The one thing I lamented about the Apple II Plus (vs. the original with Integer BASIC) was that to make room for autostart, they had to rip out the mini-assembler in the Monitor ROM. That was the closest thing I ever saw to an assembly-language REPL before this. At least they were able to keep the disassembler.
It's common in the C64 world to use freezer cartridges that allow you to interrupt normal operation and invoke a monitor program. These typically include rudimentary assemblers and disassemblers as well. Fun and useful for patching and reverse engineering!
Btw, I've only had a rough idea of how these are actually implemented, non-maskable interrupt something-something, and never knew how they make sure that the interrupt vector points to some persistent routine that can invoke the monitor. I found this great article explaining at least how The Final Cartridge 3 does it (in the first two sections): https://www.pagetable.com/282
I remember being so confused, as a 10 y/o kid, trying to follow examples from an Apple II-era book on my public library's Apple IIe computers. I didn't understand why I couldn't get the mini assembler to work. The idea that features would be removed on future machines simply didn't occur to me. I guess I really didn't even understand that the IIe was a different machine than what the book was written for since most everything else worked as-expected.
There was another downside, that the Applesoft BASIC license gave Microsoft a hostage which they later used to kill https://en.wikipedia.org/wiki/MacBASIC. Hard to tell how much it mattered that the Mac came with no user programmability, which had been expected for PCs till then.
The BBC Micro and the Atom that came before it had an assembler built into BASIC. Just think, in the early 80s you could do inline assembly if your BASIC code wasn't fast enough...
You should support symbols... For example, the monitor on my 6800 / 6809 Exorciser simulator has this (the "Offset at..." is printed when it fixes forward references):
~/exorsim$ exor -d
% a 1000
1000: start bra skip
1002: nop
1003: nop
1004: skip ldx #10
Offset at 1001 set to 02
1007: loop dex
1008: nop
1009: bne loop
100b: rts
100c:
% sy
1007 loop
1004 skip
1000 start
% u 1000
1000: 20 02 BRA $1004
1002: 01 NOP
1003: 01 NOP
1004: CE 00 0A LDX #$000A
1007: 09 DEX
1008: 01 NOP
1009: 26 FC BNE $1007
100B: 39 RTS
What would be even better: "a" brings up a built-in mini-editor. When you exit, the code is assembled.
I also wish I could select from the completion list with Tab without typing more of the instruction, fish-style. I.e., you press Tab to go through completion 1, completion 2, ..., completion N, completion 1 again with Enter to confirm your pick.
There was another downside, that the Applesoft BASIC license gave Microsoft a hostage which they later used to kill https://en.wikipedia.org/wiki/MacBASIC. Hard to tell how much it mattered that the Mac came with no user programmability, which had been expected for PCs till then.
With that said, I love the idea.
https://github.com/jhallen/exorsim