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From: ForthNet@willett.pgh.pa.us (ForthNet articles from GEnie)
Newsgroups: comp.lang.forth
Subject: PUZZLES AND PROBLEMS
Message-ID: <1512.UUL1.3#5129@willett.pgh.pa.us>
Date: 13 Aug 90 03:45:18 GMT
Organization: String, Scotch tape, and Paperclips.  (in Pgh, PA)
Lines: 61

Category 3,  Topic 35
Message 133       Sun Aug 12, 1990
W.BADEN1 [Wil]               at 13:54 PDT
 
Another candidate for readable Forth was:

 : |x| ( x - clippedx ) 
    ( clip x to left edge of screen and one screen less than
      edge on the right )
    maxx viewwidth - min 0max
 ;

There's no doubt that to the author this is very readable, because the author
knows the meaning of all constituent words and how they are used.  A good
Forth programmer who doesn't know the meaning of constituent words can analyze
the code and make reasonable guesses, which for this definition luck out. 
Thus the code is decipherable but I would not call it readable.

Let's show how the analysis might proceed.  

We are told that at the end "clippedx" is left on the stack.

    maxx viewwidth - min 0max ( clippedx) ;

"0max" is a would-be standard word whose meaning is known.  It may change the
value of the top stack element, but it leaves the state status unchanged.

    maxx viewwidth - min ( clippedx) 0MAX    ;

"min" is a standard word whose meaning is known.  Its stack effect is ( n1 n2 -
- n3).  n3 is clippedx, and we make the reasonable guess that n1 is x.

    maxx viewwidth - ( x n2) MIN ( clippedx) 0MAX    ;

The stack effect of "viewwidth" is unknown.  The "view" part suggests that it
displays something, but a luckier guess is that it behaves like a constant.
This moves the stack comment left.  Extra spacing is left to show that the
stack status recurs.

    maxx ( x n2) ViewWidth -    MIN ( clippedx) 0MAX    ;

A reasonable guess is that "maxx" also behaves like a constant.  This gives a
better stack comment.

    MaxX ( x xLimit) ViewWidth -    MIN ( clippedX) 0MAX    ;

The analysis would be much easier if the definition is written:

 : |x| ( x -- clippedX ) 
    ( Clip x to left edge of screen and one screen less than
      edge on the right. )
    MaxX ( x xLimit) ViewWidth -   MIN ( clippedX) 0MAX
 ;

The definition would be even a little more readable if it had a name something
like "clipX" or "xClipped".

(Is the hornet's nest still stirring?)
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