1 files changed, 50 insertions, 0 deletions
diff --git a/nfa/src/desrec.rs b/nfa/src/desrec.rs
new file mode 100644
index 0000000..c57d313
--- /dev/null
+++ b/nfa/src/desrec.rs
@@ -0,0 +1,50 @@
+//! This file defines the expected behaviours of a recursive descent
+//! parser.
+
+use super::Regex;
+use graph::GraphLabel;
+
+/// A thin wrapper of the parse output, to simplify the function
+/// signature a little.
+pub type ParseOutput<'a, T> = Option<(T, &'a str)>;
+
+/// Types implementing this trait provide a method to be recursively
+/// parsed.
+pub trait DesRec {
+    /// The type of labels of the resulting regular expression.
+    type Label: GraphLabel;
+
+    /// The type of the resulting regular expression.
+    type Regex: Regex<Self::Label>;
+
+    /// The type of errors encountered when parsing.
+    type Error: std::error::Error;
+
+    /// Auxiliary data when parsing
+    type Aux;
+
+    /// The type of a scanner that classifies inputs into tokens.
+    ///
+    /// The return type indicates the result of classification:
+    ///
+    /// - `Err(_)`: the classification fails
+    ///
+    /// - `Ok(None)`: the classification succeeds, and the parsing
+    /// should stop here
+    ///
+    /// - `Ok(Some(_))`: the classification succeeds and the parsing
+    /// should continue.
+    type Scanner<'a>: FnMut(&'a str) -> Result<Option<(usize, Self::Label, Self::Aux)>, Self::Error>;
+
+    /// Parse a string into a regular expression with the aid of this
+    /// type.
+    ///
+    /// Accept a slice of string and return either a parsing error, or
+    /// a pair of correctly parsed regular expression and the
+    /// remaining slice.
+    fn parse<'a>(
+        input: &'a str,
+        scanner: Self::Scanner<'a>,
+        post_p: bool,
+    ) -> Result<ParseOutput<'a, Self::Regex>, Self::Error>;
+}