Helix uses tree-sitter to correctly indent new lines. This requires a tree-
sitter grammar and an indent.scm query file placed in runtime/queries/
{language}/indents.scm. The indentation for a line is calculated by traversing
the syntax tree from the lowest node at the beginning of the new line (see
Indent queries). 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.
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 Capture types.
By default, Helix uses the hybrid 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
:set indent-heuristic tree-sitter. The rest of this guide assumes that
the tree-sitter heuristic is used.
When Helix is inserting a new line through o, O, or <ret>, to determine
the indent level for the new line, the query in indents.scm 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.
For o, the inserted line is the line below the cursor, so that starting
position of the query is the end of the current line.
fn need_hero(some_hero: Hero, life: Life) -> {
matches!(some_hero, Hero { // ←─────────────────╮
strong: true,//←╮ ↑ ↑ │
fast: true, // │ │ ╰── query start │
sure: true, // │ ╰───── cursor ├─ traversal
soon: true, // ╰──────── new line inserted │ start node
}) && // │
// ↑ │
// ╰───────────────────────────────────────────────╯
some_hero > life
}
For O, the newly inserted line is the current line, so the starting position
of the query is the end of the line above the cursor.
fn need_hero(some_hero: Hero, life: Life) -> { // ←─╮
matches!(some_hero, Hero { // ←╮ ↑ │
strong: true,// ↑ ╭───╯ │ │
fast: true, // │ │ query start ─╯ │
sure: true, // ╰───┼ cursor ├─ traversal
soon: true, // ╰ new line inserted │ start node
}) && // │
some_hero > life // │
} // ←──────────────────────────────────────────────╯
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 capture types and scopes to a final indent
level for the line.
@indent (default scope tail):
Increase the indent level by 1. Multiple occurrences in the same line do not
stack. If there is at least one @indent and one @outdent capture on the
same line, the indent level isn't changed at all.@outdent (default scope all):
Decrease the indent level by 1. The same rules as for @indent apply.@indent.always (default scope tail):
Increase the indent level by 1. Multiple occurrences on the same line do
stack. The final indent level is @indent.always – @outdent.always. If
an @indent and an @indent.always are on the same line, the @indent is
ignored.@outdent.always (default scope all):
Decrease the indent level by 1. The same rules as for @indent.always apply.@align (default scope all):
Align everything inside this node to some anchor. The anchor is given
by the start of the node captured by @anchor in the same pattern.
Every pattern with an @align should contain exactly one @anchor.
Indent (and outdent) for nodes below (in terms of their starting line)
the @align node is added to the indentation required for alignment.@extend:
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.@extend.prevent-once:
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 @extend 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).
Consider this example:
fn shout(things: Vec<Thing>) {
// ↑
// ├───────────────────────╮ indent level
// @indent ├┄┄┄┄┄┄┄┄┄┄┄┄┄┄
// │
let it_all = |out| { things.filter(|thing| { // │ 1
// ↑ ↑ │
// ├───────────────────────┼─────┼┄┄┄┄┄┄┄┄┄┄┄┄┄┄
// @indent @indent │
// │ 2
thing.can_do_with(out) // │
})}; // ├┄┄┄┄┄┄┄┄┄┄┄┄┄┄
//↑↑↑ │ 1
} //╰┼┴──────────────────────────────────────────────┴┄┄┄┄┄┄┄┄┄┄┄┄┄┄
// 3x @outdent
((block) @indent)
["}" ")"] @outdent
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 }s are part of the
@indent captures, but the 3 @outdents also combine into 1 and result in that
line losing one indent level.
For an example of where @extend can be useful, consider Python, which is
whitespace-sensitive.
]
(parenthesized_expression)
(function_definition)
(class_definition)
] @indent
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 > life # │
) # ←─────────────────────────────────────────╯
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.
To address this case, @extend 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.
(parenthesized_expression) @indent
]
(function_definition)
(class_definition)
] @indent @extend
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 > life
)
Furthermore, there are some cases where extending to everything with a greater
indent level may not be desirable. Consider the need_hero function above. If
our cursor is on the last line of the returned expression.
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 > life
) # ←─── cursor
#←────────── where cursor should go on new line
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 @outdent would not help us here
either, since it would cause everything in the parentheses to become outdented
as well.
To help, we need to signal an end to the extension. We can do this with
@extend.prevent-once.
(parenthesized_expression) @indent
]
(function_definition)
(class_definition)
] @indent @extend
(return_statement) @extend.prevent-once
As mentioned before, normally if there is more than one @indent or @outdent
capture on the same line, they are combined.
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.
- 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
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 @indent.always helps.
((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
)
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 #set! declarations work:
(some_kind
(child_kind) @indent
(#predicate? arg1 arg2 ...)
)
The number of arguments depends on the predicate that's used.
Each argument is either a capture (@name) or a string ("some string").
The following predicates are supported by tree-sitter:
-
#eq?/#not-eq?:
The first argument (a capture) must/must not be equal to the second argument
(a capture or a string).
-
#match?/#not-match?:
The first argument (a capture) must/must not match the regex given in the
second argument (a string).
Additionally, we support some custom predicates for indent queries:
-
#not-kind-eq?:
The kind of the first argument (a capture) must not be equal to the second
argument (a string).
-
#same-line?/#not-same-line?:
The captures given by the 2 arguments must/must not start on the same line.
-
#one-line?/#not-one-line?:
The captures given by the fist argument must/must span a total of one line.
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):
tail:
This scope applies to everything except for the first line of the
captured node.all:
This scope applies to the whole captured node. This is only different from
tail when the captured node is the first node on its line.
For example, imagine we have the following function
fn aha() { // ←─────────────────────────────────────╮
let take = "on me"; // ←──────────────╮ scope: │
let take = "me on"; // ├─ "tail" ├─ (block) @indent
let ill = be_gone_days(1 || 2); // │ │
} // ←───────────────────────────────────┴──────────┴─ "}" @outdent
// scope: "all"
We can write the following query with the #set! declaration:
scm
((block) @indent
(#set! "scope" "tail"))
("}" @outdent
(#set! "scope" "all"))
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 @indent and @outdent—they are
written explicitly for demonstration.)