they’re practically two sides of
the same coin. I keep thinking it should be possible to build a toy language that has both a forth-like mode and a lisp-like mode, that use the same standard library
I think you'd have to hobble Forth pretty badly to make it stand on the same level as Lisp. Lisp has too much hidden machinery- a garbage collector, environment structures, etc. You couldn't even represent forth's "/mod" without resorting to packing results into tuples. Forth is good because every piece of the compiler/interpreter is exposed, usable, and extensible. Lisp wants to keep its innards sealed off and abstracted from the hardware.
There was a language for calculators called Reverse Polish Lisp. (That's all I know about it.)
If I were to design a language matching that name, I guess it'd be basically Logo written backwards. This might be interesting for a live programming environment, because in Lisp you type a whole expression and and nothing happens till you hit enter; but on e.g. an old HP RPN calculator you see the state of the data change after every keypress. You could design an "RPL" to work that way too, though after every word instead of every key.
Forth is more low level, ie. there is no object memory/GC at what one would describe as implementation level. On the other hand all the Lisp Machines essentially are hardware implementations of something vaguely Forth-like with some level of hardware support for lisp-style tagged pointers.
A good way to jump over the low level implementation details is to look at Factor. Forth style but much better library and most of the low level constructs are baked in.
I can't say the documentation / tutorials that I've found are the best but it's definitely something.
