Clojure HTML
Switch branches/tags
Nothing to show
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.


Example based coding

Converging Tracks

Build Status

We become what we behold. We shape our tools, and thereafter our tools shape us.

― Marshall McLuhan


Add the following line to your leiningen dependencies:

Clojars Project

Require tracks in your namespace header:

(:require [tracks.core :as t :refer [track]])


This is a library dedicated to the concept of shape.

shape n.
    - the external form, contours, or outline of something.
    - the correct or original form or contours of something.
    - an example of something that has a particular form.

shape v.
    - to give definite form, organization, or character to.
    - fashion or form.

It's common to grapple with large maps whose shapes are uncomfortable to reason about.

tracks simplifies transformations and destructuring of Clojure datastructures. Instead of describing how to do a transformation, tracks allows the user to create those transformations by example. This makes writing complex code that takes one shape and transforms them to another dead simple.



Destructuring complex nested data structures can be a real pain. Tracks makes this easy. Much like clojure.core/let, symbols in the track pattern will be bound to the value and available the body. Unlike clojure.core/let we supply a binding form of the same shape as the data we are interested in.

(t/let [{:a {:b [greeting person]}} ;;<- binding form
        {:a {:b ["Hello" "World"]}} ;;<- data we want to get at
  (str greeting " " person "!"))

;;=> "Hello World!"

(t/let [{:a {:b x} :c {:d y}}
        {:a {:b 1} :c {:d 2}}]
  (+ x y))

;;=> 3

track/track for building functions

track returns a function which takes data of the shape of its first argument.

Below, the function returned by track will take a map with keys :a and :b and move the value at :a to :b, and the value at :b to :a:

(track {:a one :b two}
       {:a two :b one})

;;=> anonymous fn

(def swap-a-b (track {:a one :b two}
              {:a two :b one}))
(swap-a-b {:a 100 :b 3000})

;;=> {:a 3000 :b 100}

deftrack does the same thing, but binds it too:

(deftrack swap-a-b {:a one :b two} {:a two :b one})
(swap-a-b {:a 100 :b 3000})

;;=> {:a 3000 :b 100}

We can move positions in vectors and deeply nested maps in exactly the same way:

((track {:a [zero one]}
        {:b [one zero]})
  {:a [:zero :one]})

;; => {:b [:one :zero]}

Arbitrary nesting levels

Deep thinking about deeply nested shapes is a bygone era:

(deftrack deeptx
  {0 zero, 1 one, 2 two, 3 three} ;; <- deeptx takes a map with this shape
  {:a zero :b {:c one :d {:e two :f {:g three}}}} ;; <- deeptx then returns one with this shape

(deeptx {0 "first" 1 "second" 2 "third" 3 "fourth"})
;;=> {:a "first", :b {:c "second", :d {:e "third", :f {:g "fourth"}}}}

Complex leaf values

track greatly simplifies rotating values, too:

Let's simulate a game where there's an active player, and all other players wait in line to become the active one. Once a player has played their turn, they go to the back of the line.

;;; Setup the function that moves around players,
;;; no matter what datastructure the players are
;;; represented as:

(deftrack move-players
  {:active-player p1 :players [p2 p3 p4]}
  {:active-player p2 :players [p3 p4 p1]})

;;; Here's the datastructure that represents the state of the game.
;;; Notice that the players are more than scalar values!

(defonce game (atom {:active-player {:name "A"}
                     :players [{:name "B"}
                               {:name "C"}
                               {:name "D"}]}))

(swap! game move-players)
;;=>  {:active-player {:name "B"}
;;     :players [{:name "C"}
;;               {:name "D"}
;;               {:name "A"}]}

(swap! game move-players)
;;=>  {:active-player {:name "C"}
;;     :players [{:name "D"}
;;               {:name "A"}
;;               {:name "B"}]}

(swap! game move-players)
;;=>  {:active-player {:name "D"}
;;     :players [{:name "A"}
;;               {:name "B"}
;;               {:name "C"}]}

Multiple endpoints

Like a train track, sometimes one track can split into many. With track the values can be duplicated.

(deftrack one-to-many x {:a x :b {:c [x x]}})

(one-to-many "?")

;;=> {:a "?", :b {:c ["?" "?"]}}

How it works

track is implemented in terms of let

(def move-a-key (track {:x one} {:y one}))

(move-a-key {:x "MoveMe"})

;;=> {:y "MoveMe"}

(move-a-key {:x [:a :b :c]})

;;=> {:y [:a :b :c]}

We see it moves any value from keypath [:x] to keypath [:y].

The way it does it is by moving {:x one} into a let like so:

          ;; vvvvvvvv---- this is the first arg to track
(tracks/let [{:x one} input]
    ;; so now one is bound to (get input :x)
 ;;  vvvvvv---- this is the 2nd arg to track
    {:y one})

Want more examples?

Check the test namespace!


Copyright © 2016 Bryan Maass

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.