Diffstat (limited to 'src/builtins/walker/hint.rs')
| -rw-r--r-- | src/builtins/walker/hint.rs | 45 |
1 files changed, 45 insertions, 0 deletions
diff --git a/src/builtins/walker/hint.rs b/src/builtins/walker/hint.rs new file mode 100644 index 0000000..42234e9 --- /dev/null +++ b/src/builtins/walker/hint.rs @@ -0,0 +1,45 @@ +//! Protocol for giving a hint to a walker. +//! +//! Sometimes a walker has multiple protocols it could use, +//! this module gives a protocol by which a visitor can give a hint +//! to the walker about what it is expecting. + +use crate::protocol::{Implementer, Protocol}; + +/// Protocol for giving a hint to a walker. +/// +/// A hint is for a particular protocol `P`. +pub struct Hint<P: Meta>(P); + +/// Meta information for the hint. +/// +/// This gives the visitor more information to work from when selecting a hint. +pub trait Meta: Protocol { + /// Information known by the walker. + /// + /// This should be information easy to get without changing the state of the walker + /// in an irreversable way. + type Known<'a, 'ctx: 'a>; + + /// Extra information the visitor can give to the walker about what it is expecting. + type Hint<'a, 'ctx: 'a>; +} + +/// Object implementing the [`Hint`] protocol. +pub trait Object<'ctx, P: Meta> { + /// Hint to the walker to use the `P` protocol. + /// + /// This should only be called once per [`RequestHint`]. + fn hint( + &mut self, + visitor: &mut dyn Implementer<'ctx>, + hint: P::Hint<'_, 'ctx>, + ) -> Result<(), ()>; + + /// Ask the walker for information about it's support of the protocol. + fn known(&mut self, hint: &P::Hint<'_, 'ctx>) -> Result<P::Known<'_, 'ctx>, ()>; +} + +impl<P: Meta> Protocol for Hint<P> { + type Object<'a, 'ctx: 'a> = &'a mut dyn Object<'ctx, P>; +} |