;;;; derrida.lisp

(in-package #:derrida)

(defmacro with-plist (keys plist &body body)
  "KEYS is a list, each member of which is either a symbol or a pair of symbols.

If a member is just a symbol, say KEY, then it is treated as the name
of a symbol-macro (defined using symbol-macrolet) that expands to the
expression (getf PLIST KEY).  In this case, KEY is not allowed to be a
keyword symbol.

If a member is a pair of symbols, it is of the form (VAR KEY). Here,
key is a valid key into the PLIST and VAR is the name of the symbol
macrolet that will be bound to (getf PLIST KEY).

EXAMPLE:

(let ((pl 
        (list 'name \"colin\" :age 40 :|currentJob| :crumb-bum)))
  (hq:with-plist (name (age :age) (job :|currentJob|)) pl 
    (setf age (1+ age)) 
    (format t \"~a the ~a had a birthday, and is now ~a years old~%\" 
            name job age) 
    pl))

The above would print out:
colin the CRUMB-BUM had a birthday, and is now 41 years old

And would return 
(NAME \"colin\" :AGE 41 :|currentJob| :CRUMB-BUM)"  

  (let* ((plist-var
           (gensym))
         (macrolet-bindings
           (loop for term in keys
                 when (consp term )
                   collect (destructuring-bind (var key) term
                               `(,var (getf ,plist-var ',key)))
                 else
                   collect `(,term (getf ,plist-var ',term)))))
    `(let ((,plist-var ,plist)) (symbol-macrolet ,macrolet-bindings ,@body))))