Require iteratable to be passed to foreach

Author: istathar

src/problem/messages.rs

@@ -1126,8 +1126,15 @@ of the document doesn't take ant parameters.
             "#.trim_ascii().to_string(),
         ),
         RunnerError::NotIterable => (
-            "Value is not a list".to_string(),
-            "The foreach keyword requires a list to iterate over, but the value suppliedisn't one.".to_string(),
+            "Iteration requires a list".to_string(),
+            r#"
+The foreach loop control structure requires a list to iterate over, but the
+value supplied isn't one. A tablet is a dictonary, not a sequence. If you want
+to use the values from a tablet convert them into a list first with the
+values() function. There is also a labels() function to get each of the
+tablet's labels, and pairs() to get a sequence of tuples of labels and values
+you can iterate over.
+            "#.trim_ascii().to_string(),
         ),
         RunnerError::UserQuit => (
             "Interrupted".to_string(),

src/runner/checks/runner.rs

@@ -909,9 +909,10 @@ fn foreach_widens_primitive_to_singleton() {
 #[test]
 fn foreach_over_non_list_or_unbound_errors() {
     // foreach item in source, where `source` is supplied by the caller's
-    // environment. A tuple or tablet source is `NotIterable` (only lists
-    // iterate and only scalars widen); an unbound source propagates
-    // `UnboundVariable` rather than being swallowed.
+    // environment. A tuple or tablet source is `NotIterable` (lists iterate
+    // and scalars widen, but a tablet is a record that must be projected via
+    // values()/labels()/pairs() first, and a tuple does neither); an unbound
+    // source propagates `UnboundVariable` rather than being swallowed.
     let names = [Identifier::new("item")];
     let loop_op = Operation::Loop {
         names: &names,
@@ -948,7 +949,8 @@ fn foreach_over_non_list_or_unbound_errors() {
         other => panic!("expected NotIterable, got {:?}", other),
     }
 
-    // A tablet bound to `source` is not a list either.
+    // A tablet bound to `source` is a record, not a sequence: it must be
+    // projected with values()/labels()/pairs() rather than iterated directly.
     let mut tablet_fixture = StoreFixture::new("foreach-tablet");
     let mut env = Environment::new();
     env.extend(

src/runner/runner.rs

@@ -263,6 +263,8 @@ impl<'i, P: Prompt> Runner<'i, P> {
                     Value::Arraeum(items) => items,
                     // A scalar in list context is a singleton list.
                     value @ (Value::Literali(_) | Value::Quanticle(_)) => vec![value],
+                    // A tablet is a record, not a sequence, so it does not
+                    // iterate directly.
                     _ => return Err(RunnerError::NotIterable),
                 };
                 for (i, item) in items