simplified rule interface

src/rule/bad_state_error.rs

@@ -0,0 +1,11 @@
+use std::{fmt, error::Error};
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct BadState(Vec<String>);
+
+impl fmt::Display for BadState {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "The following key(s) weren't produced by the matching pattern: {:?}", self.0)
+    }
+}
+impl Error for BadState {}

src/rule/executor.rs

@@ -0,0 +1,14 @@
+use crate::expression::Expr;
+
+use super::{Rule, BadState};
+
+pub fn execute<Src, Tgt>(src: &Src, tgt: &Tgt, mut input: Vec<Expr>)
+-> Result<(Vec<Expr>, bool), BadState> where Src: Rule, Tgt: Rule {
+    let (range, state) = match src.scan_slice(&input) {
+        Some(res) => res,
+        None => return Ok((input, false))
+    };
+    let output = tgt.write(&state)?;
+    input.splice(range, output);
+    Ok((input, true))
+}

src/rule/mod.rs

@@ -1 +1,6 @@
 mod rule;
+mod executor;
+mod bad_state_error;
+
+pub use rule::Rule;
+pub use bad_state_error::BadState;

src/rule/name.rs

@@ -0,0 +1,3 @@
+struct Name {
+    qualified: Vec<String>
+}

src/rule/rule.rs

@@ -1,38 +1,50 @@
-use std::cmp::{min, max};
+use std::{cmp::{min, max}, error::Error, fmt, ops::Range};
 
-use hashbrown::HashSet;
+use hashbrown::HashMap;
 
 use crate::expression::Expr;
 
+use super::BadState;
+
+type State = HashMap<String, Expr>;
+
 pub trait Rule {
-    type OutIter: Iterator<Item = Option<Expr>>;
+    type Out: Iterator<Item = Expr>;
     /// The minimum and maximum set of symbols this rule may match.
     fn len(&self) -> (Option<usize>, Option<usize>);
-    /// The exact tokens the pattern consumes (None if varies)
-    fn consumes(&self) -> Option<HashSet<Vec<String>>>;
-    /// The exact tokens the pattern produces (None if varies)
-    fn produces(&self) -> Option<HashSet<Vec<String>>>;
-    /// Check if the slice matches, and produce the necessary transformations
-    fn produce(&self, base: &[Expr]) -> Option<Self::OutIter>;
+    /// Check if the slice matches, and extract data
+    fn read(&self, input: &[Expr]) -> Option<State>;
+    /// Construct item from state
+    fn write(&self, state: &State) -> Result<Self::Out, BadState>;
+    /// Placeholders present in this pattern (all consumed must also be provided)
+    fn placeholders(&'_ self) -> &'_ [&'_ str];
     /// Try all subsections of Vec of appropriate size, longest first, front-to-back
-    /// Match the first, execute the substitution, return the vector and whether any
-    /// substitutions happened
-    fn apply(&self, mut base: Vec<Expr>) -> (Vec<Expr>, bool) {
+    /// Match the first, return the position and produced state
+    fn scan_slice(&self, input: &[Expr]) -> Option<(Range<usize>, State)> {
         let len_range = self.len();
         let lo = max(len_range.0.unwrap_or(1), 1);
-        let hi = min(len_range.1.unwrap_or(base.len()), base.len());
+        let hi = min(len_range.1.unwrap_or(input.len()), input.len());
         for width in (lo..hi).rev() {
-            let starts = (0..base.len() - width).into_iter();
+            let starts = (0..input.len() - width).into_iter();
             let first_match = starts.filter_map(|start| {
-                self.produce(&base[start..start+width])
-                    .map(|res| (start, res))
+                let res = self.read(&input[start..start+width])?;
+                Some((start..start+width, res))
             }).next();
-            if let Some((start, substitution)) = first_match {
-                let diff = substitution.enumerate().filter_map(|(i, opt)| opt.map(|val| (i, val)));
-                for (idx, item) in diff { base[start + idx] = item }
-                return (base, true)
+            if first_match.is_some() {
+                return first_match;
             }
         }
-        (base, false)
+        None
     }
 }
+
+pub fn verify<Src, Tgt>(src: &Src, tgt: &Tgt) -> Option<Vec<String>> where Src: Rule, Tgt: Rule {
+    let mut amiss: Vec<String> = Vec::new();
+    for ent in tgt.placeholders() {
+        if src.placeholders().iter().find(|x| x == &ent).is_none() {
+            amiss.push(ent.to_string())
+        }
+    }
+    if amiss.len() > 0 { Some(amiss) }
+    else { None }
+}