Segments: The Atomic Unit

A segment is a sequence of bits in a binary. These sequences need not be aligned on byte boundaries—you can have a 3-bit segment followed by a 5-bit segment, and the system will pack them tightly together because it’s not 1970 anymore and we have the technology to handle bits that don’t come in groups of eight.

lfe> (binary (5 (size 4)) (5 (size 4)))
#B(85)

Wait, what? Where did 85 come from?

Let’s examine this with a magnifying glass and possibly a stiff drink: The integer 5 in binary is 0101. We’ve asked for two 4-bit segments, each containing 5. So we get: 0101 0101, which when interpreted as a byte is… 85 in decimal. Or if you’re feeling hexadecimal, that’s 0x55. Or if you’re feeling ASCII, that’s “U”.

The REPL, in its infinite helpfulness, notices that 85 is a printable ASCII character and shows us <<"U">> (in Erlang notation) or #"U" (in LFE notation). The bits haven’t changed—only our perception of them has been subject to the REPL’s well-meaning interference.

lfe> (list-to-binary (integer-to-list 85))
#"U"

Yes, indeed. 85 is “U”. This is the kind of thing you either find fascinating or deeply troubling, depending on your relationship with the arbitrary nature of character encodings and numeric representations.