Optionally strongly typed array indexes

[Mason McGill via Digitalmars-d]

Interesting idea. I run into these sorts of bugs often doing 
multimedia processing. There's definitely a tension between 
writing generic code (NumPy-style, operating on abstract 
dimensions) and safe code (operating on dimensions with 
semantics; OpenCV supports this to an extent). I've never used 
Ada so I may be missing the point, but here are my impressions:

It seems to me that the users who may benefit most from this 
feature are likely to be using non-built-in data structures. For 
example, you mention interpreting the first level of array 
nesting as rows and the second as columns, implying you are 
treating the nested arrays as a matrix (rather than a list of 
lists, boxes of kittens, etc.). I'm inclined to believe use-cases 
for strongly-typed indices would often overlap with use-cases for 
strongly-typed containers. If that is true, then the language 
already provides decent support for annotating indices with 
"units":

   /* Deep in the belly of some image-processing library... */
   alias Row = Dimension!"row";
   alias Column = Dimension!"column";
   alias Channel = Dimension!"channel";
   alias Image = DenseGrid!(ubyte, "row column channel");

   /* In user code... */
   Image image = drawPrettyPicture();
   ubyte r = image[Row(0), Column(0), Channel(0)];
   ubyte g = image[Row(0), Column(0), Channel(1)];
   ubyte b = image[Row(0), Column(0), Channel(2)];

   /* Or if you want to get really fancy... */
   enum red = Channel(0), green = Channel(1), blue = Channel(2);
   ubyte r = image[Row(0), Column(0), red];
   ubyte g = image[Row(0), Column(0), green];
   ubyte b = image[Row(0), Column(0), blue];

   /* Depending on how DenseGrid.opIndex is defined,
      these statements could fail to compile... */
   image[Column(0), Row(0), red];
   image[blue, Row(0), Column(0)];

Speculation: A good behavior for such a `DenseGrid.opIndex` might 
allow untyped (size_t) indices for generic algorithms and users 
who can't be bothered, but reject incorrectly-typed indices (e.g. 
a `BoxOfKittens` should not be indexed with a `Row`, just as an 
`Image` should not be indexed with a `CatName`.