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This library is inspired by Clojure and implements threading macros. They are useful when you need to chain some calls, passing result from one call to another.

There are two other CL libraries similar to arrows:

  • cl-arrows
  • arrow-macros

but arrows have more features.

Here is an example of two simples arrows:

POFTHEDAY> (arrows:-> (list :foo
                            (list :bar
                                   (list 1 2 3)
                                   (list 'a 'b 'c))))
             (getf :foo)
             (getf :bar)
             (arrows:->> (mapcar #'reverse)))
((3 2 1) (C B A))

;; The code above is translated into:
POFTHEDAY> (mapcar #'reverse
                     (list :foo
                           (list :bar
                                 (list (list 1 2 3)
                                       (list 'a 'b 'c))))
((3 2 1) (C B A))

There is also ->> which adds an argument to the end, and ->* which uses the last form as an initial value. They could be used in tandem like this:

POFTHEDAY> (arrows:->> '((:foo . ((:bar . 8))))
             (assoc :foo)
             (assoc :bar)
              (mod 3)
              (expt 2)))
4 (3 bits, #x4, #o4, #b100)

;; Real operations will be:
;; (expt (mod 8 3)
;;       2)
;; and here is the full macroexpansion:

             (cdr (assoc :bar
               (cdr (assoc :foo
                           '((:foo (:bar . 8)))))))

Another interesting macro is as->. It creates a lexical binding and a variable can be used in any place of the nested code:

POFTHEDAY> (arrows:-> '(:foo (:bar 15))
             (getf :foo)
             (getf :bar)
             (arrows:as-> $
                   (format t "Var value: ~A~%" $)
             (/ 75))
Var value: 15
1/5 (0.2, 20%)

There are a few other interesting types of arrows. Go, read about them in the documentation!

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