Variadic arguments should be coerced to nil when none are provided (#802)

Fixes #801

Author: chrisrink10

CHANGELOG.md

@@ -28,6 +28,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
  * Fix issue with keywords throwing `TypeError` when used as a function on vectors (#770)
  * Fix an issue where the constructors of types created by `deftype` and `defrecord` could not be called if they contained `-` characters (#777) 
  * Fix issue with the variadic ampersand operator treated as a binding in macros (#772)
+ * Fix a bug the variadic arg symbol was not correctly bound to `nil` when no variadic arguments were provided (#801)
 
 ### Removed
  * Removed support for PyPy 3.8 (#785)

src/basilisp/core.lpy

@@ -26,7 +26,7 @@
 (def ^{:doc      "Create a list from the arguments."
        :arglists '([& args])}
   list
-  (fn* list [& args] args))
+  (fn* list [& args] (if args args '())))
 
 (def
   ^{:doc      "If ``seq`` is a Seq, return the first element from ``seq``. If ``seq``

src/basilisp/lang/compiler/generator.py

@@ -694,7 +694,6 @@ def _var_ns_as_python_sym(name: str) -> str:
 _ATTR_CLASS_DECORATOR_NAME = _load_attr("attr.define")
 _ATTR_FROZEN_DECORATOR_NAME = _load_attr("attr.frozen")
 _ATTRIB_FIELD_FN_NAME = _load_attr("attr.field")
-_COLLECT_ARGS_FN_NAME = _load_attr(f"{_RUNTIME_ALIAS}._collect_args")
 _COERCE_SEQ_FN_NAME = _load_attr(f"{_RUNTIME_ALIAS}.to_seq")
 _BASILISP_FN_FN_NAME = _load_attr(f"{_RUNTIME_ALIAS}._basilisp_fn")
 _FN_WITH_ATTRS_FN_NAME = _load_attr(f"{_RUNTIME_ALIAS}._with_attrs")
@@ -1627,10 +1626,14 @@ def __fn_args_to_py_ast(
             fn_body_ast.append(
                 ast.Assign(
                     targets=[ast.Name(id=safe_local, ctx=ast.Store())],
-                    value=ast.Call(
-                        func=_COLLECT_ARGS_FN_NAME,
-                        args=[ast.Name(id=arg_name, ctx=ast.Load())],
-                        keywords=[],
+                    value=ast.IfExp(
+                        test=ast.Name(id=arg_name, ctx=ast.Load()),
+                        body=ast.Call(
+                            func=_NEW_LIST_FN_NAME,
+                            args=[ast.Name(id=arg_name, ctx=ast.Load())],
+                            keywords=[],
+                        ),
+                        orelse=ast.Constant(None),
                     ),
                 )
             )

src/basilisp/lang/runtime.py

@@ -1638,17 +1638,6 @@ def repl_completions(text: str) -> Iterable[str]:
 ####################
 
 
-@functools.singledispatch
-def _collect_args(args) -> ISeq:
-    """Collect Python starred arguments into a Basilisp list."""
-    raise TypeError("Python variadic arguments should always be a tuple")
-
-
-@_collect_args.register(tuple)
-def _collect_args_tuple(args: tuple) -> ISeq:
-    return llist.list(args)
-
-
 class _TrampolineArgs:
     __slots__ = ("_has_varargs", "_args", "_kwargs")
 

tests/basilisp/compiler_test.py

@@ -2313,6 +2313,12 @@ def test_variadic_arity_fn_method_has_variadic_argument(
         with pytest.raises(compiler.CompilerException):
             lcompile("(fn* ([] :a) ([m &] m))")
 
+    def test_variadic_arity_fn_collects_args_correctly(self, lcompile: CompileFn):
+        f = lcompile("(fn* [& args] args)")
+        assert f() is None
+        assert f(1) == llist.l(1)
+        assert f(1, 2, 3) == llist.l(1, 2, 3)
+
     def test_fn_argument_vector_is_vector(self, lcompile: CompileFn):
         with pytest.raises(compiler.CompilerException):
             lcompile("(fn* () :a)")