Specter

Most of Clojure programming involves creating, manipulating, and transforming immutable values. However, as soon as your values become more complicated than a simple map or list – like a list of maps of maps – transforming these data structures becomes extremely cumbersome.

Specter is a library (for both Clojure and ClojureScript) for doing these queries and transformations concisely, elegantly, and efficiently. These kinds of manipulations are so common when using Clojure – and so cumbersome without Specter – that Specter is in many ways Clojure's missing piece.

Specter is fully extensible. At its core, its just a protocol for how to navigate within a data structure. By extending this protocol, you can use Specter to navigate any data structure or object you have.

Specter does not sacrifice performance to achieve its elegance. Actually, Specter is faster than the limited facilities Clojure provides for doing nested operations. For example: the Specter equivalent to get-in runs 30% faster than get-in, and the Specter equivalent to update-in runs 5x faster than update-in. In each case the Specter code is equally as convenient.

Latest Version

The latest release version of Specter is hosted on Clojars:

Learn Specter

• Introductory blog post: Functional-navigational programming in Clojure(Script) with Specter
• Presentation about Specter: Specter: Powerful and Simple Data Structure Manipulation
• API docs
• Performance guide: The Specter 0.11.0 announcement post provides a comprehensive overview of how Specter achieves its performance and what you need to know as a user to enable Specter to perform its optimizations.

Specter's API is contained in three files:

• macros.clj: This contains the core select/transform/etc. operations as well as macros for defining new navigators.
• specter.cljx: This contains the build-in navigators and functional versions of select/transform/etc.
• zippers.cljx: This integrates zipper-based navigation into Specter.

Questions?

You can ask questions about Specter by opening an issue on Github.

You can also find help in the #specter channel on Clojurians.

Examples

Here's how to increment all the even values for :a keys in a sequence of maps:

user> (use 'com.rpl.specter)
user> (use 'com.rpl.specter.macros)
user> (transform [ALL :a even?]
              inc
              [{:a 1} {:a 2} {:a 4} {:a 3}])
[{:a 1} {:a 3} {:a 5} {:a 3}]

Here's how to retrieve every number divisible by 3 out of a sequence of sequences:

user> (select [ALL ALL #(= 0 (mod % 3))]
              [[1 2 3 4] [] [5 3 2 18] [2 4 6] [12]])
[3 3 18 6 12]

Here's how to increment the last odd number in a sequence:

user> (transform [(filterer odd?) LAST]
              inc
              [2 1 3 6 9 4 8])
[2 1 3 6 10 4 8]

Here's how to increment all the odd numbers between indexes 1 (inclusive) and 4 (exclusive):

user> (transform [(srange 1 4) ALL odd?] inc [0 1 2 3 4 5 6 7])
[0 2 2 4 4 5 6 7]

Here's how to replace the subsequence from index 2 to 4 with [:a :b :c :d :e]:

user> (setval (srange 2 4) [:a :b :c :d :e] [0 1 2 3 4 5 6 7 8 9])
[0 1 :a :b :c :d :e 4 5 6 7 8 9]

Here's how to concatenate the sequence [:a :b] to every nested sequence of a sequence:

user> (setval [ALL END] [:a :b] [[1] '(1 2) [:c]])
[[1 :a :b] (1 2 :a :b) [:c :a :b]]

Here's how to get all the numbers out of a map, no matter how they're nested:

user> (select (walker number?)
              {2 [1 2 [6 7]] :a 4 :c {:a 1 :d [2 nil]}})
[2 1 2 1 2 6 7 4]

Here's now to navigate via non-keyword keys:

user> (select [(keypath "a") (keypath "b")]
              {"a" {"b" 10}})
[10]

Here's how to reverse the positions of all even numbers between indexes 4 and 11:

user> (transform [(srange 4 11) (filterer even?)]
              reverse
              [0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15])
[0 1 2 3 10 5 8 7 6 9 4 11 12 13 14 15]

Here's how to decrement every value in a map:

user> (transform [ALL LAST]
              dec
              {:a 1 :b 3})
{:b 2 :a 0}

Here's how to append [:c :d] to every subsequence that has at least two even numbers:

user> (setval [ALL
               (selected? (filterer even?) (view count) #(>= % 2))
               END]
              [:c :d]
              [[1 2 3 4 5 6] [7 0 -1] [8 8] []])
[[1 2 3 4 5 6 :c :d] [7 0 -1] [8 8 :c :d] []]

When doing more involved transformations, you often find you lose context when navigating deep within a data structure and need information "up" the data structure to perform the transformation. Specter solves this problem by allowing you to collect values during navigation to use in the transform function. Here's an example which transforms a sequence of maps by adding the value of the :b key to the value of the :a key, but only if the :a key is even:

user> (transform [ALL (collect-one :b) :a even?]
              +
              [{:a 1 :b 3} {:a 2 :b -10} {:a 4 :b 10} {:a 3}])
[{:b 3, :a 1} {:b -10, :a -8} {:b 10, :a 14} {:a 3}]

The transform function receives as arguments all the collected values followed by the navigated to value. So in this case + receives the value of the :b key followed by the value of the :a key, and the transform is performed to :a's value.

The four built-in ways for collecting values are VAL, collect, collect-one, and putval. VAL just adds whatever element it's currently on to the value list, while collect and collect-one take in a selector to navigate to the desired value. collect works just like select by finding a sequence of values, while collect-one expects to only navigate to a single value. Finally, putval adds an external value into the collected values list.

Here's how to increment the value for :a key by 10:

user> (transform [:a (putval 10)]
              +
              {:a 1 :b 3})
{:b 3 :a 11}

For every map in a sequence, increment every number in :c's value if :a is even or increment :d if :a is odd:

user> (transform [ALL (if-path [:a even?] [:c ALL] :d)]
              inc
              [{:a 2 :c [1 2] :d 4} {:a 4 :c [0 10 -1]} {:a -1 :c [1 1 1] :d 1}])
[{:c [2 3], :d 4, :a 2} {:c [1 11 0], :a 4} {:c [1 1 1], :d 2, :a -1}]

"Protocol paths" can be used to navigate on polymorphic data. For example, if you have two ways of storing "account" information:

(defrecord Account [funds])
(defrecord User [account])
(defrecord Family [accounts-list])

You can make an "AccountPath" that dynamically chooses its path based on the type of element it is currently navigated to:

(use 'com.rpl.specter.macros)
(defprotocolpath AccountPath [])
(extend-protocolpath AccountPath
  User :account
  Family [:accounts-list ALL])

Then, here is how to select all the funds out of a list of User and Family:

user> (select [ALL AccountPath :funds]
        [(->User (->Account 50))
         (->User (->Account 51))
         (->Family [(->Account 1)
                    (->Account 2)])
         ])
[50 51 1 2]

The next examples demonstrate recursive navigation. Here's how to double all the even numbers in a tree:

(defprotocolpath TreeWalker [])

(extend-protocolpath TreeWalker
  Object nil
  clojure.lang.PersistentVector [ALL TreeWalker])

(transform [TreeWalker number? even?] #(* 2 %) [:a 1 [2 [[[3]]] :e] [4 5 [6 7]]])
;; => [:a 1 [4 [[[3]]] :e] [8 5 [12 7]]]

Here's how to reverse the positions of all even numbers in a tree (with order based on a depth first search). This example uses conditional navigation instead of protocol paths to do the walk:

(declarepath TreeValues)

(providepath TreeValues
  (if-path vector?
    [ALL TreeValues]
    STAY
    ))


(transform (subselect TreeValues even?)
  reverse
  [1 2 [3 [[4]] 5] [6 [7 8] 9 [[10]]]]
  )

;; => [1 10 [3 [[8]] 5] [6 [7 4] 9 [[2]]]]

Future work

• Integrate Specter with other kinds of data structures, such as graphs. Desired navigations include: reduction in topological order, navigate to outgoing/incoming nodes, to a subgraph (with metadata indicating how to attach external edges on transformation), to node attributes, to node values, to specific nodes.
• Make it possible to parallelize selects/transforms
• Any connection to transducers?

License

Copyright 2015-2016 Red Planet Labs, Inc. Specter is licensed under Apache License v2.0.