# Invalid escape sequence "\xCE" — even though "\xCE\x92" is valid UTF-8.
# ("\u{392}" should be used instead)
capital_beta = "\xCE\x92"
# Invalid escape sequence "\x00" / Invalid character in value (null character)
C_string = "Hello, world!\x00"
# Stray characters after closing "
name = "Andy" B
# Duplicate key 'tab-size'
tab-size = 4
tab-size = 8
</code></pre>
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