Adding indent queries

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.

Indent queries

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.

#![allow(unused)]
-fn main() {
-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.

#![allow(unused)]
-fn main() {
-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.

Capture types

@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).

`@indent` / `@outdent`

Consider this example:

#![allow(unused)]
-fn main() {
-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.

`@extend` / `@extend.prevent-once`

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
-

`@indent.always` / `@outdent.always`

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
-)
-

Predicates

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).
-
-
#any-of?/#not-any-of?: -The first argument (a capture) must/must not be one of the other arguments -(strings).
-

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.
-

Scopes

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

#![allow(unused)]
-fn main() {
-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:

((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.)

- -