retroforth/example/accumulator.forth

# Accumulator

## Description

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:

~~~
:acc (ns-)
  d:create , [ [ fetch ] [ v:inc ] bi ] does ;
~~~

This removes the primitive stack shuffling, and leaves something that expresses the intent more clearly.

Finally, here's a little test case:

```
  #10 'foo acc
  foo
  foo
  foo
```