RE: orthogonality and generalized references (was Re: Zen of Python)

[Guy Steele - Sun Microsystems Labs <address@hidden>]

   Date: Mon, 27 May 2002 17:57:55 -0700
   From: "Todd Proebsting" <toddpro@microsoft.com>

   
   Ages ago I posted to this list about goal-directed evaluation in Icon.
   This "ifAbsentPut" discussion will, I hope, allow me to express another
   advantage of goal-directed evaluation.  In Icon, the ifAbsentPut idiom
   is the following simple expression---Icon has only expressions, and not
   statements.
   
   /dict["foo"] := 42
   
   To understand this statement, you must understand the unary '/' operator
   and the notion of goal-directed evaluation.  The '/' operator computes
   the l-value (if one exists, otherwise the r-value) of its operand IFF
   the r-value is not null.  If the r-value is null, then the operation
   fails.  
   
Did you get the sense of null vs. non-null backwards in
your description here?   I believe that / fails iff the
r-value is not null.  See

  http://www.cs.arizona.edu/icon/refernce/prefix.htm#null

It is the \ operator that fails if its operand is null, right?
So

  \dict["foo"] := 42

replaces the hash table entry with 42 iff the entry is non-null,
but leaves it alone if it is null.

--Guy Steele