The Unit Specification

Now we arrive at the unit specification, which is where things become delightfully mathematical. The unit specifies a multiplier for the size, allowing you to work in convenient chunks:

Value:Size/TypeSpec-unit:N

The total number of bits used is: Size × Unit

Default Units

• integer, float, bitstring: unit of 1 (counting individual bits)
• binary: unit of 8 (counting bytes, as civilized beings do)

Why Units Matter

Consider the case of reading 4-byte chunks of data:

lfe> (defun read-4byte-chunks (bin)
       (binary-comp
         ((<< (chunk (size 4) binary) >> (binary-gen (<= bin))))
         chunk))

lfe> (read-4byte-chunks #B(1 2 3 4 5 6 7 8 9 10 11 12))
(#B(1 2 3 4) #B(5 6 7 8) #B(9 10 11 12))

Or, more explicitly using unit:

lfe> (set msg #B(72 101 108 108 111))  ; "Hello"
lfe> (binary ((msg (size 5) (unit 8))))  ; 5 units of 8 bits = 40 bits total
#B(72 101 108 108 111)