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# Adding Indent Queries
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Helix uses tree-sitter to correctly indent new lines. This requires
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a tree-sitter grammar and an `indent.scm` query file placed in
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`runtime/queries/{language}/indents.scm`. The indentation for a line
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is calculated by traversing the syntax tree from the lowest node at the
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beginning of the new line. Each of these nodes contributes to the total
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indent when it is captured by the query (in what way depends on the name
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of the capture).
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Note that it matters where these added indents begin. For example,
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multiple indent level increases that start on the same line only increase
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the total indent level by 1.
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## Scopes
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Added indents don't always apply to the whole node. For example, in most
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cases when a node should be indented, we actually only want everything
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except for its first line to be indented. For this, there are several
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scopes (more scopes may be added in the future if required):
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- `all`:
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This scope applies to the whole captured node. This is only different from
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`tail` when the captured node is the first node on its line.
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- `tail`:
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This scope applies to everything except for the first line of the
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captured node.
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Every capture type has a default scope which should do the right thing
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in most situations. When a different scope is required, this can be
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changed by using a `#set!` declaration anywhere in the pattern:
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```scm
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(assignment_expression
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right: (_) @indent
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(#set! "scope" "all"))
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```
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## Capture Types
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- `@indent` (default scope `tail`):
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Increase the indent level by 1. Multiple occurrences in the same line
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don't stack. If there is at least one `@indent` and one `@outdent`
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capture on the same line, the indent level isn't changed at all.
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- `@outdent` (default scope `all`):
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Decrease the indent level by 1. The same rules as for `@indent` apply.
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## Predicates
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In some cases, an S-expression cannot express exactly what pattern should be matched.
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For that, tree-sitter allows for predicates to appear anywhere within a pattern,
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similar to how `#set!` declarations work:
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```scm
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(some_kind
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(child_kind) @indent
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(#predicate? arg1 arg2 ...)
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)
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```
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The number of arguments depends on the predicate that's used.
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Each argument is either a capture (`@name`) or a string (`"some string"`).
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The following predicates are supported by tree-sitter:
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- `#eq?`/`#not-eq?`:
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The first argument (a capture) must/must not be equal to the second argument
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(a capture or a string).
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- `#match?`/`#not-match?`:
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The first argument (a capture) must/must not match the regex given in the
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second argument (a string).
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Additionally, we support some custom predicates for indent queries:
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- `#not-kind-eq?`:
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The kind of the first argument (a capture) must not be equal to the second
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argument (a string).
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- `#same-line?`/`#not-same-line?`:
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The captures given by the 2 arguments must/must not start on the same line.
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