Implement a very basic first stab at lisp parser.

src/compare/sexp.rs

@@ -0,0 +1,65 @@
+use nom::branch::alt;
+use nom::bytes::complete::tag;
+use nom::bytes::complete::take_till1;
+use nom::character::complete::multispace0;
+use nom::character::complete::multispace1;
+use nom::multi::separated_list1;
+
+use super::error::Res;
+
+#[derive(Debug)]
+pub enum Token<'s> {
+    Atom(&'s str),
+    List(Vec<Token<'s>>),
+}
+
+#[tracing::instrument(ret, level = "debug")]
+pub fn sexp<'s>(input: &'s str) -> Res<&'s str, Token<'s>> {
+    let (remaining, _) = multispace0(input)?;
+    let (remaining, tkn) = token(remaining)?;
+    let (remaining, _) = multispace0(remaining)?;
+    Ok((remaining, tkn))
+}
+
+#[tracing::instrument(ret, level = "debug")]
+fn token<'s>(input: &'s str) -> Res<&'s str, Token<'s>> {
+    alt((list, atom))(input)
+}
+
+#[tracing::instrument(ret, level = "debug")]
+fn list<'s>(input: &'s str) -> Res<&'s str, Token<'s>> {
+    let (remaining, opening_paren) = tag("(")(input)?;
+    let (remaining, children) = separated_list1(multispace1, token)(remaining)?;
+    let (remaining, closing_paren) = tag(")")(remaining)?;
+    Ok((remaining, Token::List(children)))
+}
+
+#[tracing::instrument(ret, level = "debug")]
+fn atom<'s>(input: &'s str) -> Res<&'s str, Token<'s>> {
+    unquoted_atom(input)
+}
+
+#[tracing::instrument(ret, level = "debug")]
+fn unquoted_atom<'s>(input: &'s str) -> Res<&'s str, Token<'s>> {
+    let (remaining, body) = take_till1(|c| match c {
+        ' ' | '\t' | '\r' | '\n' => true,
+        _ => false,
+    })(input)?;
+    Ok((remaining, Token::Atom(body)))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn simple() {
+        let input = "  (foo bar baz )  ";
+        let (remaining, parsed) = sexp(input).expect("Parse the input");
+        assert_eq!(remaining, "");
+        assert!(match parsed {
+            Token::Atom(_) => false,
+            Token::List(_) => true,
+        });
+    }
+}