fix: normalise frontmatter list keys

src/plain_parser/plain_file.py

@@ -448,10 +448,12 @@ def process_imports(
 
         if REQUIRED_CONCEPTS_DIRECTIVE in plain_file_parse_result.plain_source_obj.metadata:
             for item in plain_file_parse_result.plain_source_obj.metadata[REQUIRED_CONCEPTS_DIRECTIVE]:
-                assert isinstance(
-                    item, str
-                ), f"Syntax error: Invalid {REQUIRED_CONCEPTS_DIRECTIVE} metadata. Expected a string."
-                new_concepts, _ = concept_utils.extract_concepts_from_definition(item)
+                new_concepts, errors = concept_utils.extract_concepts_from_definition(item)
+                if errors:
+                    raise PlainSyntaxError(
+                        f"Plain syntax error: Module '{module_name}' requires an invalid concept (`{item}`). "
+                        f"Expected a concept token, e.g. :ConceptName:."
+                    )
                 required_concepts.extend(new_concepts)
 
             required_concepts.extend(plain_file_parse_result.required_concepts)
@@ -480,11 +482,17 @@ def read_plain_source_metadata(plain_source_text):
     except Exception as e:
         raise PlainSyntaxError(f"Plain syntax error: Invalid frontmatter: {e}")
 
+    for directive in [IMPORT_DIRECTIVE, REQUIRES_DIRECTIVE, EXPORTED_CONCEPTS_DIRECTIVE, REQUIRED_CONCEPTS_DIRECTIVE]:
+        if directive in plain_source_obj.metadata and not isinstance(plain_source_obj.metadata[directive], list):
+            plain_source_obj.metadata[directive] = [plain_source_obj.metadata[directive]]
+
+    for directive in [IMPORT_DIRECTIVE, REQUIRES_DIRECTIVE]:
+        for module_name in plain_source_obj.metadata.get(directive, []):
+            if not isinstance(module_name, str):
+                raise PlainSyntaxError(f"Plain syntax error: Invalid {directive} metadata. Expected a module name.")
+
     for directive in [EXPORTED_CONCEPTS_DIRECTIVE, REQUIRED_CONCEPTS_DIRECTIVE]:
         if directive in plain_source_obj.metadata:
-            assert isinstance(
-                plain_source_obj.metadata[directive], list
-            ), f"Syntax error: Invalid {directive} metadata. Expected a list."
             prepared_metadata = []
             for item in plain_source_obj.metadata[directive]:
                 if isinstance(item, dict):

tests/data/imports/scalar_import/lib.plain

@@ -0,0 +1,3 @@
+***implementation reqs***
+
+- The main executable file should be called "hello_world.py".

tests/data/imports/scalar_import/main.plain

@@ -0,0 +1,11 @@
+---
+import: lib
+---
+
+***implementation reqs***
+
+- The main executable file should be called "hello_world.py".
+
+***functional specs***
+
+- Display "hello, world"

tests/data/imports/scalar_import_single_char/l.plain

@@ -0,0 +1,3 @@
+***implementation reqs***
+
+- The main executable file should be called "hello_world.py".

tests/data/imports/scalar_import_single_char/main.plain

@@ -0,0 +1,11 @@
+---
+import: l
+---
+
+***implementation reqs***
+
+- The main executable file should be called "hello_world.py".
+
+***functional specs***
+
+- Display "hello, world"

tests/data/requires/scalar_requires_main.plain

@@ -0,0 +1,11 @@
+---
+requires: chain_base
+---
+
+***implementation reqs***
+
+- Scalar requires implementation requirement.
+
+***functional specs***
+
+- A scalar requires functionality.

tests/test_imports.py

@@ -1,7 +1,7 @@
 import pytest
 
-from plain_parser import plain_file
-from plain_parser.exceptions import ImportedModuleWithFunctionalitiesError
+from plain_parser import concept_utils, plain_file
+from plain_parser.exceptions import ImportedModuleWithFunctionalitiesError, PlainSyntaxError
 
 
 def test_imported_module_with_functionalities(load_test_data, get_test_data_path):
@@ -51,3 +51,55 @@ def test_diamond_imports(load_test_data, get_test_data_path):
         ],
         "functional specs": [{"markdown": '- Display "hello, world"'}],
     }
+
+
+@pytest.mark.parametrize("fixture", ["scalar_import", "scalar_import_single_char"])
+def test_scalar_import_is_a_list_of_one(load_test_data, get_test_data_path, fixture):
+    plain_source_text = load_test_data(f"data/imports/{fixture}/main.plain")
+    template_dirs = [get_test_data_path(f"data/imports/{fixture}")]
+    plain_file_parse_result = plain_file.parse_plain_source(plain_source_text, {}, template_dirs, [], [])
+
+    assert plain_file.marshall_plain_source(plain_file_parse_result.plain_source)["implementation reqs"] == [
+        {"markdown": '- The main executable file should be called "hello_world.py".'},
+        {"markdown": '- The main executable file should be called "hello_world.py".'},
+    ]
+
+
+@pytest.mark.parametrize("directive", ["import", "requires"])
+def test_non_string_module_entry_is_a_syntax_error(directive):
+    plain_source_text = f"---\n{directive}: [1]\n---\n\n***functional specs***\n\n- Display text\n"
+    with pytest.raises(PlainSyntaxError, match=f"Invalid {directive} metadata"):
+        plain_file.parse_plain_source(plain_source_text, {}, [], [], [])
+
+
+@pytest.mark.parametrize("directive", ["exported_concepts", "required_concepts"])
+def test_scalar_concept_directive_is_a_list_of_one(directive):
+    plain_source_obj = plain_file.read_plain_source_metadata(f'---\n{directive}: ":A:"\n---\n')
+
+    assert plain_source_obj.metadata[directive] == ["- :A:"]
+
+
+@pytest.mark.parametrize("directive", ["exported_concepts", "required_concepts"])
+def test_non_string_concept_entry_is_a_syntax_error(directive):
+    with pytest.raises(PlainSyntaxError, match=f"Invalid {directive} metadata"):
+        plain_file.read_plain_source_metadata(f"---\n{directive}: [1]\n---\n")
+
+
+@pytest.mark.parametrize("value", ["foo", "[foo]"])
+def test_imported_required_concept_without_concept_token_is_a_syntax_error(tmp_path, value):
+    (tmp_path / "lib.plain").write_text(f"---\nrequired_concepts: {value}\n---\n\n***implementation reqs***\n\n- x\n")
+    plain_source_text = (
+        "---\nimport: [lib]\n---\n\n***implementation reqs***\n\n- y\n\n***functional specs***\n\n- Display text\n"
+    )
+    with pytest.raises(PlainSyntaxError, match="requires an invalid concept"):
+        plain_file.parse_plain_source(plain_source_text, {}, [str(tmp_path)], [], [])
+
+
+@pytest.mark.parametrize("directive", ["exported_concepts", "required_concepts"])
+@pytest.mark.parametrize("entry, concepts", [(":A:", [":A:"]), (":A:, :B:", [":A:", ":B:"])])
+def test_unquoted_concept_in_block_list(directive, entry, concepts):
+    # YAML reads an unquoted `- :A:` as a mapping; real modules rely on it still naming :A:.
+    plain_source_obj = plain_file.read_plain_source_metadata(f"---\n{directive}:\n  - {entry}\n---\n")
+
+    [item] = plain_source_obj.metadata[directive]
+    assert concept_utils.extract_concepts_from_definition(item) == (concepts, [])

tests/test_requires.py

@@ -37,6 +37,12 @@ def test_parse_module_chain_orders_required_modules_first(get_test_data_path):
         assert "functional specs" in plain_source_tree
 
 
+def test_scalar_requires_is_a_list_of_one(get_test_data_path):
+    chain = plain_file.parse_module_chain("scalar_requires_main.plain", [get_test_data_path("data/requires")])
+
+    assert [module_name for module_name, _ in chain] == ["chain_base", "scalar_requires_main"]
+
+
 def test_parse_module_chain_single_module(get_test_data_path):
     chain = plain_file.parse_module_chain("chain_base.plain", [get_test_data_path("data/requires")])