pub trait Peek<'a>: Sized {
const PEEK_KINDSET: KindSet = KindSet::ANY;
// Provided method
fn peek(_: &Parser<'a>, c: Cursor) -> bool { ... }
}Expand description
This trait allows AST nodes to indicate whether the Parser is in the right position to potentially
Parse the node. Returning true from Peek is not a guarantee that a node will successfully
parse, instead it offers an indication that the node can successfully parse the first node. This is useful for
cheaply comparing a set of Nodes to see which one might viably parse, rather than calling Parser::try_parse() on
each.
Nodes that implement this trait are entitled to peek any number of Cursors ahead from the Parser, to
determine if those Cursors are viable to begin parsing, however there is a cost involved in peeking, so
it is worth being conservative; peek the minimum amount ahead to determine this. Most implementations can peek just
1 Cursor ahead - this is provided as the second argument. To peek further, use the Parser::peek_n() method.
Calling peek_n(2) will return the Cursor after the provided one peek_n(3) will return the second Cursor
after, and so on.
For simple implementations it may be sufficient to just check the Kind of the given Cursor. Rather than implementing Peek::peek(), supplying Peek::PEEK_KINDSET and relying on the provided Peek::peek() method will work well.
However it is likely that more complex checks will be needed. In order to reason about the given Cursor (or other cursors ahead) an implementation might want to call Parser::parse_str() - which takes a Cursor and returns the underlying string to reason about. When comparing lots of strings, consider implementing a phf::Map. If comparing just one string, consider Parser::eq_ignore_ascii_case() which can fail-fast, rather than parsing a whole string.
When peeking child nodes, implementations should not call Peek::peek() directly. Instead - call
Parser::peek<T>(). Parser::parse_if_peek<T>() also exists to conveniently parse a Node if it passes the peek
test.
If a Node can construct itself from a single Cursor it should also implement Build, then it will get Parse for free.
Provided Associated Constants§
const PEEK_KINDSET: KindSet = KindSet::ANY
Provided Methods§
Dyn Compatibility§
This trait is not dyn compatible.
In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.