McCarthy's Ambiguous Operator
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Back in 1961, John McCarthy (the inventor of LISP) described
an
interesting mathematical operator called amb
. Essentially, amb
hates
to be called with no arguments, and can look
into the future to keep that from happening. Here's how it might look in
Ruby.
# amb will (appear to) choose values # for x and y that prevent future # trouble. x = amb 1, 2, 3 y = amb 4, 5, 6 # Ooops! If x*y isn't 8, amb would # get angry. You wouldn't like # amb when it's angry. amb if x*y != 8 # Sure enough, x is 2 and y is 4. puts x, y
Of course, amb
can't actually see the future. However, it can
rewind into the past whenever it sees trouble, and try a
different coice.
So, how could we implement this function? As it turns out, we need continuations. Here's a basic implementation in Ruby.
# A list of places we can "rewind" to # if we encounter amb with no # arguments. $backtrack_points = [] # Rewind to our most recent backtrack # point. def backtrack if $backtrack_points.empty? raise "Can't backtrack" else $backtrack_points.pop.call end end # Recursive implementation of the # amb operator. def amb *choices # Fail if we have no arguments. backtrack if choices.empty? callcc {|cc| # cc contains the "current # continuation". When called, # it will make the program # rewind to the end of this block. $backtrack_points.push cc # Return our first argument. return choices[0] } # We only get here if we backtrack # using the stored value of cc, # above. We call amb recursively # with the arguments we didn't use. amb *choices[1...choices.length] end # Backtracking beyond a call to cut # is strictly forbidden. def cut $backtrack_points = [] end
If you'd like a fun, non-technical overview of continuations, see the explanation at RubyGarden .
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