2 files changed, 32 insertions, 9 deletions

diff --git a/src/func/macros.rs b/src/func/macros.rs
index 9e931ea2..1083e53c 100644
--- a/src/func/macros.rs
+++ b/src/func/macros.rs
@@ -75,6 +75,8 @@ macro_rules! function {
         parse($args:ident, $body:pat, $ctx:pat, $metadata:pat) $code:block
         $($rest:tt)*
     ) => {
+        use $crate::func::prelude::*;
+
         impl $crate::func::ParseFunc for $type {
             type Meta = $meta;
 
@@ -88,7 +90,8 @@ macro_rules! function {
                 let mut $args = args;
                 let val = $code;
                 if !$args.is_empty() {
-                    error!(unexpected_argument);
+                    return Err($crate::TypesetError
+                        ::with_message("unexpected arguments"));
                 }
                 Ok(val)
             }
@@ -109,6 +112,8 @@ macro_rules! function {
 
     // (2-arg) Parse a layout-definition with all arguments.
     (@layout $type:ident | layout($this:ident, $ctx:pat) $code:block) => {
+        use $crate::func::prelude::*;
+
         impl LayoutFunc for $type {
             fn layout<'a, 'life0, 'life1, 'async_trait>(
                 &'a $this,
@@ -138,13 +143,13 @@ macro_rules! function {
 macro_rules! parse {
     (forbidden: $body:expr) => {
         if $body.is_some() {
-            error!("unexpected body");
+            return Err($crate::TypesetError::with_message("unexpected body"));
         }
     };
 
     (optional: $body:expr, $ctx:expr) => (
         if let Some(body) = $body {
-            Some($crate::syntax::parse(body, $ctx))
+            Some($crate::syntax::parse(body, $ctx).0)
         } else {
             None
         }
@@ -152,9 +157,9 @@ macro_rules! parse {
 
     (expected: $body:expr, $ctx:expr) => (
         if let Some(body) = $body {
-            $crate::syntax::parse(body, $ctx)?
+            $crate::syntax::parse(body, $ctx).0
         } else {
-            error!("expected body");
+            Err($crate::TypesetError::with_message("unexpected body"))
         }
     )
 }
diff --git a/src/func/mod.rs b/src/func/mod.rs
index 5f4918d9..90b2a31d 100644
--- a/src/func/mod.rs
+++ b/src/func/mod.rs
@@ -119,6 +119,7 @@ pub enum Command<'a> {
 /// A map from identifiers to function parsers.
 pub struct Scope {
     parsers: HashMap<String, Box<Parser>>,
+    debug: Option<Box<Parser>>
 }
 
 /// A function which parses the source of a function into a function type which
@@ -129,11 +130,30 @@ type Parser = dyn Fn(
     ParseContext
 ) -> ParseResult<Box<dyn LayoutFunc>>;
 
+fn make_parser<F>(metadata: <F as ParseFunc>::Meta) -> Box<Parser>
+where F: ParseFunc + LayoutFunc + 'static {
+    Box::new(move |a, b, c| {
+        F::parse(a, b, c, metadata.clone())
+            .map(|f| Box::new(f) as Box<dyn LayoutFunc>)
+    })
+}
+
 impl Scope {
     /// Create a new empty scope.
     pub fn new() -> Scope {
         Scope {
             parsers: HashMap::new(),
+            debug: None,
+        }
+    }
+
+    /// Create a new scope with a debug parser that is invoked if not other
+    /// match is found.
+    pub fn with_debug<F>() -> Scope
+    where F: ParseFunc<Meta=()> + LayoutFunc + 'static {
+        Scope {
+            parsers: HashMap::new(),
+            debug: Some(make_parser::<F>(())),
         }
     }
 
@@ -154,16 +174,14 @@ impl Scope {
     where F: ParseFunc + LayoutFunc + 'static {
         self.parsers.insert(
             name.to_owned(),
-            Box::new(move |a, b, c| {
-                F::parse(a, b, c, metadata.clone())
-                    .map(|f| Box::new(f) as Box<dyn LayoutFunc>)
-            })
+            make_parser::<F>(metadata),
         );
     }
 
     /// Return the parser with the given name if there is one.
     pub(crate) fn get_parser(&self, name: &str) -> Option<&Parser> {
         self.parsers.get(name).map(|x| &**x)
+            .or(self.debug.as_ref().map(|x| &**x))
     }
 }