args.py: Show a proper usage error on invalid arguments.

Addresses issue #277.

frontend/args.py

@@ -169,6 +169,8 @@ def OnMatch(self, prefix, suffix, arg_r, out):
 
     #log('SetToArg arg %r', arg)
 
+    # TODO: better location information for all these errors.
+
     typ = self.arg_type
     if isinstance(typ, list):
       if arg not in typ:
@@ -179,9 +181,17 @@ def OnMatch(self, prefix, suffix, arg_r, out):
       if typ == Str:
         value = arg
       elif typ == Int:
-        value = int(arg)  # TODO: check errors
+        try:
+          value = int(arg)
+        except ValueError:
+          raise UsageError(
+              'Expected integer after %r, got %r' % ('-' + self.name, arg))
       elif typ == Float:
-        value = float(arg)  # TODO: check errors
+        try:
+          value = float(arg)
+        except ValueError:
+          raise UsageError(
+              'Expected numeric after %r, got %r' % ('-' + self.name, arg))
       else:
         raise AssertionError
 

spec/builtin-io.test.sh

@@ -398,3 +398,10 @@ argv.py "${arguments[@]}"
 ## END
 ## N-I dash/mksh/zsh/ash status: 2
 ## N-I dash/mksh/zsh/ash stdout-json: ""
+
+#### read -n with invalid arg
+read -n not_a_number
+echo status=$?
+## stdout: status=2
+## OK bash stdout: status=1
+## N-I zsh stdout-json: ""