retroforth/example/variables-and-formulas.retro

From my mastodon feed:

    @millihertz@oldbytes.space:

    so i just saw this example given as a reason why
    concatenative languages tend to be called "write-only":

    f x y z = y¬≤ + x¬- |y|

    : f
      drop dup dup *
      swap abs
      rot3 dup *
      swap - + ;

... snipped ...

He ends up writing the following:

    lastly, nobody would ever write Forth like that anyway!

    : ²+  dup * swap dup * + ;
    : y²+x²-|y|  ( y x -- result )
      tuck ²+ swap abs - ;

Given a dislike of stack shufflers, a Retro implementation could be:

    :f [ [ n:square ] bi@ + ] sip n:abs - ;

Assuming one knows the combinators, this is pretty straightforward,
but doesn't really resemble the original formula. I decided to write
a quick set of words to let me convert the original formula into
something more recognizable while still being RPN.

    :f /xyz
       x: n:square y: n:square + y: n:abs - ;

This adds a / sigil that maps stack values to an internal set of
variables, whose values are returned by words with the variable names
followed by a colon.

The code follows.

~~~
'Vars d:create #256 allot

'abcdefghijklmnopqrstuvwxyz s:keep
[ dup ':%c:_&Vars_$%c_+_fetch_; s:format s:evaluate ] s:for-each

:sigil:/
  s:reverse [ 'liliadst i , &Vars , ] s:for-each ; immediate
~~~

And the test case:

```
:f /xyz
  x: n:square y: n:square + y: n:abs - ;

#33 #12 #301 f n:put
```