Working example

src/foreign_macros/externfn_impl.rs

@@ -0,0 +1,43 @@
+#[allow(unused)] // for the doc comments
+use crate::{atomic_impl, atomic_redirect};
+#[allow(unused)] // for the doc comments
+use crate::representations::Primitive;
+#[allow(unused)] // for the doc comments
+use crate::foreign::{Atomic, ExternFn};
+#[allow(unused)] // for the doc comments
+use std::any::Any;
+#[allow(unused)] // for the doc comments
+use std::hash::Hash;
+#[allow(unused)] // for the doc comments
+use dyn_clone::DynClone;
+#[allow(unused)] // for the doc comments
+use std::fmt::Debug;
+
+/// Implement [ExternFn] with a closure that produces an [Atomic] from a reference to self
+/// and a closure. This can be used in conjunction with [atomic_impl] and [atomic_redirect]
+/// to normalize the argument automatically before using it.
+#[macro_export]
+macro_rules! externfn_impl {
+  ($typ:ident, $next_atomic:expr) => {
+    impl $crate::foreign::ExternFn for $typ {
+      fn name(&self) -> &str {stringify!($typ)}
+      fn apply(&self,
+        c: $crate::representations::interpreted::Clause
+      ) -> Result<
+        $crate::representations::interpreted::Clause,
+        std::rc::Rc<dyn $crate::foreign::ExternError>
+      > {
+        match ($next_atomic)(self, c) { // ? casts the result but we want to strictly forward it
+          Ok(r) => Ok(
+            $crate::representations::interpreted::Clause::P(
+              $crate::representations::Primitive::Atom(
+                $crate::foreign::Atom::new(r)
+              )
+            )
+          ),
+          Err(e) => Err(e)
+        }
+      }
+    }
+  };
+}