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feat: support querying missing nodes

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Co-authored-by: Amaan Qureshi <amaanq12@gmail.com>
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Riley Bruins 2024-12-14 11:57:36 -08:00 • committed by GitHub
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23
docs/section-2-using-parsers.md
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@ -617,6 +617,29 @@ For example, this pattern would match any node inside a call:
(call (_) @call.inner)
```
#### Special Nodes
When the parser encounters text it does not recognize, it represents this node
as `(ERROR)` in the syntax tree. These error nodes can be queried just like
normal nodes:
```scheme
(ERROR) @error-node
```
Similarly, if a parser is able to recover from erroneous text by inserting a missing token and then reducing, it will insert that missing node in the final tree so long as that tree has the lowest error cost. These missing nodes appear as seemingly normal nodes in the tree, but they are zero tokens wide, and are a property of the actual terminal node that was inserted, instead of being its own kind of node. These special missing nodes can be queried using `(MISSING)`:
```scheme
(MISSING) @missing-node
```
This is useful when attempting to detect all syntax errors in a given parse tree, since these missing node are not captured by `(ERROR)` queries. Specific missing node types can also be queried:
```scheme
(MISSING identifier) @missing-identifier
(MISSING ";") @missing-semicolon
```
#### Anchors
The anchor operator, `.`, is used to constrain the ways in which child patterns are matched. It has different behaviors depending on where it's placed inside a query.