This implements a function that takes an initial value and constructs a new function that returns the value before incrementing the stored value by 1.
So, given an initial value of 1, the first time the function is called, 1 is returned. The second, 2, and so on.
In traditional Forth, this would be done using a CREATE/DOES> construct. RETRO allows for something similar using the does combinator.
An example in a traditional Forth:
: acc ( n "name" -- ) create , does> dup >r @ dup 1+ r> ! ;
In RETRO, we could begin by rewriting this using the RETRO words:
:acc (ns-) d:create , [ dup push fetch n:inc pop store ] does ;
The dup push ... pop pattern is the sip combinator, so we can simplify it:
:acc (ns-) d:create , [ [ fetch n:inc ] sip store ] does ;
This is better, but not quite done. RETRO has a v:inc for incrementing variables, which would eliminate the n:inc and store. And a bi combinator to run two quotes against a value. So we could simplify yet again, resulting in:
This removes the primitive stack shuffling, and leaves something that expresses the intent more clearly.
Finally, here's a little test case: