very elegant extension API and parts of it used in std as POC

orchid-extension/src/fun.rs

@@ -0,0 +1,35 @@
+use std::borrow::Cow;
+
+use dyn_clone::{clone_box, DynClone};
+use never::Never;
+use trait_set::trait_set;
+
+use crate::atom::{AtomCard, OwnedAtom};
+use crate::expr::{ExprHandle, GenClause};
+use crate::system::SysCtx;
+
+trait_set! {
+  trait FunCB = FnOnce(ExprHandle) -> GenClause + DynClone + Send + Sync + 'static;
+}
+
+pub struct Fun(Box<dyn FunCB>);
+impl Fun {
+  pub fn new(f: impl FnOnce(ExprHandle) -> GenClause + Clone + Send + Sync + 'static) -> Self {
+    Self(Box::new(f))
+  }
+}
+impl Clone for Fun {
+  fn clone(&self) -> Self { Self(clone_box(&*self.0)) }
+}
+impl AtomCard for Fun {
+  type Data = ();
+  type Req = Never;
+}
+impl OwnedAtom for Fun {
+  fn val(&self) -> Cow<'_, Self::Data> { Cow::Owned(()) }
+  fn call_ref(&self, arg: ExprHandle) -> GenClause { self.clone().call(arg) }
+  fn call(self, arg: ExprHandle) -> GenClause { (self.0)(arg) }
+  fn handle_req(&self, _ctx: SysCtx, req: Self::Req, _rep: &mut (impl std::io::Write + ?Sized)) {
+    match req {}
+  }
+}