Indentation rework

book/src/SUMMARY.md

@@ -14,3 +14,4 @@
 - [Guides](./guides/README.md)
   - [Adding Languages](./guides/adding_languages.md)
   - [Adding Textobject Queries](./guides/textobject.md)
+  - [Adding Indent Queries](./guides/indent.md)

book/src/guides/indent.md

@@ -0,0 +1,79 @@
+# 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.