diff --git a/mycpp/TEST.sh b/mycpp/TEST.sh
index d271a54867..f2e645fdb4 100755
--- a/mycpp/TEST.sh
+++ b/mycpp/TEST.sh
@@ -232,6 +232,9 @@ test-invalid-examples() {
       */invalid_global.py)
         expected_status=2
         ;;
+      */invalid_switch.py)
+        expected_status=5
+        ;;
     esac
 
     if test $status -ne $expected_status; then
diff --git a/mycpp/cppgen_pass.py b/mycpp/cppgen_pass.py
index 50b9735492..6474d79788 100644
--- a/mycpp/cppgen_pass.py
+++ b/mycpp/cppgen_pass.py
@@ -2,6 +2,7 @@
 cppgen.py - AST pass to that prints C++ code
 """
 import io
+import itertools
 import json  # for "C escaping"
 
 from typing import overload, Union, Optional, Dict
@@ -385,8 +386,8 @@ def GetCReturnType(t) -> Tuple[str, bool, Optional[str]]:
 
 def PythonStringLiteral(s: str) -> str:
     """
-  Returns a properly quoted string.
-  """
+    Returns a properly quoted string.
+    """
     # MyPy does bad escaping. Decode and push through json to get something
     # workable in C++.
     return json.dumps(format_strings.DecodeMyPyString(s))
@@ -1912,6 +1913,13 @@ def _collect_cases(self, if_node, out):
         """
         The MyPy AST has a recursive structure for if-elif-elif rather than a
         flat one.  It's a bit confusing.
+
+        Appends (expr, block) cases to out param, and returns the default
+        block, which has no expression.
+
+        default block may be None.
+
+        Returns False if there is no default block.
         """
         assert isinstance(if_node, IfStmt), if_node
         assert len(if_node.expr) == 1, if_node.expr
@@ -1920,6 +1928,11 @@ def _collect_cases(self, if_node, out):
         expr = if_node.expr[0]
         body = if_node.body[0]
 
+        if not isinstance(expr, CallExpr):
+            self.report_error(expr,
+                              'Expected call like case(x), got %s' % expr)
+            return
+
         out.append((expr, body))
 
         if if_node.else_body:
@@ -1930,30 +1943,44 @@ def _collect_cases(self, if_node, out):
             #   if 0:
 
             if isinstance(first_of_block, IfStmt):
-                self._collect_cases(first_of_block, out)
+                return self._collect_cases(first_of_block, out)
             else:
                 # default case - no expression
-                out.append((None, if_node.else_body))
+                return if_node.else_body
+
+        return False  # NO DEFAULT BLOCK - Different than None
 
-    def _write_cases(self, cases):
+    def _write_cases(self, switch_expr, cases, default_block):
         """ Write a list of (expr, block) pairs """
 
         for expr, body in cases:
-            if expr is not None:
-                assert isinstance(expr, CallExpr), expr
-                for i, arg in enumerate(expr.args):
-                    if i != 0:
-                        self.def_write('\n')
-                    self.def_write_ind('case ')
-                    self.accept(arg)
-                    self.def_write(': ')
+            assert expr is not None, expr
+            if not isinstance(expr, CallExpr):
+                self.report_error(expr,
+                                  'Expected call like case(x), got %s' % expr)
+                return
 
-                self.accept(body)
-                self.def_write_ind('  break;\n')
-            else:
-                self.def_write_ind('default: ')
-                self.accept(body)  # the whole block
-                # don't write 'break'
+            for i, arg in enumerate(expr.args):
+                if i != 0:
+                    self.def_write('\n')
+                self.def_write_ind('case ')
+                self.accept(arg)
+                self.def_write(': ')
+
+            self.accept(body)
+            self.def_write_ind('  break;\n')
+
+        if default_block is None:
+            # an error occurred
+            return
+        if default_block is False:
+            self.report_error(switch_expr,
+                              'switch got no else: for default block')
+            return
+
+        self.def_write_ind('default: ')
+        self.accept(default_block)
+        # don't write 'break'
 
     def _write_switch(self, expr, o):
         """Write a switch statement over integers."""
@@ -1969,8 +1996,8 @@ def _write_switch(self, expr, o):
 
         self.indent += 1
         cases = []
-        self._collect_cases(if_node, cases)
-        self._write_cases(cases)
+        default_block = self._collect_cases(if_node, cases)
+        self._write_cases(expr, cases, default_block)
 
         self.indent -= 1
         self.def_write_ind('}\n')
@@ -1989,20 +2016,57 @@ def _write_tag_switch(self, expr, o):
 
         self.indent += 1
         cases = []
-        self._collect_cases(if_node, cases)
-        self._write_cases(cases)
+        default_block = self._collect_cases(if_node, cases)
+        self._write_cases(expr, cases, default_block)
 
         self.indent -= 1
         self.def_write_ind('}\n')
 
+    def _str_switch_cases(self, cases):
+        cases2 = []
+        for expr, body in cases:
+            if not isinstance(expr, CallExpr):
+                # non-fatal check from _collect_cases
+                break
+
+            args = expr.args
+            if len(args) != 1:
+                self.report_error(
+                    expr,
+                    'str_switch can only have case("x"), not case("x", "y")' %
+                    args)
+                break
+
+            if not isinstance(args[0], StrExpr):
+                self.report_error(
+                    expr,
+                    'str_switch can only be used with constant strings, got %s'
+                    % args[0])
+                break
+
+            s = args[0].value
+            cases2.append((len(s), s, body))
+
+        # Sort by string length
+        cases2.sort(key=lambda pair: pair[0])
+        grouped = itertools.groupby(cases2, key=lambda pair: pair[0])
+        return grouped
+
     def _write_str_switch(self, expr, o):
         """Write a switch statement over strings."""
         assert len(expr.args) == 1, expr.args
 
-        self.def_write_ind('switch (len(')
-        self.accept(expr.args[0])
-        self.def_write(')) {\n')
+        switch_var = expr.args[0]
+        if not isinstance(switch_var, NameExpr):
+            self.report_error(
+                expr.args[0],
+                'str_switch(x) accepts only a variable name, got %s' %
+                switch_var)
+            return
 
+        self.def_write_ind('switch (len(%s)) {\n' % switch_var.name)
+
+        # There can only be one thing under 'with str_switch'
         assert len(o.body.body) == 1, o.body.body
         if_node = o.body.body[0]
         assert isinstance(if_node, IfStmt), if_node
@@ -2010,13 +2074,40 @@ def _write_str_switch(self, expr, o):
         self.indent += 1
 
         cases = []
-        self._collect_cases(if_node, cases)
+        default_block = self._collect_cases(if_node, cases)
+
+        grouped_cases = self._str_switch_cases(cases)
+        # Warning: this consumes internal iterator
+        #self.log('grouped %s', list(grouped_cases))
+
+        for str_len, group in grouped_cases:
+            self.def_write_ind('case %s: {\n' % str_len)
+            if_num = 0
+            for _, case_str, block in group:
+                self.indent += 1
 
-        # TODO:
-        # - Every element must be a constant string
-        # - group by length
+                else_str = '' if if_num == 0 else 'else '
+                self.def_write_ind('%sif (str_equals_c(%s, %s, %d)) ' %
+                                   (else_str, switch_var.name,
+                                    PythonStringLiteral(case_str), str_len))
+                self.accept(block)
 
-        self.log('CASES %s', cases)
+                self.indent -= 1
+                if_num += 1
+
+            self.indent += 1
+            self.def_write_ind('else {\n')
+            self.def_write_ind('  goto str_switch_default;\n')
+            self.def_write_ind('}\n')
+            self.indent -= 1
+
+            self.def_write_ind('}\n')
+            self.def_write_ind('  break;\n')
+
+        self.def_write('\n')
+        self.def_write_ind('str_switch_default:\n')
+        self.def_write_ind('default: ')
+        self.accept(default_block)
 
         self.indent -= 1
         self.def_write_ind('}\n')
diff --git a/mycpp/examples/invalid_switch.py b/mycpp/examples/invalid_switch.py
new file mode 100644
index 0000000000..fa73dd150c
--- /dev/null
+++ b/mycpp/examples/invalid_switch.py
@@ -0,0 +1,83 @@
+#!/usr/bin/env python2
+"""
+invalid_except.py
+"""
+from __future__ import print_function
+
+from mycpp.mylib import switch, str_switch, tagswitch
+
+
+def NoDefault():
+    # type: () -> None
+
+    with switch(42) as case:
+        if case(42):
+            print('42')
+
+
+def TagSwitch():
+    # type: () -> None
+
+    s = "foo"
+    with tagswitch(s) as case:
+        if 42:
+            print('ONE')
+            print('dupe')
+
+        elif 43:
+            print('TWO')
+
+        else:
+            print('neither')
+
+
+def SwitchMustHaveCase():
+    # type: () -> None
+
+    i = 49
+    with switch(i) as case:
+        if 42:
+            print('ONE')
+            print('dupe')
+
+        elif 43:
+            print('TWO')
+
+        else:
+            print('neither')
+
+
+def StrSwitchNoTuple():
+    # type: () -> None
+
+    s = "foo"
+    with str_switch(s) as case:
+        # Problem: if you switch on length, do you duplicate the bogies
+        if case('spam', 'different len'):
+            print('ONE')
+            print('dupe')
+
+        elif case('foo'):
+            print('TWO')
+
+        else:
+            print('neither')
+
+
+def StrSwitchNoInt():
+    # type: () -> None
+
+    s = "foo"
+    with str_switch(s) as case:
+        # integer not allowed
+        if case(42):
+            print('ONE')
+            print('dupe')
+
+        else:
+            print('neither')
+
+
+def run_tests():
+    # type: () -> None
+    pass
diff --git a/mycpp/examples/test_switch.py b/mycpp/examples/test_switch.py
index 86952f46a2..89e1cd29e0 100755
--- a/mycpp/examples/test_switch.py
+++ b/mycpp/examples/test_switch.py
@@ -7,34 +7,38 @@
 import os
 
 from mycpp.mylib import switch, str_switch, log
-#from mycpp.mylib import switch, log
 
 
 def TestString(s):
     # type: (str) -> None
 
-    print('''
+    #print('''
     with str_switch(s) as case:
         # Problem: if you switch on length, do you duplicate the bogies
-        if case('spam', 'different len'):
-            print('ONE')
-            print('dupe')
+        if case('spam'):
+            print('== %s ==' % s)
+            print('SPAM')
+            print('yes')
 
         elif case('foo'):
-            print('TWO')
+            print('== %s ==' % s)
+            print('FOO')
+
+        elif case('bar'):  # same length
+            print('== %s ==' % s)
+            print('BAR')
 
         else:
+            print('== %s ==' % s)
             print('neither')
-            ''')
+            #''')
+    print('--')
+    print('')
 
 
-def run_tests():
+def TestNumSwitch():
     # type: () -> None
 
-    TestString('spam')
-    TestString('foo')
-    TestString('zzz')
-
     x = 5
     with switch(x) as case:
         if case(0):
@@ -52,6 +56,17 @@ def run_tests():
             print('another')
 
 
+def run_tests():
+    # type: () -> None
+
+    TestString('spam')
+    TestString('bar')
+    TestString('zzz')  # same length as bar
+    TestString('different len')
+
+    TestNumSwitch()
+
+
 def run_benchmarks():
     # type: () -> None
     raise NotImplementedError()
diff --git a/mycpp/gc_builtins.cc b/mycpp/gc_builtins.cc
index 769c113a03..fc23c523e4 100644
--- a/mycpp/gc_builtins.cc
+++ b/mycpp/gc_builtins.cc
@@ -419,7 +419,7 @@ bool keys_equal(Tuple2<BigStr*, int>* t1, Tuple2<BigStr*, int>* t2) {
   return are_equal(t1, t2);
 }
 
-bool str_equals_c(const char* c_string, int c_len, BigStr* s) {
+bool str_equals_c(BigStr* s, const char* c_string, int c_len) {
   // Needs SmallStr change
   if (len(s) == c_len) {
     return memcmp(s->data_, c_string, c_len) == 0;
diff --git a/mycpp/gc_builtins.h b/mycpp/gc_builtins.h
index e2e66a33f6..de758db84a 100644
--- a/mycpp/gc_builtins.h
+++ b/mycpp/gc_builtins.h
@@ -155,8 +155,8 @@ inline bool to_bool(int i) {
 
 bool str_contains(BigStr* haystack, BigStr* needle);
 
-// Used by 'with switch(s)'
-bool str_equals_c(const char* c_string, int c_len, BigStr* s);
+// Used by 'with str_switch(s)'
+bool str_equals_c(BigStr* s, const char* c_string, int c_len);
 
 // Only used by unit tests
 bool str_equals0(const char* c_string, BigStr* s);