join/1 folded each element as `(acc + $x) + elem`, with the element
branch ending in `.//""`. Both leave a second live reference to the
accumulator: the nested + feeds the same `.` to both operands, and the
`//` leaves a forkpoint below it on the stack. `jvp_string_append`
therefore cannot grow the accumulator in place, and copies the whole
string once per element.
Pipe `acc + $x` into `. + elem` and drop the `//`, since adding null to
a string is already a no-op. join/1 is now linear: joining 8000
strings of 1000 chars goes from 6.5s to 0.06s, with output and error
messages unchanged.
Artem Lytkin
9a75bb0d43