data: add Nothing type

sx/__init__.py

@@ -4,7 +4,7 @@ from .core.io import Stream, Segment
 from .core.meta import Wrapper, Generic
 from .core.expr import Expr
 
-from .types.data import Data, data
+from .types.data import Nothing, Data, data
 from .types.num import *
 from .types.struct import StructType, Struct
 from .types.seq import Arr, Tuple
@@ -16,7 +16,7 @@ __all__ = [x.__name__ for x in {
     Context, Type, Stream, Segment, Expr,
     Wrapper, Default, Sized, Ref, Transform, Mapped, Enum, Switch,
     AlignTo, AlignedTo,
-    Data,
+    Nothing, Data,
     Int, Bool, Float,
     Arr, Tuple, 
     StructType, Struct, Generic,

sx/types/data.py

@@ -1,6 +1,29 @@
 from typing import Optional as O, Union as U
 from ..core.base import Type, Context, PossibleDynamic as D
-from ..core.io import Stream
+from ..core.io import Stream, Pos
+
+
+class Nothing(Type[None]):
+    def __init__(self) -> None:
+        pass
+
+    def parse(self, context: Context, stream: Stream) -> None:
+        return None
+
+    def dump(self, context: Context, stream: Stream, value: None) -> None:
+        pass
+
+    def sizeof(self, context: Context, value: None) -> O[Pos]:
+        return 0
+
+    def default(self, context: Context) -> None:
+        return None
+
+    def __str__(self) -> str:
+        return 'Nothing'
+
+    def __repr__(self) -> str:
+        return '{__name__}.Nothing()'
 
 class Data(Type[bytes]):
     __slots__ = ('size',)
@@ -23,7 +46,7 @@ class Data(Type[bytes]):
             size = 0
         return bytes(size)
 
-    def sizeof(self, context: Context, value: O[bytes]) -> O[int]:
+    def sizeof(self, context: Context, value: O[bytes]) -> O[Pos]:
         return context.peek(self.size)
 
 data = Data()