From 915d95659619e85113b36a6571e7205eeef7c05f Mon Sep 17 00:00:00 2001
From: Andrew Cowie <istathar@gmail.com>
Date: Thu, 11 Jun 2026 18:07:08 +1000
Subject: [PATCH] Defer external invocation targets in translation stage

---
 src/language/types.rs         | 2 +-
 src/problem/messages.rs       | 5 +----
 src/program/types.rs          | 4 +++-
 src/runner/runner.rs          | 5 +++++
 src/translation/translator.rs | 2 +-
 5 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/src/language/types.rs b/src/language/types.rs
index a1f68dd1..da453bbb 100644
--- a/src/language/types.rs
+++ b/src/language/types.rs
@@ -131,7 +131,7 @@ impl<'i> Identifier<'i> {
     }
 }
 
-#[derive(Eq, Debug)]
+#[derive(Clone, Copy, Eq, Debug)]
 pub struct External<'i> {
     pub value: &'i str,
     pub span: Span,
diff --git a/src/problem/messages.rs b/src/problem/messages.rs
index c5496131..79e38357 100644
--- a/src/problem/messages.rs
+++ b/src/problem/messages.rs
@@ -1137,10 +1137,7 @@ functions calls:
             function: Identifier { value: name, .. },
         } => (
             format!("Unknown function {}()", name),
-            format!(
-                "The function {}() is neither builtin nor provided by the selected domain.",
-                name
-            ),
+            "The function is neither builtin nor provided by the selected domain.".to_string(),
         ),
     }
 }
diff --git a/src/program/types.rs b/src/program/types.rs
index 401f7a75..3a9c560d 100644
--- a/src/program/types.rs
+++ b/src/program/types.rs
@@ -144,11 +144,13 @@ pub struct Invocable<'i> {
 /// subroutine into `Program.subroutines`; the resolve pass walks the
 /// translated tree replacing matching `Unresolved` references with
 /// `Resolved`. Names that don't match any declared subroutine become a
-/// translation error.
+/// translation error. `Deferred` references (ie URLs) are left as-is; they
+/// are resolved at a later phase or at runtime.
 #[derive(Debug, Eq, PartialEq)]
 pub enum SubroutineRef<'i> {
     Unresolved(language::Identifier<'i>),
     Resolved(SubroutineId),
+    Deferred(language::External<'i>),
 }
 
 /// Lowered form of `language::Function`. Functions live in a separate
diff --git a/src/runner/runner.rs b/src/runner/runner.rs
index 293d9d9c..d4f8eaf3 100644
--- a/src/runner/runner.rs
+++ b/src/runner/runner.rs
@@ -314,6 +314,11 @@ impl<'i, P: Prompt> Runner<'i, P> {
                     .announce(&format!("<{}>", id.value));
                 Ok(Outcome::Done(Value::Unitus))
             }
+            SubroutineRef::Deferred(ext) => {
+                self.driver
+                    .announce(&format!("<{}>", ext.value));
+                Ok(Outcome::Done(Value::Unitus))
+            }
         }
     }
 
diff --git a/src/translation/translator.rs b/src/translation/translator.rs
index 5ce5f100..0a68afa1 100644
--- a/src/translation/translator.rs
+++ b/src/translation/translator.rs
@@ -743,7 +743,7 @@ impl<'i> Translator<'i> {
     fn translate_invocation(&mut self, invocation: &'i language::Invocation<'i>) -> Invocable<'i> {
         let target = match &invocation.target {
             language::Target::Local(id) => SubroutineRef::Unresolved(*id),
-            language::Target::Remote(_) => todo!("remote invocation target"),
+            language::Target::Remote(external) => SubroutineRef::Deferred(*external),
         };
         let arguments = match &invocation.parameters {
             Some(params) => params