work around operator left recursion

src/frontend.rs

@@ -178,6 +178,7 @@ fn lower_expression(
             module.add_cell(cell);
             Ok(output_gen_wire)
         }
+        // operations should really just desugar to callables
         parser::Expression::Operation(_op) => todo!("operators not yet implemented"),
         parser::Expression::Literal(lit) => {
             Ok(rtlil::SigSpec::Const(*lit as i64, TODO_WIDTH))

src/parser.rs

@@ -61,8 +61,8 @@ pub struct Assign<'a> {
 
 #[derive(Debug)]
 pub enum Operation<'a> {
-    And { a: String, b: Expression<'a> },
-    Or { a: String, b: Expression<'a> },
+    And { a: Expression<'a>, b: Expression<'a> },
+    Or { a: Expression<'a>, b: Expression<'a> },
     Not(Expression<'a>),
 }
 
@@ -100,16 +100,16 @@ fn operation(input: Span) -> IResult<Span, Operation> {
     // temporarily given up on before I learn the shunting yard algorithm
     alt((
         map(
-            separated_pair(ws0(identifier), char('&'), ws0(expression)),
+            separated_pair(ws0(expression_nonrecurse), char('&'), ws0(expression)),
             |(a, b)| Operation::And {
-                a: (*a.fragment()).into(),
+                a,
                 b,
             },
         ),
         map(
-            separated_pair(ws0(identifier), char('|'), ws0(expression)),
+            separated_pair(ws0(expression_nonrecurse), char('|'), ws0(expression)),
             |(a, b)| Operation::Or {
-                a: (*a.fragment()).into(),
+                a,
                 b,
             },
         ),
@@ -134,14 +134,25 @@ fn call_item(input: Span) -> IResult<Span, Call> {
     )(input)
 }
 
+/// parser combinators can not parse left-recursive grammars. To work around this, we split
+/// expressions into a recursive and non-recursive portion.
+/// Parsers reachable from this point must call expression_nonrecurse instead
 fn expression(input: Span) -> IResult<Span, Expression> {
     alt((
-        map(ws0(decimal), |lit| Expression::Literal(lit)),
         map(ws0(operation), |op| Expression::Operation(Box::new(op))),
+        expression_nonrecurse
+    ))(input)
+}
+
+/// the portion of the expression grammar that can be parsed without left recursion
+fn expression_nonrecurse(input: Span) -> IResult<Span, Expression> {
+    alt((
+        map(ws0(decimal), |lit| Expression::Literal(lit)),
         map(ws0(call_item), |call| Expression::Call(Box::new(call))),
         map(ws0(identifier), |ident| {
             Expression::Ident(*ident.fragment())
         }),
+        delimited(char('('), expression, char(')')),
     ))(input)
 }