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# 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. 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.
## 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):
- `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.
- `tail`:
This scope applies to everything except for the first line of the
captured node.
Every capture type has a default scope which should do the right thing
in most situations. When a different scope is required, this can be
changed by using a `#set!` declaration anywhere in the pattern:
```scm
(assignment_expression
right: (_) @indent
(#set! "scope" "all"))
```
## Capture Types
- `@indent` (default scope `tail`):
Increase the indent level by 1. Multiple occurences in the same line
don't 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.
## 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:
```scm
(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.
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