2016-07-10

Re: Querying plists

Zach's Querying plists blog post showcases a neat little querying DSL for plists. I couldn't shake the feeling that it looked an awful lot like pattern matching. I've often been impressed by optima, but I barely get to use it, so I thought I should try and see what querying plists looked like using pattern matching.

Here's what I came up with. Zach's example

(query-plists '(:and (:= :first-name "Zach") (:= :state "ME")
                     (:not (:= :last-name "Beane")))
              *people*)
becomes:
(remove-if-not (lambda-match ((plist :first-name "Zach" :state "ME"
                                     :last-name (not "Beane")) t))
               *people*)

It turned out more succinct than I initially expected! Also, it's trivially adaptable to other kinds of objects. E.g., given the following class:
(defclass person ()
  ((first-name :initarg :first-name)
   (last-name  :initarg :last-name)
   (state      :initarg :state)))
all we have to do is swap plist with the class name person and we're all set:
(remove-if-not (lambda-match ((person :first-name "Zach" :state "ME"
                                      :last-name (not "Beane")) t))
               *people*)

We can't quite define something exactly like Zach's query-plists because, AFAICT, optima's patterns are not first-class objects but perhaps we can cheat a little bit.
;; naming things is hard. :-/
(defmacro matchp (pattern) `(lambda-match (,pattern t)))
(defun filter (predicate list) (remove-if-not predicate list))

(filter (matchp (plist :first-name "Zach" :state "ME"
                       :last-name (not "Beane")))
        *people*)

Making this equally succinct when the query criteria are not constant is a challenge for another day and makes it clear that matchp is a lousy abstraction. ;-)

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