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+ <h1 id="adding-indent-queries"><a class="header" href="#adding-indent-queries">Adding indent queries</a></h1>
+<p>Helix uses tree-sitter to correctly indent new lines. This requires a tree-
+sitter grammar and an <code>indent.scm</code> query file placed in <code>runtime/queries/ {language}/indents.scm</code>. The indentation for a line is calculated by traversing
+the syntax tree from the lowest node at the beginning of the new line (see
+<a href="#indent-queries">Indent queries</a>). Each of these nodes contributes to the total
+indent when it is captured by the query (in what way depends on the name of
+the capture.</p>
+<p>Note that it matters where these added indents begin. For example,
+multiple indent level increases that start on the same line only increase
+the total indent level by 1. See <a href="#capture-types">Capture types</a>.</p>
+<p>By default, Helix uses the <code>hybrid</code> indentation heuristic. This means that
+indent queries are not used to compute the expected absolute indentation of a
+line but rather the expected difference in indentation between the new and an
+already existing line. This difference is then added to the actual indentation
+of the already existing line. Since this makes errors in the indent queries
+harder to find, it is recommended to disable it when testing via
+<code>:set indent-heuristic tree-sitter</code>. The rest of this guide assumes that
+the <code>tree-sitter</code> heuristic is used.</p>
+<h2 id="indent-queries"><a class="header" href="#indent-queries">Indent queries</a></h2>
+<p>When Helix is inserting a new line through <code>o</code>, <code>O</code>, or <code>&lt;ret&gt;</code>, to determine
+the indent level for the new line, the query in <code>indents.scm</code> is run on the
+document. The starting position of the query is the end of the line above where
+a new line will be inserted.</p>
+<p>For <code>o</code>, the inserted line is the line below the cursor, so that starting
+position of the query is the end of the current line.</p>
+<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
+</span><span class="boring">fn main() {
+</span>fn need_hero(some_hero: Hero, life: Life) -&gt; {
+ matches!(some_hero, Hero { // ←─────────────────╮
+ strong: true,//←╮ ↑ ↑ │
+ fast: true, // │ │ ╰── query start │
+ sure: true, // │ ╰───── cursor ├─ traversal
+ soon: true, // ╰──────── new line inserted │ start node
+ }) &amp;&amp; // │
+// ↑ │
+// ╰───────────────────────────────────────────────╯
+ some_hero &gt; life
+}
+<span class="boring">}</span></code></pre></pre>
+<p>For <code>O</code>, the newly inserted line is the <em>current</em> line, so the starting position
+of the query is the end of the line above the cursor.</p>
+<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
+</span><span class="boring">fn main() {
+</span>fn need_hero(some_hero: Hero, life: Life) -&gt; { // ←─╮
+ matches!(some_hero, Hero { // ←╮ ↑ │
+ strong: true,// ↑ ╭───╯ │ │
+ fast: true, // │ │ query start ─╯ │
+ sure: true, // ╰───┼ cursor ├─ traversal
+ soon: true, // ╰ new line inserted │ start node
+ }) &amp;&amp; // │
+ some_hero &gt; life // │
+} // ←──────────────────────────────────────────────╯
+<span class="boring">}</span></code></pre></pre>
+<p>From this starting node, the syntax tree is traversed up until the root node.
+Each indent capture is collected along the way, and then combined according to
+their <a href="#capture-types">capture types</a> and <a href="#scopes">scopes</a> to a final indent
+level for the line.</p>
+<h3 id="capture-types"><a class="header" href="#capture-types">Capture types</a></h3>
+<ul>
+<li><code>@indent</code> (default scope <code>tail</code>):
+Increase the indent level by 1. Multiple occurrences in the same line <em>do not</em>
+stack. If there is at least one <code>@indent</code> and one <code>@outdent</code> capture on the
+same line, the indent level isn't changed at all.</li>
+<li><code>@outdent</code> (default scope <code>all</code>):
+Decrease the indent level by 1. The same rules as for <code>@indent</code> apply.</li>
+<li><code>@indent.always</code> (default scope <code>tail</code>):
+Increase the indent level by 1. Multiple occurrences on the same line <em>do</em>
+stack. The final indent level is <code>@indent.always</code> – <code>@outdent.always</code>. If
+an <code>@indent</code> and an <code>@indent.always</code> are on the same line, the <code>@indent</code> is
+ignored.</li>
+<li><code>@outdent.always</code> (default scope <code>all</code>):
+Decrease the indent level by 1. The same rules as for <code>@indent.always</code> apply.</li>
+<li><code>@align</code> (default scope <code>all</code>):
+Align everything inside this node to some anchor. The anchor is given
+by the start of the node captured by <code>@anchor</code> in the same pattern.
+Every pattern with an <code>@align</code> should contain exactly one <code>@anchor</code>.
+Indent (and outdent) for nodes below (in terms of their starting line)
+the <code>@align</code> node is added to the indentation required for alignment.</li>
+<li><code>@extend</code>:
+Extend the range of this node to the end of the line and to lines that are
+indented more than the line that this node starts on. This is useful for
+languages like Python, where for the purpose of indentation some nodes (like
+functions or classes) should also contain indented lines that follow them.</li>
+<li><code>@extend.prevent-once</code>:
+Prevents the first extension of an ancestor of this node. For example, in Python
+a return expression always ends the block that it is in. Note that this only
+stops the extension of the next <code>@extend</code> capture. If multiple ancestors are
+captured, only the extension of the innermost one is prevented. All other
+ancestors are unaffected (regardless of whether the innermost ancestor would
+actually have been extended).</li>
+</ul>
+<h4 id="indent--outdent"><a class="header" href="#indent--outdent"><code>@indent</code> / <code>@outdent</code></a></h4>
+<p>Consider this example:</p>
+<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
+</span><span class="boring">fn main() {
+</span>fn shout(things: Vec&lt;Thing&gt;) {
+ // ↑
+ // ├───────────────────────╮ indent level
+ // @indent ├┄┄┄┄┄┄┄┄┄┄┄┄┄┄
+ // │
+ let it_all = |out| { things.filter(|thing| { // │ 1
+ // ↑ ↑ │
+ // ├───────────────────────┼─────┼┄┄┄┄┄┄┄┄┄┄┄┄┄┄
+ // @indent @indent │
+ // │ 2
+ thing.can_do_with(out) // │
+ })}; // ├┄┄┄┄┄┄┄┄┄┄┄┄┄┄
+ //↑↑↑ │ 1
+} //╰┼┴──────────────────────────────────────────────┴┄┄┄┄┄┄┄┄┄┄┄┄┄┄
+// 3x @outdent
+<span class="boring">}</span></code></pre></pre>
+<pre><code class="language-scm">((block) @indent)
+["}" ")"] @outdent
+</code></pre>
+<p>Note how on the second line, we have two blocks begin on the same line. In this
+case, since both captures occur on the same line, they are combined and only
+result in a net increase of 1. Also note that the closing <code>}</code>s are part of the
+<code>@indent</code> captures, but the 3 <code>@outdent</code>s also combine into 1 and result in that
+line losing one indent level.</p>
+<h4 id="extend--extendprevent-once"><a class="header" href="#extend--extendprevent-once"><code>@extend</code> / <code>@extend.prevent-once</code></a></h4>
+<p>For an example of where <code>@extend</code> can be useful, consider Python, which is
+whitespace-sensitive.</p>
+<pre><code class="language-scm">]
+ (parenthesized_expression)
+ (function_definition)
+ (class_definition)
+] @indent
+
+</code></pre>
+<pre><code class="language-python">class Hero:
+ def __init__(self, strong, fast, sure, soon):# ←─╮
+ self.is_strong = strong # │
+ self.is_fast = fast # ╭─── query start │
+ self.is_sure = sure # │ ╭─ cursor │
+ self.is_soon = soon # │ │ │
+ # ↑ ↑ │ │ │
+ # │ ╰──────╯ │ │
+ # ╰─────────────────────╯ │
+ # ├─ traversal
+ def need_hero(self, life): # │ start node
+ return ( # │
+ self.is_strong # │
+ and self.is_fast # │
+ and self.is_sure # │
+ and self.is_soon # │
+ and self &gt; life # │
+ ) # ←─────────────────────────────────────────╯
+</code></pre>
+<p>Without braces to catch the scope of the function, the smallest descendant of
+the cursor on a line feed ends up being the entire inside of the class. Because
+of this, it will miss the entire function node and its indent capture, leading
+to an indent level one too small.</p>
+<p>To address this case, <code>@extend</code> tells helix to "extend" the captured node's span
+to the line feed and every consecutive line that has a greater indent level than
+the line of the node.</p>
+<pre><code class="language-scm">(parenthesized_expression) @indent
+
+]
+ (function_definition)
+ (class_definition)
+] @indent @extend
+
+</code></pre>
+<pre><code class="language-python">class Hero:
+ def __init__(self, strong, fast, sure, soon):# ←─╮
+ self.is_strong = strong # │
+ self.is_fast = fast # ╭─── query start ├─ traversal
+ self.is_sure = sure # │ ╭─ cursor │ start node
+ self.is_soon = soon # │ │ ←───────────────╯
+ # ↑ ↑ │ │
+ # │ ╰──────╯ │
+ # ╰─────────────────────╯
+ def need_hero(self, life):
+ return (
+ self.is_strong
+ and self.is_fast
+ and self.is_sure
+ and self.is_soon
+ and self &gt; life
+ )
+</code></pre>
+<p>Furthermore, there are some cases where extending to everything with a greater
+indent level may not be desirable. Consider the <code>need_hero</code> function above. If
+our cursor is on the last line of the returned expression.</p>
+<pre><code class="language-python">class Hero:
+ def __init__(self, strong, fast, sure, soon):
+ self.is_strong = strong
+ self.is_fast = fast
+ self.is_sure = sure
+ self.is_soon = soon
+
+ def need_hero(self, life):
+ return (
+ self.is_strong
+ and self.is_fast
+ and self.is_sure
+ and self.is_soon
+ and self &gt; life
+ ) # ←─── cursor
+ #←────────── where cursor should go on new line
+</code></pre>
+<p>In Python, the are a few tokens that will always end a scope, such as a return
+statement. Since the scope ends, so should the indent level. But because the
+function span is extended to every line with a greater indent level, a new line
+would just continue on the same level. And an <code>@outdent</code> would not help us here
+either, since it would cause everything in the parentheses to become outdented
+as well.</p>
+<p>To help, we need to signal an end to the extension. We can do this with
+<code>@extend.prevent-once</code>.</p>
+<pre><code class="language-scm">(parenthesized_expression) @indent
+
+]
+ (function_definition)
+ (class_definition)
+] @indent @extend
+
+(return_statement) @extend.prevent-once
+</code></pre>
+<h4 id="indentalways--outdentalways"><a class="header" href="#indentalways--outdentalways"><code>@indent.always</code> / <code>@outdent.always</code></a></h4>
+<p>As mentioned before, normally if there is more than one <code>@indent</code> or <code>@outdent</code>
+capture on the same line, they are combined.</p>
+<p>Sometimes, there are cases when you may want to ensure that every indent capture
+is additive, regardless of how many occur on the same line. Consider this
+example in YAML.</p>
+<pre><code class="language-yaml"> - foo: bar
+# ↑ ↑
+# │ ╰─────────────── start of map
+# ╰───────────────── start of list element
+ baz: quux # ←─── cursor
+ # ←───────────── where the cursor should go on a new line
+ garply: waldo
+ - quux:
+ bar: baz
+ xyzzy: thud
+ fred: plugh
+</code></pre>
+<p>In YAML, you often have lists of maps. In these cases, the syntax is such that
+the list element and the map both start on the same line. But we really do want
+to start an indentation for each of these so that subsequent keys in the map
+hang over the list and align properly. This is where <code>@indent.always</code> helps.</p>
+<pre><code class="language-scm">((block_sequence_item) @item @indent.always @extend
+ (#not-one-line? @item))
+
+((block_mapping_pair
+ key: (_) @key
+ value: (_) @val
+ (#not-same-line? @key @val)
+ ) @indent.always @extend
+)
+</code></pre>
+<h2 id="predicates"><a class="header" href="#predicates">Predicates</a></h2>
+<p>In some cases, an S-expression cannot express exactly what pattern should be matched.
+For that, tree-sitter allows for predicates to appear anywhere within a pattern,
+similar to how <code>#set!</code> declarations work:</p>
+<pre><code class="language-scm">(some_kind
+ (child_kind) @indent
+ (#predicate? arg1 arg2 ...)
+)
+</code></pre>
+<p>The number of arguments depends on the predicate that's used.
+Each argument is either a capture (<code>@name</code>) or a string (<code>"some string"</code>).
+The following predicates are supported by tree-sitter:</p>
+<ul>
+<li>
+<p><code>#eq?</code>/<code>#not-eq?</code>:
+The first argument (a capture) must/must not be equal to the second argument
+(a capture or a string).</p>
+</li>
+<li>
+<p><code>#match?</code>/<code>#not-match?</code>:
+The first argument (a capture) must/must not match the regex given in the
+second argument (a string).</p>
+</li>
+<li>
+<p><code>#any-of?</code>/<code>#not-any-of?</code>:
+The first argument (a capture) must/must not be one of the other arguments
+(strings).</p>
+</li>
+</ul>
+<p>Additionally, we support some custom predicates for indent queries:</p>
+<ul>
+<li>
+<p><code>#not-kind-eq?</code>:
+The kind of the first argument (a capture) must not be equal to the second
+argument (a string).</p>
+</li>
+<li>
+<p><code>#same-line?</code>/<code>#not-same-line?</code>:
+The captures given by the 2 arguments must/must not start on the same line.</p>
+</li>
+<li>
+<p><code>#one-line?</code>/<code>#not-one-line?</code>:
+The captures given by the fist argument must/must span a total of one line.</p>
+</li>
+</ul>
+<h3 id="scopes"><a class="header" href="#scopes">Scopes</a></h3>
+<p>Added indents don't always apply to the whole node. For example, in most
+cases when a node should be indented, we actually only want everything
+except for its first line to be indented. For this, there are several
+scopes (more scopes may be added in the future if required):</p>
+<ul>
+<li><code>tail</code>:
+This scope applies to everything except for the first line of the
+captured node.</li>
+<li><code>all</code>:
+This scope applies to the whole captured node. This is only different from
+<code>tail</code> when the captured node is the first node on its line.</li>
+</ul>
+<p>For example, imagine we have the following function</p>
+<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
+</span><span class="boring">fn main() {
+</span>fn aha() { // ←─────────────────────────────────────╮
+ let take = "on me"; // ←──────────────╮ scope: │
+ let take = "me on"; // ├─ "tail" ├─ (block) @indent
+ let ill = be_gone_days(1 || 2); // │ │
+} // ←───────────────────────────────────┴──────────┴─ "}" @outdent
+ // scope: "all"
+<span class="boring">}</span></code></pre></pre>
+<p>We can write the following query with the <code>#set!</code> declaration:</p>
+<pre><code class="language-scm">((block) @indent
+ (#set! "scope" "tail"))
+("}" @outdent
+ (#set! "scope" "all"))
+</code></pre>
+<p>As we can see, the "tail" scope covers the node, except for the first line.
+Everything up to and including the closing brace gets an indent level of 1.
+Then, on the closing brace, we encounter an outdent with a scope of "all", which
+means the first line is included, and the indent level is cancelled out on this
+line. (Note these scopes are the defaults for <code>@indent</code> and <code>@outdent</code>—they are
+written explicitly for demonstration.)</p>
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