report unexposed values

compiler/reporting/src/error/canonicalize.rs

@@ -344,6 +344,11 @@ fn pretty_runtime_error<'b>(
             unreachable!("")
         }
 
+        RuntimeError::UnresolvedTypeVar | RuntimeError::ErroneousType => {
+            // only generated during layout generation
+            unreachable!("")
+        }
+
         RuntimeError::Shadowing {
             original_region,
             shadow,
@@ -443,7 +448,20 @@ fn pretty_runtime_error<'b>(
         RuntimeError::UnsupportedPattern(_) => {
             todo!("unsupported patterns are currently not parsed!")
         }
-        RuntimeError::ValueNotExposed { .. } => todo!("value not exposed"),
+        RuntimeError::ValueNotExposed { module_name, ident, region } => { 
+            alloc.stack(vec![
+                alloc.concat(vec![
+                    alloc.reflow("The "),
+                    alloc.module_name(module_name),
+                    alloc.reflow(" module does not expose a "),
+                    alloc.string(ident.to_string()),
+                    alloc.reflow(" value:"),
+                ]),
+                alloc.region(region),
+            ])
+        }
+
+
         RuntimeError::ModuleNotImported {
             module_name,
             imported_modules,

compiler/reporting/src/report.rs

@@ -1,6 +1,6 @@
 use inlinable_string::InlinableString;
 use roc_module::ident::Ident;
-use roc_module::ident::{Lowercase, TagName, Uppercase};
+use roc_module::ident::{Lowercase, ModuleName, TagName, Uppercase};
 use roc_module::symbol::{Interns, ModuleId, Symbol};
 use std::fmt;
 use std::path::PathBuf;
@@ -316,6 +316,17 @@ impl<'a> RocDocAllocator<'a> {
         self.text(name).annotate(Annotation::Module)
     }
 
+    pub fn module_name(&'a self, name: ModuleName) -> DocBuilder<'a, Self, Annotation> {
+        let name = if name.is_empty() {
+            // Render the app module as "app"
+            "app".to_string()
+        } else {
+            name.as_str().to_string()
+        };
+
+        self.text(name).annotate(Annotation::Module)
+    }
+
     pub fn inlinable_string(&'a self, s: InlinableString) -> DocBuilder<'a, Self, Annotation> {
         self.text(format!("{}", s)).annotate(Annotation::Module)
     }

compiler/reporting/tests/test_reporting.rs

@@ -216,6 +216,27 @@ mod test_reporting {
             .replace(UNDERLINE_CODE, "<underline>")
     }
 
+    #[test]
+    fn value_not_exposed() {
+        report_problem_as(
+            indoc!(
+                r#"
+                List.foobar 1 2
+            "#
+            ),
+            indoc!(
+                r#"
+                ── SYNTAX PROBLEM ──────────────────────────────────────────────────────────────
+
+                The List module does not expose a foobar value:
+
+                1│  List.foobar 1 2
+                    ^^^^^^^^^^^
+                "#
+            ),
+        )
+    }
+
     #[test]
     fn report_unused_def() {
         report_problem_as(