update: custom_parser.rs

src/bin/custom_parser.rs

@@ -1,3 +1,50 @@
 fn main() {
     println!("This is the start of making a custom parser in Rust.")
 }
+
+/**
+ * Given a grammar, writing a parser boils down to finding a method to translate each production rule into a function.
+ * It's convenient to name these functions after the production rules they express.
+ * The parser for the Star language would have two functions named string and unit, respectively.
+ * By translating syntax features into programming language control statements, we can arrive at a method to translate
+ * the Star grammar into a validating parser.
+ */
+pub fn string(string: &str) -> bool {
+    let mut state = State {
+        cursor: 0,
+        characters: string.chars().collect::<Vec<_>>(),
+    };
+
+    loop {
+        if !unit(&mut state) {
+            break;
+        }
+    }
+
+    state.cursor > 0 && state.cursor == state.characters.len()
+}
+/**
+ * To test whether a str encodes a member of the Star language, we call the string function.
+ * A return value of true indicates membership.
+ * Notice that Rust's Boolean type serves as the data structure returned from this parser.
+ * This return value can be as simple or complex as you'd like.
+ */
+fn unit(state: &mut State) -> bool {
+    match state.characters.get(state.cursor) {
+        Some(character) => {
+            if character == &'*' {
+                state.cursor += 1;
+
+                true
+            } else {
+                false
+            }
+        }
+        None => false,
+    }
+}
+
+struct State {
+    cursor: usize,
+    characters: Vec<char>,
+}