Limited Types

Limited types provide a way to define new, restricted types based on existing types in a way that is different from subclassing. For example, there is no built-in <byte> type, but it could be defined as follows:

define constant <byte> = limited(<integer>, min: 0, max: 255);

Open Dylan can often provide improved error checking and optimizations for limited types. For example, this code could generate a compile time error:

define function add1 (b :: <byte>) => (b2 :: <byte>) b + 1 end;
add1(255);

The set of limited types that are supported depends on the compiler.

Collection classes can be limited to containing a specific type of element:

define constant <int-vector> = limited(<vector>, of: <integer>);

Open Dylan will optimize accesses into an <int-vector> defined as above.

Open Dylan can avoid doing bounds checking or type checking, and can use an efficient representation of the vectors of floating point numbers for this code:

define constant $audio-buffer-size = 2048;
define constant <audio-buffer> =
  limited(<vector>,
          of: <single-float>,
          size: $audio-buffer-size);

define function mix-buffers
    (input1 :: <audio-buffer>, input2 :: <audio-buffer>,
     output :: <audio-buffer>)
 => ()
  for (i from 0 below $audio-buffer-size)
    output[i] = 0.5 * input1[i] + 0.5 * input2[i];
  end;
end;