# Buggy code
def macroexpand(tree, module, compiler=None, once=False):
    """Expand the toplevel macros for the given Hy AST tree.

    Load the macros from the given `module`, then expand the (top-level) macros
    in `tree` until we no longer can.

    `HyExpression` resulting from macro expansions are assigned the module in
    which the macro function is defined (determined using `inspect.getmodule`).
    If the resulting `HyExpression` is itself macro expanded, then the
    namespace of the assigned module is checked first for a macro corresponding
    to the expression's head/car symbol.  If the head/car symbol of such a
    `HyExpression` is not found among the macros of its assigned module's
    namespace, the outer-most namespace--e.g.  the one given by the `module`
    parameter--is used as a fallback.

    Parameters
    ----------
    tree: HyObject or list
        Hy AST tree.

    module: str or types.ModuleType
        Module used to determine the local namespace for macros.

    compiler: HyASTCompiler, optional
        The compiler object passed to expanded macros.

    once: boolean, optional
        Only expand the first macro in `tree`.

    Returns
    ------
    out: HyObject
        Returns a mutated tree with macros expanded.
    """
    if not inspect.ismodule(module):
        module = importlib.import_module(module)

    assert not compiler or compiler.module == module

    while True:
        if not isinstance(tree, HyExpression) or tree == []:
            break

        fn = tree[0]
        if fn in ("quote", "quasiquote") or not isinstance(fn, HySymbol):
            break

        fn = mangle(fn)
        expr_modules = ([] if not hasattr(tree, "module") else [tree.module]) + [module]

        # Choose the first namespace with the macro.
        m = next(
            (mod.__macros__[fn] for mod in expr_modules if fn in mod.__macros__), None
        )
        if not m:
            break

        opts = {}
        if m._hy_macro_pass_compiler:
            if compiler is None:
                from hy.compiler import HyASTCompiler

                compiler = HyASTCompiler(module)
            opts["compiler"] = compiler

        with macro_exceptions(module, tree, compiler):
            m_copy = make_empty_fn_copy(m)
            m_copy(module.__name__, *tree[1:], **opts)
            obj = m(module.__name__, *tree[1:], **opts)

        if isinstance(obj, HyExpression):
            obj.module = inspect.getmodule(m)

        tree = replace_hy_obj(obj, tree)

        if once:
            break

    tree = wrap_value(tree)
    return tree


# Traceback (hy.errors.HyCompileError)
Traceback (most recent call last):
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/importer.py", line 180, in hy_eval
_ast, expr = hy_compile(hytree, module_name, get_expr=True)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 2213, in hy_compile
result = compiler.compile(tree)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 460, in compile
ret = self.compile_atom(_type, tree)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 447, in compile_atom
else _compile_table[atom_type](self, atom))
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 1731, in compile_expression
ret = self.compile_atom(fn, expression)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 447, in compile_atom
else _compile_table[atom_type](self, atom))
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 819, in compile_do
return self._compile_branch(expression)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 545, in _compile_branch
return _branch(self.compile(expr) for expr in exprs)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 318, in _branch
results = list(results)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 545, in <genexpr>
return _branch(self.compile(expr) for expr in exprs)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 460, in compile
ret = self.compile_atom(_type, tree)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 447, in compile_atom
else _compile_table[atom_type](self, atom))
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 1731, in compile_expression
ret = self.compile_atom(fn, expression)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 447, in compile_atom
else _compile_table[atom_type](self, atom))
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 372, in checker
return fn(self, expression)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 873, in compile_try_expression
name)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 1002, in _compile_catch_expression
body = self._compile_branch(expr)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 545, in _compile_branch
return _branch(self.compile(expr) for expr in exprs)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 318, in _branch
results = list(results)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 545, in <genexpr>
return _branch(self.compile(expr) for expr in exprs)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 460, in compile
ret = self.compile_atom(_type, tree)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 447, in compile_atom
else _compile_table[atom_type](self, atom))
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 1731, in compile_expression
ret = self.compile_atom(fn, expression)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 447, in compile_atom
else _compile_table[atom_type](self, atom))
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 372, in checker
return fn(self, expression)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 858, in compile_try_expression
body += self.compile(expr.pop(0))
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 472, in compile
raise_empty(HyCompileError, e, sys.exc_info()[2])
File "<string>", line 1, in raise_empty
hy.errors.HyCompileError: Internal Compiler Bug 😱
⤷ ImportError: cannot require names: [HySymbol('let')]
Compilation traceback:
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 460, in compile
ret = self.compile_atom(_type, tree)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 447, in compile_atom
else _compile_table[atom_type](self, atom))
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 1731, in compile_expression
ret = self.compile_atom(fn, expression)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 447, in compile_atom
else _compile_table[atom_type](self, atom))
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/compiler.py", line 1486, in compile_require
require(module, self.module_name, assignments=assignments)
File "/home/johnzhuang/.virtualenvs/hy/lib/python3.6/site-packages/hy/macros.py", line 106, in require
raise ImportError("cannot require names: " + repr(list(unseen)))

# Fixed code
def macroexpand(tree, module, compiler=None, once=False):
    """Expand the toplevel macros for the given Hy AST tree.

    Load the macros from the given `module`, then expand the (top-level) macros
    in `tree` until we no longer can.

    `HyExpression` resulting from macro expansions are assigned the module in
    which the macro function is defined (determined using `inspect.getmodule`).
    If the resulting `HyExpression` is itself macro expanded, then the
    namespace of the assigned module is checked first for a macro corresponding
    to the expression's head/car symbol.  If the head/car symbol of such a
    `HyExpression` is not found among the macros of its assigned module's
    namespace, the outer-most namespace--e.g.  the one given by the `module`
    parameter--is used as a fallback.

    Parameters
    ----------
    tree: HyObject or list
        Hy AST tree.

    module: str or types.ModuleType
        Module used to determine the local namespace for macros.

    compiler: HyASTCompiler, optional
        The compiler object passed to expanded macros.

    once: boolean, optional
        Only expand the first macro in `tree`.

    Returns
    ------
    out: HyObject
        Returns a mutated tree with macros expanded.
    """
    if not inspect.ismodule(module):
        module = importlib.import_module(module)

    assert not compiler or compiler.module == module

    while True:
        if not isinstance(tree, HyExpression) or tree == []:
            break

        fn = tree[0]
        if fn in ("quote", "quasiquote") or not isinstance(fn, HySymbol):
            break

        fn = mangle(fn)
        expr_modules = ([] if not hasattr(tree, "module") else [tree.module]) + [module]

        # Choose the first namespace with the macro.
        m = next(
            (mod.__macros__[fn] for mod in expr_modules if fn in mod.__macros__), None
        )
        if not m:
            break

        opts = {}
        if m._hy_macro_pass_compiler:
            if compiler is None:
                from hy.compiler import HyASTCompiler

                compiler = HyASTCompiler(module)
            opts["compiler"] = compiler

        with macro_exceptions(module, tree, compiler):
            obj = m(module.__name__, *tree[1:], **opts)

        if isinstance(obj, HyExpression):
            obj.module = inspect.getmodule(m)

        tree = replace_hy_obj(obj, tree)

        if once:
            break

    tree = wrap_value(tree)
    return tree