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> I implemented Common Lisp once on the Lisp machine, and I'm not all that happy with it. One thing I don't like terribly much is keyword arguments (8). They d

• kazinator 8y ago
  Wow, I so agree with that and the (8) footnote: "I don't mind if a very complex and heavyweight function takes keyword arguments. What bothers me is making simple basic functions such as “member” use them." I kept keyword arguments out of TXR Lisp. That is, out of the function call mechanism. It doesn't seem like a good idea to deal with a dictionary like mechanism in function calls. I invented something called "parameter list macros". A parameter list macro can be written which adds keyword argument ability to a function (and such a macro is provided). You can do (lambda (:key a b c -- x y) ...) and now you have keyword parameters x and y. They support the (sym dfl-val present-p) syntax and all. The :key parameter macro implements them via a source transformation. :key is able to rewrite both the parameter list and the body of the function to make this work. One reason Common Lisp programmers use keyword macros is that there is no way to invoke the default value of an optional parameter which precedes another optional parameter to which an argument is being specified. If a function has optional arguments X and Y, the caller cannot default X if it specifies Y. I fixed that in TXR Lisp: the symbol : (colon) can be passed to an optional parameter to activate its default value. So if a binary function f has two optional parameters we can invoke (f : 3) to default the first optional, and specify the second one as 3. This special feature is only supported in function calls, not in macro/destructuring parameter lists. That : symbol also serves as the &optional keyword, separating the required parameters from the optionals. It harmonizes with the consing dot that demarcates the &rest parameter: (lambda (x y : z w . r) ...): x y required, z w optional, r rest. The : symbol is nothing more than the symbol named "" (empty string) in the keyword package. It's provides a very useful third value to the nil and t duo. Common Lisps have this symbol! Unfortunately, they tend to print it funny back at you like :||, and neglect to get any mileage out of its notational convenience. As for those member functions, I provide a memq, memql and memqual that use the three different equalities. The more general member function takes a key and test function, but not as keyword arguments but positional optional parameters. If you want the default test, but custom key: [member foo bar : mykey].