Add better error reporting for repl.

dm/src/common.rs

@@ -0,0 +1,43 @@
+use codespan_reporting::diagnostic::Label;
+use dmc_lib::diagnostic::Diagnostic;
+
+pub fn to_csr_diagnostic(
+    file_id: usize,
+    diagnostic: &Diagnostic,
+) -> codespan_reporting::diagnostic::Diagnostic<usize> {
+    let mut primary_label = Label::primary(file_id, diagnostic.start()..diagnostic.end());
+    if let Some(primary_label_message) = diagnostic.primary_label_message() {
+        primary_label = primary_label.with_message(primary_label_message);
+    }
+
+    let secondary_labels: Vec<Label<usize>> = diagnostic
+        .secondary_labels()
+        .iter()
+        .map(|secondary_label| {
+            let mut label =
+                Label::secondary(file_id, secondary_label.start()..secondary_label.end());
+            if let Some(message) = secondary_label.message() {
+                label = label.with_message(message);
+            }
+            label
+        })
+        .collect();
+
+    let mut csr_diagnostic = codespan_reporting::diagnostic::Diagnostic::error()
+        .with_message(diagnostic.message())
+        .with_label(primary_label)
+        .with_labels(secondary_labels);
+
+    if let Some(error_code) = diagnostic.error_code() {
+        csr_diagnostic = csr_diagnostic.with_code(format!("E{:04}", error_code));
+    }
+
+    if let Some((reporter_file, reporter_line)) = diagnostic.reporter() {
+        csr_diagnostic = csr_diagnostic.with_note(format!(
+            "Reported by (Rust) source: {}, line {}",
+            reporter_file, reporter_line
+        ));
+    }
+
+    csr_diagnostic
+}

dm/src/main.rs

@@ -1,9 +1,11 @@
+mod common;
 mod repl;
 mod run;
 
 use crate::repl::repl;
 use crate::run::compile_and_run_script;
 use clap::{Parser, Subcommand};
+use codespan_reporting::term::termcolor::{ColorChoice, StandardStream};
 use std::io;
 use std::path::PathBuf;
 
@@ -47,7 +49,7 @@ fn main() {
             repl(
                 &mut io::stdin().lock(),
                 &mut io::stdout().lock(),
-                &mut io::stderr().lock(),
+                &mut StandardStream::stderr(ColorChoice::Always),
                 register_count,
             );
         }

dm/src/repl.rs

@@ -1,3 +1,6 @@
+use crate::common::to_csr_diagnostic;
+use codespan_reporting::files::SimpleFiles;
+use codespan_reporting::term::{Config, WriteStyle, emit_to_write_style};
 use dmc_lib::SyntheticFunctionSession;
 use dmc_lib::constants_table::ConstantsTable;
 use dmc_lib::diagnostic::Diagnostics;
@@ -11,9 +14,13 @@ use std::io::{BufRead, Write};
 pub fn repl(
     read: &mut impl BufRead,
     out: &mut impl Write,
-    err: &mut impl Write,
+    err: &mut impl WriteStyle,
     register_count: usize,
 ) {
+    let diagnostics_config = Config::default();
+    let mut files: SimpleFiles<&str, String> = SimpleFiles::new();
+    let mut current_file_id;
+
     let mut buffer = String::new();
 
     let mut session = SyntheticFunctionSession::new("__repl");
@@ -48,8 +55,10 @@ pub fn repl(
                     .insert(function.name_owned(), function);
             }
             Err(diagnostics) => {
+                current_file_id = files.add("input", String::from(input));
                 for diagnostic in diagnostics {
-                    writeln!(err, "{}", diagnostic.message()).unwrap();
+                    let csr_diagnostic = to_csr_diagnostic(current_file_id, &diagnostic);
+                    emit_to_write_style(err, &diagnostics_config, &files, &csr_diagnostic).unwrap();
                 }
                 buffer.clear();
                 continue 'repl;

dm/src/run.rs

@@ -1,4 +1,4 @@
-use codespan_reporting::diagnostic::Label;
+use crate::common::to_csr_diagnostic;
 use codespan_reporting::files::SimpleFiles;
 use codespan_reporting::term;
 use codespan_reporting::term::termcolor::{ColorChoice, StandardStream};
@@ -88,43 +88,7 @@ fn report_and_exit(
     let writer = StandardStream::stderr(ColorChoice::Always);
     let config = term::Config::default();
     for diagnostic in diagnostics {
-        let mut primary_label =
-            Label::primary(script_file_id, diagnostic.start()..diagnostic.end());
-        if let Some(primary_label_message) = diagnostic.primary_label_message() {
-            primary_label = primary_label.with_message(primary_label_message);
-        }
-
-        let secondary_labels: Vec<Label<usize>> = diagnostic
-            .secondary_labels()
-            .iter()
-            .map(|secondary_label| {
-                let mut label = Label::secondary(
-                    script_file_id,
-                    secondary_label.start()..secondary_label.end(),
-                );
-                if let Some(message) = secondary_label.message() {
-                    label = label.with_message(message);
-                }
-                label
-            })
-            .collect();
-
-        let mut csr_diagnostic = codespan_reporting::diagnostic::Diagnostic::error()
-            .with_message(diagnostic.message())
-            .with_label(primary_label)
-            .with_labels(secondary_labels);
-
-        if let Some(error_code) = diagnostic.error_code() {
-            csr_diagnostic = csr_diagnostic.with_code(format!("E{:04}", error_code));
-        }
-
-        if let Some((reporter_file, reporter_line)) = diagnostic.reporter() {
-            csr_diagnostic = csr_diagnostic.with_note(format!(
-                "Reported by (Rust) source: {}, line {}",
-                reporter_file, reporter_line
-            ));
-        }
-
+        let csr_diagnostic = to_csr_diagnostic(script_file_id, diagnostic);
         term::emit_to_write_style(&mut writer.lock(), &config, files, &csr_diagnostic).unwrap();
     }
     if diagnostics.len() == 1 {

dmc-lib/src/parser.rs

@@ -62,7 +62,7 @@ pub fn parse_statement(input: &str) -> ParseResult<Option<Statement>> {
     (statement, diagnostics)
 }
 
-pub fn parse_expression(input: &str) -> ParseResult<Expression> {
+pub fn parse_expression(input: &str) -> ParseResult<Option<Expression>> {
     let mut parser = Parser::new(input);
     let mut diagnostics = Vec::new();
     diagnostics.append(&mut parser.advance());
@@ -918,9 +918,9 @@ impl<'a> Parser<'a> {
 
         let initializer = if self.current.is_some() && self.peek_current(TokenKind::Equals) {
             diagnostics.append(&mut self.advance()); // equals
-            let (expression, mut ds) = self.expression();
+            let (maybe_expression, mut ds) = self.expression();
             diagnostics.append(&mut ds);
-            Some(expression)
+            maybe_expression
         } else {
             None
         };
@@ -961,10 +961,7 @@ impl<'a> Parser<'a> {
                     Some(let_statement) => (Some(Statement::Let(let_statement)), diagnostics),
                 }
             }
-            _ => {
-                let (statement, diagnostics) = self.expression_statement_or_assign_statement();
-                (Some(statement), diagnostics)
-            }
+            _ => self.expression_statement_or_assign_statement(),
         }
     }
 
@@ -987,10 +984,22 @@ impl<'a> Parser<'a> {
         let (_, mut ds) = self.expect_advance(TokenKind::Equals);
         diagnostics.append(&mut ds);
 
-        let (expression, mut ds) = self.expression();
+        let (maybe_expression, mut ds) = self.expression();
         diagnostics.append(&mut ds);
 
-        if let Some(identifier) = maybe_identifier {
+        let identifier = match maybe_identifier {
+            Some(identifier) => identifier,
+            None => {
+                return (None, diagnostics);
+            }
+        };
+        let expression = match maybe_expression {
+            Some(expression) => expression,
+            None => {
+                return (None, diagnostics);
+            }
+        };
+
         let let_statement = LetStatement::new(
             self.next_node_id(),
             self.token_text(&identifier),
@@ -999,53 +1008,76 @@ impl<'a> Parser<'a> {
             expression,
         );
         (Some(let_statement), diagnostics)
+    }
+
+    fn expression_statement_or_assign_statement(&mut self) -> ParseResult<Option<Statement>> {
+        let mut diagnostics = Vec::new();
+        let (maybe_base, mut ds) = self.expression();
+        diagnostics.append(&mut ds);
+        let base = match maybe_base {
+            Some(base) => base,
+            None => {
+                return (None, diagnostics);
+            }
+        };
+
+        if self.current.is_some() && self.peek_current(TokenKind::Equals) {
+            let (maybe_assign_statement, mut ds) = self.assign_rhs(base);
+            diagnostics.append(&mut ds);
+            if let Some(assign_statement) = maybe_assign_statement {
+                (Some(Statement::Assign(assign_statement)), diagnostics)
             } else {
                 (None, diagnostics)
             }
-    }
-
-    fn expression_statement_or_assign_statement(&mut self) -> ParseResult<Statement> {
-        let mut diagnostics = Vec::new();
-        let (base, mut ds) = self.expression();
-        diagnostics.append(&mut ds);
-
-        if self.current.is_some() && self.peek_current(TokenKind::Equals) {
-            let (assign_statement, mut ds) = self.assign_rhs(base);
-            diagnostics.append(&mut ds);
-            (Statement::Assign(assign_statement), diagnostics)
         } else {
             (
-                Statement::Expression(ExpressionStatement::new(base)),
+                Some(Statement::Expression(ExpressionStatement::new(base))),
                 diagnostics,
             )
         }
     }
 
-    fn assign_rhs(&mut self, destination: Expression) -> ParseResult<AssignStatement> {
+    fn assign_rhs(&mut self, destination: Expression) -> ParseResult<Option<AssignStatement>> {
         let mut diagnostics = Vec::new();
         let (_, mut ds) = self.expect_advance(TokenKind::Equals);
         diagnostics.append(&mut ds);
 
-        let (value, mut ds) = self.expression();
+        let (maybe_value, mut ds) = self.expression();
         diagnostics.append(&mut ds);
-
-        (AssignStatement::new(destination, value), diagnostics)
+        if let Some(value) = maybe_value {
+            (Some(AssignStatement::new(destination, value)), diagnostics)
+        } else {
+            (None, diagnostics)
+        }
     }
 
-    fn expression(&mut self) -> ParseResult<Expression> {
+    fn expression(&mut self) -> ParseResult<Option<Expression>> {
         self.bitwise_or_expression()
     }
 
-    fn bitwise_or_expression(&mut self) -> ParseResult<Expression> {
+    fn bitwise_or_expression(&mut self) -> ParseResult<Option<Expression>> {
         let mut diagnostics = Vec::new();
 
-        let (mut base, mut ds) = self.bitwise_xor_expression();
+        let (maybe_base, mut ds) = self.bitwise_xor_expression();
         diagnostics.append(&mut ds);
+        let mut base = match maybe_base {
+            Some(base) => base,
+            None => {
+                return (None, diagnostics);
+            }
+        };
 
         while self.current.is_some() && self.peek_current(TokenKind::Bar) {
             diagnostics.append(&mut self.advance()); // |
-            let (rhs, mut ds) = self.bitwise_xor_expression();
+            let (maybe_rhs, mut ds) = self.bitwise_xor_expression();
             diagnostics.append(&mut ds);
+            let rhs = match maybe_rhs {
+                Some(rhs) => rhs,
+                None => {
+                    return (None, diagnostics);
+                }
+            };
+
             let source_range =
                 SourceRange::new(base.source_range().start(), rhs.source_range().end());
             base = Expression::Binary(BinaryExpression::new(
@@ -1056,19 +1088,32 @@ impl<'a> Parser<'a> {
                 source_range,
             ));
         }
-        (base, diagnostics)
+        (Some(base), diagnostics)
     }
 
-    fn bitwise_xor_expression(&mut self) -> ParseResult<Expression> {
+    fn bitwise_xor_expression(&mut self) -> ParseResult<Option<Expression>> {
         let mut diagnostics = Vec::new();
 
-        let (mut base, mut ds) = self.bitwise_and_expression();
+        let (maybe_base, mut ds) = self.bitwise_and_expression();
         diagnostics.append(&mut ds);
+        let mut base = match maybe_base {
+            Some(base) => base,
+            None => {
+                return (None, diagnostics);
+            }
+        };
 
         while self.current.is_some() && self.peek_current(TokenKind::Caret) {
             diagnostics.append(&mut self.advance()); // ^
-            let (rhs, mut ds) = self.bitwise_and_expression();
+            let (maybe_rhs, mut ds) = self.bitwise_and_expression();
             diagnostics.append(&mut ds);
+            let rhs = match maybe_rhs {
+                Some(rhs) => rhs,
+                None => {
+                    return (None, diagnostics);
+                }
+            };
+
             let source_range =
                 SourceRange::new(base.source_range().start(), rhs.source_range().end());
             base = Expression::Binary(BinaryExpression::new(
@@ -1079,19 +1124,31 @@ impl<'a> Parser<'a> {
                 source_range,
             ));
         }
-        (base, diagnostics)
+        (Some(base), diagnostics)
     }
 
-    fn bitwise_and_expression(&mut self) -> ParseResult<Expression> {
+    fn bitwise_and_expression(&mut self) -> ParseResult<Option<Expression>> {
         let mut diagnostics = Vec::new();
 
-        let (mut base, mut ds) = self.shift_expression();
+        let (maybe_base, mut ds) = self.shift_expression();
         diagnostics.append(&mut ds);
+        let mut base = match maybe_base {
+            Some(base) => base,
+            None => {
+                return (None, diagnostics);
+            }
+        };
 
         while self.current.is_some() && self.peek_current(TokenKind::Ampersand) {
             diagnostics.append(&mut self.advance()); // &
-            let (rhs, mut ds) = self.shift_expression();
+            let (maybe_rhs, mut ds) = self.shift_expression();
             diagnostics.append(&mut ds);
+            let rhs = match maybe_rhs {
+                Some(rhs) => rhs,
+                None => {
+                    return (None, diagnostics);
+                }
+            };
 
             let source_range =
                 SourceRange::new(base.source_range().start(), rhs.source_range().end());
@@ -1103,14 +1160,20 @@ impl<'a> Parser<'a> {
                 source_range,
             ));
         }
-        (base, diagnostics)
+        (Some(base), diagnostics)
     }
 
-    fn shift_expression(&mut self) -> ParseResult<Expression> {
+    fn shift_expression(&mut self) -> ParseResult<Option<Expression>> {
         let mut diagnostics = Vec::new();
 
-        let (mut base, mut ds) = self.additive_expression();
+        let (maybe_base, mut ds) = self.additive_expression();
         diagnostics.append(&mut ds);
+        let mut base = match maybe_base {
+            Some(base) => base,
+            None => {
+                return (None, diagnostics);
+            }
+        };
 
         while self.current.is_some() {
             let current = self.current.as_ref().unwrap();
@@ -1122,8 +1185,14 @@ impl<'a> Parser<'a> {
                         self.expect_immediately_after_advance(TokenKind::Lt, &previous_cloned); // second <
                     diagnostics.append(&mut ds);
 
-                    let (rhs, mut ds) = self.additive_expression();
+                    let (maybe_rhs, mut ds) = self.additive_expression();
                     diagnostics.append(&mut ds);
+                    let rhs = match maybe_rhs {
+                        Some(rhs) => rhs,
+                        None => {
+                            return (None, diagnostics);
+                        }
+                    };
 
                     let source_range =
                         SourceRange::new(base.source_range().start(), rhs.source_range().end());
@@ -1142,8 +1211,14 @@ impl<'a> Parser<'a> {
                         self.expect_immediately_after_advance(TokenKind::Gt, &previous_cloned); // second >
                     diagnostics.append(&mut ds);
 
-                    let (rhs, mut ds) = self.additive_expression();
+                    let (maybe_rhs, mut ds) = self.additive_expression();
                     diagnostics.append(&mut ds);
+                    let rhs = match maybe_rhs {
+                        Some(rhs) => rhs,
+                        None => {
+                            return (None, diagnostics);
+                        }
+                    };
 
                     let source_range =
                         SourceRange::new(base.source_range().start(), rhs.source_range().end());
@@ -1159,22 +1234,34 @@ impl<'a> Parser<'a> {
             }
         }
 
-        (base, diagnostics)
+        (Some(base), diagnostics)
     }
 
-    fn additive_expression(&mut self) -> ParseResult<Expression> {
+    fn additive_expression(&mut self) -> ParseResult<Option<Expression>> {
         let mut diagnostics = Vec::new();
 
-        let (mut base, mut ds) = self.multiplicative_expression();
+        let (maybe_base, mut ds) = self.multiplicative_expression();
         diagnostics.append(&mut ds);
+        let mut base = match maybe_base {
+            Some(base) => base,
+            None => {
+                return (None, diagnostics);
+            }
+        };
 
         while self.current.is_some() {
             let current = self.current.as_ref().unwrap();
             match current.kind() {
                 TokenKind::Plus => {
                     diagnostics.append(&mut self.advance()); // plus
-                    let (rhs, mut ds) = self.multiplicative_expression();
+                    let (maybe_rhs, mut ds) = self.multiplicative_expression();
                     diagnostics.append(&mut ds);
+                    let rhs = match maybe_rhs {
+                        Some(rhs) => rhs,
+                        None => {
+                            return (None, diagnostics);
+                        }
+                    };
 
                     let source_range =
                         SourceRange::new(base.source_range().start(), rhs.source_range().end());
@@ -1188,8 +1275,14 @@ impl<'a> Parser<'a> {
                 }
                 TokenKind::Minus => {
                     diagnostics.append(&mut self.advance()); // minus
-                    let (rhs, mut ds) = self.multiplicative_expression();
+                    let (maybe_rhs, mut ds) = self.multiplicative_expression();
                     diagnostics.append(&mut ds);
+                    let rhs = match maybe_rhs {
+                        Some(rhs) => rhs,
+                        None => {
+                            return (None, diagnostics);
+                        }
+                    };
 
                     let source_range =
                         SourceRange::new(base.source_range().start(), rhs.source_range().end());
@@ -1204,22 +1297,34 @@ impl<'a> Parser<'a> {
                 _ => break,
             }
         }
-        (base, diagnostics)
+        (Some(base), diagnostics)
     }
 
-    fn multiplicative_expression(&mut self) -> ParseResult<Expression> {
+    fn multiplicative_expression(&mut self) -> ParseResult<Option<Expression>> {
         let mut diagnostics = Vec::new();
 
-        let (mut base, mut ds) = self.prefix_expression();
+        let (maybe_base, mut ds) = self.prefix_expression();
         diagnostics.append(&mut ds);
+        let mut base = match maybe_base {
+            Some(base) => base,
+            None => {
+                return (None, diagnostics);
+            }
+        };
 
         while self.current.is_some() {
             let current = self.current.as_ref().unwrap();
             match current.kind() {
                 TokenKind::Star => {
                     diagnostics.append(&mut self.advance()); // multiply
-                    let (rhs, mut ds) = self.prefix_expression();
+                    let (maybe_rhs, mut ds) = self.prefix_expression();
                     diagnostics.append(&mut ds);
+                    let rhs = match maybe_rhs {
+                        Some(rhs) => rhs,
+                        None => {
+                            return (None, diagnostics);
+                        }
+                    };
 
                     let source_range =
                         SourceRange::new(base.source_range().start(), rhs.source_range().end());
@@ -1233,8 +1338,14 @@ impl<'a> Parser<'a> {
                 }
                 TokenKind::Slash => {
                     diagnostics.append(&mut self.advance()); // slash
-                    let (rhs, mut ds) = self.prefix_expression();
+                    let (maybe_rhs, mut ds) = self.prefix_expression();
                     diagnostics.append(&mut ds);
+                    let rhs = match maybe_rhs {
+                        Some(rhs) => rhs,
+                        None => {
+                            return (None, diagnostics);
+                        }
+                    };
 
                     let source_range =
                         SourceRange::new(base.source_range().start(), rhs.source_range().end());
@@ -1248,8 +1359,14 @@ impl<'a> Parser<'a> {
                 }
                 TokenKind::Modulo => {
                     diagnostics.append(&mut self.advance()); // modulo
-                    let (rhs, mut ds) = self.prefix_expression();
+                    let (maybe_rhs, mut ds) = self.prefix_expression();
                     diagnostics.append(&mut ds);
+                    let rhs = match maybe_rhs {
+                        Some(rhs) => rhs,
+                        None => {
+                            return (None, diagnostics);
+                        }
+                    };
 
                     let source_range =
                         SourceRange::new(base.source_range().start(), rhs.source_range().end());
@@ -1264,10 +1381,10 @@ impl<'a> Parser<'a> {
                 _ => break,
             }
         }
-        (base, diagnostics)
+        (Some(base), diagnostics)
     }
 
-    fn prefix_expression(&mut self) -> ParseResult<Expression> {
+    fn prefix_expression(&mut self) -> ParseResult<Option<Expression>> {
         let mut diagnostics = Vec::new();
 
         // first, collect all consecutive operators
@@ -1286,8 +1403,14 @@ impl<'a> Parser<'a> {
         // now go in reverse and build up expressions
         // the parser is currently just after the prefix operators, so we need a suffix expression
         // as a base
-        let (mut base, mut ds) = self.suffix_expression();
+        let (maybe_base, mut ds) = self.suffix_expression();
         diagnostics.append(&mut ds);
+        let mut base = match maybe_base {
+            Some(base) => base,
+            None => {
+                return (None, diagnostics);
+            }
+        };
 
         while let Some(operator_token) = operator_tokens.pop() {
             let source_range = SourceRange::new(operator_token.start(), base.source_range().end());
@@ -1302,14 +1425,20 @@ impl<'a> Parser<'a> {
                 _ => unreachable!(),
             }
         }
-        (base, diagnostics)
+        (Some(base), diagnostics)
     }
 
-    fn suffix_expression(&mut self) -> ParseResult<Expression> {
+    fn suffix_expression(&mut self) -> ParseResult<Option<Expression>> {
         let mut diagnostics = Vec::new();
 
-        let (mut base, mut ds) = self.expression_base();
+        let (maybe_base, mut ds) = self.expression_base();
         diagnostics.append(&mut ds);
+        let mut base = match maybe_base {
+            Some(base) => base,
+            None => {
+                return (None, diagnostics);
+            }
+        };
 
         while self.current.is_some() {
             let current = self.current.as_ref().unwrap();
@@ -1323,10 +1452,10 @@ impl<'a> Parser<'a> {
             }
         }
 
-        (base, diagnostics)
+        (Some(base), diagnostics)
     }
 
-    fn expression_base(&mut self) -> ParseResult<Expression> {
+    fn expression_base(&mut self) -> ParseResult<Option<Expression>> {
         let current = match self.current.as_ref() {
             None => {
                 let mut ds = Diagnostics::new();
@@ -1334,7 +1463,7 @@ impl<'a> Parser<'a> {
                     &EXPRESSION_FIRSTS,
                     self.input.len(),
                 ));
-                return (todo!(), ds);
+                return (None, ds);
             }
             Some(current) => current,
         };
@@ -1345,11 +1474,11 @@ impl<'a> Parser<'a> {
                 let source_range = SourceRange::new(current.start(), current.end());
                 diagnostics.append(&mut self.advance());
                 (
-                    Expression::Integer(IntegerLiteral::new(
+                    Some(Expression::Integer(IntegerLiteral::new(
                         self.next_node_id(),
                         i32::from_str(raw).unwrap(),
                         source_range,
-                    )),
+                    ))),
                     diagnostics,
                 )
             }
@@ -1358,11 +1487,11 @@ impl<'a> Parser<'a> {
                 let source_range = SourceRange::new(current.start(), current.end());
                 diagnostics.append(&mut self.advance());
                 (
-                    Expression::Double(DoubleLiteral::new(
+                    Some(Expression::Double(DoubleLiteral::new(
                         self.next_node_id(),
                         f64::from_str(raw).unwrap(),
                         source_range,
-                    )),
+                    ))),
                     diagnostics,
                 )
             }
@@ -1371,11 +1500,11 @@ impl<'a> Parser<'a> {
                 let source_range = SourceRange::new(current.start(), current.end());
                 diagnostics.append(&mut self.advance());
                 (
-                    Expression::String(StringLiteral::new(
+                    Some(Expression::String(StringLiteral::new(
                         self.next_node_id(),
                         &with_quotes[1..with_quotes.len() - 1],
                         source_range,
-                    )),
+                    ))),
                     diagnostics,
                 )
             }
@@ -1384,11 +1513,11 @@ impl<'a> Parser<'a> {
                 let source_range = SourceRange::new(current.start(), current.end());
                 diagnostics.append(&mut self.advance());
                 (
-                    Expression::Identifier(Identifier::new(
+                    Some(Expression::Identifier(Identifier::new(
                         self.next_node_id(),
                         declared_name,
                         source_range,
-                    )),
+                    ))),
                     diagnostics,
                 )
             }
@@ -1430,14 +1559,18 @@ impl<'a> Parser<'a> {
         let mut expressions = vec![];
         let mut diagnostics = Vec::new();
 
-        let (expression, mut ds) = self.expression();
+        let (maybe_expression, mut ds) = self.expression();
+        if let Some(expression) = maybe_expression {
             expressions.push(expression);
+        }
         diagnostics.append(&mut ds);
 
         while self.current.is_some() && self.peek_current(TokenKind::Comma) {
             diagnostics.append(&mut self.advance()); // comma
-            let (expression, mut ds) = self.expression();
+            let (maybe_expression, mut ds) = self.expression();
+            if let Some(expression) = maybe_expression {
                 expressions.push(expression);
+            }
             diagnostics.append(&mut ds);
         }
 
@@ -1677,11 +1810,15 @@ mod concrete_tests {
     }
 
     fn assert_expression(input: &str) -> Expression {
-        let (expression, diagnostics) = parse_expression(input);
+        let (maybe_expression, diagnostics) = parse_expression(input);
         if !diagnostics.is_empty() {
             report_diagnostics(&diagnostics);
         }
+        if let Some(expression) = maybe_expression {
             expression
+        } else {
+            panic!("parse_expression produced a None")
+        }
     }
 
     fn assert_function_in<'a>(