Enhancing Applesoft using ampersand routines: https://link.springer.com/content/pdf/10.3758/BF03203836.pdf
>Applesoft BASIC's ampersand (&) jump feature gives the programmer an easy and fast way to connect machine language routines to BASIC. When the Applesoft interpreter encounters an "&," it does an immediate and unconditional jump to location $3F5 (decimal 1013). This feature, originally described as being "intended for the computer's internal use only" and "not a proper APPLESOFT command" (Apple Computer, 1978, p. 123), has become the favorite procedure for executing machine language routines from a running Applesoft program.
Transfers control to a machine-language procedure.
CALL ABSOLUTE ([argumentlist,] offset%)
▪ argumentlist Arguments passed to a machine-language procedure
as offsets from the current data segment.
▪ offset% The offset from the current code segment, set by
DEF SEG, to the starting location of the procedure.
Example:
'Calls routine for printing the screen to a local printer.
DIM a%(2)
DEF SEG = VARSEG(a%(0))
FOR i% = 0 TO 2
READ d%
POKE VARPTR(a%(0)) + i%, d%
NEXT i%
DATA 205, 5, 203 : ' int 5 retf 'Machine-language code
'for printing screen.
CALL ABSOLUTE(VARPTR(a%(0)))
DEF SEG
>>> s = 'foobar'
>>> s[1:2] = 'xxx'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment
> Can you remember what it does? (no, it doesn't draw a line; it would, if not for "FB").
Yes. Draws a filled box (FB), from top-left (x1,y1) to bottom right (x2,y2) in Purple (colour 5).
Although thinking about it... I'm pretty sure that the co-ords of 0,0 were bottom left, not top left on DOS based graphic screens, so is quite probably bottom-left to top-right.
Source: One of my first 'paid' jobs was as a QB45 programmer for an accountancy firm in the mid 1990s.
Or, for example, MID$ is a function, except when it's not:
Then there's stuff like DEFINT and GOSUB and ON STRIG and ...The reason for all this is that BASIC was never designed as a language in which the standard library could mostly be written, or even described, in that language. Thus, most features were implemented as intrinsics, with magic syntax and/or semantics, which varies drastically from feature to feature. This goes even for the standard functions - e.g. MID$ is a function (except when it's not, as above) - but it can take either 2 or 3 arguments, which is not something that you can declare yourself. As a result, when language evolved, it also did so in a haphazard way, as with LINE re-purposed for other reasons.
The original BASIC was very simple (but also non-extensible). The BASIC that most people remember - at its peak - was actually more complicated than an equivalent subset of Python.