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Keyword Arguments

You can specify the arguments to a function by name. This can be useful when you need to add occasionally-used parameters to existing functions, similar to the way you can use default parameter values in languages like C++.

define function describe-list(my-list :: <list>, #key verbose?) => ()
  format(*standard-output*, "{a <list>, size: %d", my-list.size);
  if (verbose?)
    format(*standard-output*, ", elements:");
    for (item in my-list)
      format(*standard-output*, " %=", item);
    end for;
  end if;
  format(*standard-output*, "}");
end function;

This method could be invoked in one of several ways.  The first
specifies no keyword arguments, and exhibits the default behavior:
    
describe-list(#(1, 2, 3, 4, 5, 6));
// prints "{a <list>, size: 6}"

Alternatively, the verbose? keyword argument could
be used to indicate the 'verbose' behavior was desired:
describe-list(#(5, 7, 3), verbose?: #t);
// prints "{a <list>, size: 3, elements: 5 7 3}"

Dylan's format statement works just like printf, except that you can extend it to handle new types of objects.