dsl: add DslExpNs::from_exp; return Perhaps rather than Usually

dsl/src/dsl_conv.rs

@@ -6,15 +6,15 @@ pub struct DslNs<'t, T: 't>(pub &'t [(&'t str, T)]);
 /// Namespace where keys are symbols.
 pub trait DslSymNs<'t, T: 't>: 't {
     const SYMS: DslNs<'t, fn (&'t Self)->T>;
-    fn from_sym <D: Dsl> (&'t self, dsl: D) -> Usually<T> {
+    fn from_sym <D: Dsl> (&'t self, dsl: D) -> Perhaps<T> {
         if let Some(dsl) = dsl.sym()? {
             for (sym, get) in Self::SYMS.0 {
                 if dsl == *sym {
-                    return Ok(get(self))
+                    return Ok(Some(get(self)))
                 }
             }
         }
-        return Err(format!("not found: sym: {dsl:?}").into())
+        return Ok(None)
     }
 }
 
@@ -26,11 +26,21 @@ pub trait DslSymNs<'t, T: 't>: 't {
 
 pub trait DslExpNs<'t, T: 't>: 't {
     const EXPS: DslNs<'t, fn (&'t Self, &str)->T>;
+    fn from_exp <D: Dsl> (&'t self, dsl: D) -> Perhaps<T> {
+        if let Some(exp) = dsl.exp()? {
+            for (key, value) in Self::EXPS.0.iter() {
+                if exp.head() == Ok(Some(key)) {
+                    return Ok(Some(value(self, exp.tail()?.unwrap_or(""))))
+                }
+            }
+        }
+        return Ok(None)
+    }
 }
 
 #[macro_export] macro_rules! dsl_exp (
-    (|$state:ident:$State:ty|->$type:ty { $(
+    (|$state:ident:$State:ty$(, $tail:ident)?|->$type:ty { $(
-        [$key:literal $(/ $sub:ident: $Sub:ty)? $(, $arg:ident $(?)? :$argtype:ty)*] => $body:expr
+        ($key:literal $(/ $sub:ident: $Sub:ty)? $(, $arg:ident $(?)? :$argtype:ty)*) => $body:expr
     ),* $(,)? }) => {
         impl<'t> DslExpNs<'t, $type> for $State {
             const EXPS: DslNs<'t, fn (&'t $State, &str)->$type> =