WIP

temp/concepts.md

@@ -183,10 +183,10 @@ A field descriptor is a (usually) YAML description of either a single field or
 an array of fields. Each field produced by a field descriptor has exactly the
 same characteristics, but maps to a different bitrange, and uses a different
 array index on the register file interface. These bitranges can be described
-either manually as an array or by means of a repeat count and the necessary
-strides. Such repetition is useful when you have an array of similar registers,
-for instance in a DMA controller with multiple channels, where each channel has
-its own set of status and control flags.
+by means of a repeat count and the necessary strides. Such repetition is useful
+when you have an array of similar registers, for instance in a DMA controller
+with multiple channels, where each channel has its own set of status and
+control flags.
 
 Note that this means that you can have four kinds of field descriptors:
 

vhdmmio/config/choice.py

@@ -22,10 +22,11 @@ class Choice(ScalarLoader):
     something whose `str()` representation performs the inverse of the
     conversion function."""
 
-    def __init__(self, key, markdown, *choices):
-        default = choices[0][0]
-        if default is bool:
-            default = False
+    def __init__(self, key, markdown, *choices, default=Unset):
+        if default is not Unset:
+            default = choices[0][0]
+            if default is bool:
+                default = False
         if not isinstance(default, (int, str, bool)) and default is not None:
             raise ValueError('invalid default value')
         super().__init__(key, markdown, default, Unset)
@@ -223,5 +224,7 @@ def choice(method):
 
 def flag(method):
     """Convenience method for making flag `Choice`s, i.e. booleans that default
-    to `False`. The annotated method is never called."""
-    return Choice(method.__name__, method.__doc__, (bool, ''))
+    to `False`. The return value of the annotated method (cast to bool) is used
+    as the default value. The method should not take any arguments; not even
+    `self`."""
+    return Choice(method.__name__, method.__doc__, (bool, ''), bool(method()))