fix: Improve error messages for missing main pipeline

prqlc/prqlc/src/semantic/lowering.rs

@@ -40,11 +40,27 @@ pub fn lower_to_ir(
 ) -> Result<(RelationalQuery, RootModule)> {
     // find main
     log::debug!("lookup for main pipeline in {main_path:?}");
-    let (_, main_ident) = root_mod.find_main_rel(main_path).map_err(|(hint, span)| {
-        Error::new_simple("Missing main pipeline")
-            .with_code("E0001")
-            .with_hints(hint)
-            .with_span(span)
+    let (_, main_ident) = root_mod.find_main_rel(main_path).map_err(|(_hint, span)| {
+        // Provide better error messages based on what's in the module
+        let user_declared_names: Vec<_> = root_mod
+            .module
+            .names
+            .keys()
+            .filter(|name| *name != "std" && *name != "default_db")
+            .collect();
+
+        let error = if user_declared_names.is_empty() {
+            // No user declarations - empty query or only comments
+            // Message is self-explanatory, no hint needed
+            Error::new_simple("No PRQL query entered").with_code("E0001")
+        } else {
+            // Has declarations but no pipeline starting with 'from'
+            Error::new_simple("PRQL queries must begin with 'from'")
+                .with_code("E0001")
+                .push_hint("A query must start with a 'from' statement to define the main pipeline")
+        };
+
+        error.with_span(span)
     })?;
 
     // find & validate query def

prqlc/prqlc/tests/integration/error_messages.rs

@@ -348,3 +348,34 @@ fn empty_interpolations() {
     ───╯
     "#);
 }
+
+#[test]
+fn no_query_entered() {
+    // Empty query
+    assert_snapshot!(compile("").unwrap_err(), @r"
+    [E0001] Error: No PRQL query entered
+    ");
+
+    // Comment-only query
+    assert_snapshot!(compile("# just a comment").unwrap_err(), @r"
+    [E0001] Error: No PRQL query entered
+    ");
+}
+
+#[test]
+fn query_must_begin_with_from() {
+    // Query with declaration but no 'from'
+    assert_snapshot!(compile("let x = 5").unwrap_err(), @r"
+    [E0001] Error: PRQL queries must begin with 'from'
+    ↳ Hint: A query must start with a 'from' statement to define the main pipeline
+    ");
+
+    // Query with multiple declarations but no pipeline
+    assert_snapshot!(compile(r#"
+    let x = 5
+    let y = 10
+    "#).unwrap_err(), @r"
+    [E0001] Error: PRQL queries must begin with 'from'
+    ↳ Hint: A query must start with a 'from' statement to define the main pipeline
+    ");
+}