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functional-clojure-sequences.clj
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functional-clojure-sequences.clj
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;;;;
;;;; ____________________
;;;; < Functional Clojure >
;;;; --------------------
;;;; \ ^__^
;;;; \ (oo)\_______
;;;; (__)\ )\/\
;;;; ||----w |
;;;; || ||
;;;;
;;;; Functional clojure: sequences
;;;;
;;;; (c) 2011-2012 Joost Diepenmaat
;;;; Zeekat Softwareontwikkeling
;;;;
;;;; http://joost.zeekat.nl/
;;;; joost@zeekat.nl
(ns user)
;;;;
;;;; Clojure basics that you need to know
;;;
(def my-keyword :a-keyword)
(def my-vector [:a :b :c 4 "some text"])
(my-vector 2)
(def my-map {:one 1
:two 2
:sentence "some sentence"})
(my-map :one)
(:one my-map)
(defn my-function
[arg1 arg2]
(+ (* arg1 arg2) 10))
(my-function 4 5)
(defn my-other-function
[arg1 arg2]
(arg1 (* 4 arg2) 10))
(my-other-function + 5)
;;;;
;;;; Some data
;;;;
(def items [{:name "car"
:wheels [:wheel1 :wheel2
:wheel3 :wheel4]}
{:name "bike"
:wheels [:bicycle-wheel-1
:bicycle-wheel-2]}
{:name "tricycle"
:wheels [:tricycle-wheel1
:tricycle-wheel2
:tricycle-wheel2]}
{:name "unicycle"
:wheels [:unicycle-wheel]}
{:name "dog"
:legs [:dog-leg1 :dog-leg2
:dog-leg3 :dog-leg4]}])
;;;;
;;;; What is a seq
;;;;
(seq? items) ; => false
;;;;
;;;; Clojure lists are seqs, but vectors, arrays, hash-maps any many
;;;; other things are not
;;;;
(seq? (list 1 2 3 4)) ; => true
(seq? [1 2 3 4]) ; => false
(seq? {:a 1 :b 2}) ; => false
;;;;
;;;; You can *make* a seq from many things
;;;;
(seq items) ; => ({:name "car", :wheels [:wheel1 :wheel2 :wheel3 :wheel4]} {:name "bike", :wheels [:bicycle-wheel-1 :bicycle-wheel-2]} {:name "tricycle", :wheels [:tricycle-wheel1 :tricycle-wheel2 :tricycle-wheel2]} {:name "unicycle", :wheels [:unicycle-wheel]} {:name "dog", :legs [:dog-leg1 :dog-leg2 :dog-leg3 :dog-leg4]})
(seq? (seq items)) ; => true
(seq {:name "value"
:stuff 2}) ; => ([:name "value"] [:stuff 2])
(seq #{:a :b 1 2}) ; => (1 2 :a :b)
;;;;
;;;; So what is a seq?
;;;;
;;;;
;;;; What is a list?
;;;;
;;;; Cell Cell Cell Cell
;;;; +-+-+ +-+-+ +-+-+ +-+-+
;;;; |*|*+--> |*|*+--> |*|*+--> |*|*+--> ..
;;;; +++-+ +++-+ +++-+ +++-+
;;;; | | | |
;;;; V V V V
;;;; value value value value
(def v1 :my-value)
(def v2 :another-value)
(def lst (list 1 2 3 4))
(cons v1 lst) ; => (:my-value 1 2 3 4)
(cons v1 (cons v2 lst)) ; => (:my-value :another-value 1 2 3 4)
;;;;
;;;; Seqs are *abstractions* that act like immutable linked lists
;;;;
;;;; Cell Cell Cell Cell
;;;; +-+-+ +-+-+ +-+-+ +-+-+
;;;; |*|*+--> |*|*+--> |*|*+--> |*|*+--> ..
;;;; +++-+ +++-+ +++-+ +++-+
;;;; | | | |
;;;; V V V V
;;;; value value value value
;;;;
;;;; What does a seq provide
;;;;
(def short-list (list :a :b :c))
(first short-list) ; => :a
(rest short-list) ; => (:b :c)
(next short-list) ; => (:b :c)
(cons :d short-list) ; => (:d :a :b :c)
(rest '(:a)) ; => ()
(next '(:a)) ; => nil
(next ()) ; => nil
(rest ()) ; => ()
;; But do not think too hard about rest vs next right now
;; when in doubt, use next
;;;;
;;;; Seq to collections
;;;;
(def s (seq [1 2 3 4 5 6])) ; => #'user/s
;; Direct constructors
(vec s) ; => [1 2 3 4 5 6]
(set s) ; => #{1 2 3 4 5 6}
;; conj into an existing collection
(into [] s) ; => [1 2 3 4 5 6]
(into #{} s) ; => #{1 2 3 4 5 6}
(into {} (seq [[:a 1] [:b 2]])) ; => {:a 1, :b 2}
;; but note:
(into '() s) ; => (6 5 4 3 2 1)
;;;;
;;;; `seq` on empty collections returns nil
;;;;
(seq? ())
(seq nil) ; => nil
(seq ()) ; => nil
(seq []) ; => nil
(seq {}) ; => nil
(seq #{}) ; => nil
;;;;
;;;; Making a seq from a seq
;;;;
;; reminder:
(def items [{:name "car"
:wheels [:wheel1 :wheel2
:wheel3 :wheel4]}
{:name "bike"
:wheels [:bicycle-wheel-1
:bicycle-wheel-2]}
{:name "tricycle"
:wheels [:tricycle-wheel1
:tricycle-wheel2
:tricycle-wheel2]}
{:name "unicycle"
:wheels [:unicycle-wheel]}
{:name "dog"
:legs [:dog-leg1 :dog-leg2
:dog-leg3 :dog-leg4]}])
;;;;
;;;; what does this do?
;;;;
((fn self [coll]
(and (seq coll)
(cons (:name (first coll))
(self (next coll)))))
items) ; => ("car" "bike" "tricycle" "unicycle" "dog")
;;;;
;;;; what does this do?
;;;;
((fn self [coll]
(and (seq coll)
(cons (:name (first coll))
(self (next coll)))))
items)
;;;; seems like a lot of boiler plate
;;;; when we just want to extract the names
;;;;
;;;; solution: take a function as an argument
;;;;
((fn self [f coll]
(and (seq coll)
(cons (f (first coll))
(self f (next coll)))))
(fn [i] (:name i)) ; f argument
items) ; => ("car" "bike" "tricycle" "unicycle" "dog")
;;;;
;;;; solution: take a function as an argument
;;;;
((fn self [f coll]
(and (seq coll)
(cons (f (first coll))
(self f (next coll)))))
(fn [i] (:name i))
items)
;; problem:
;; still uses stack space proportional to (count coll)
;;;;
;;;; solution: use lazy-seq.
;;;;
((fn self [f coll]
(lazy-seq (and (seq coll)
(cons (f (first coll))
(self f (next coll))))))
(fn [i] (:name i))
items)
;;;;
;;;; solution: use lazy-seq.
;;;;
((fn self [f coll]
(lazy-seq (and (seq coll)
(cons (f (first coll))
(self f (next coll))))))
(fn [i] (:name i))
items)
;;;; which is basically, map:
(map (fn [item]
(:name item))
items) ; => ("car" "bike" "tricycle" "unicycle" "dog")
;;;;
;;;; solution: use lazy-seq.
;;;;
((fn self [f coll]
(lazy-seq (and (seq coll)
(cons (f (first coll))
(self f (next coll))))))
(fn [i] (:name i))
items)
;;;; which is basically, map:
(map (fn [item]
(:name item))
items)
;;;; let's shorten that a bit
(map #(:name %)
items)
;;;;
;;;; solution: use lazy-seq.
;;;;
((fn self [f coll]
(lazy-seq (and (seq coll)
(cons (f (first coll))
(self f (next coll))))))
(fn [i] (:name i))
items)
;;;; which is basically, map:
(map (fn [item]
(:name item))
items)
;;;; let's shorten that a bit
(map #(:name %)
items)
;;;; but keywords are already functions!
(:key {:key :value}) ; => :value
(map :name items)
;;;;
;;;; What else can we do with seqs
;;;;
;; get all the things with wheels:
(filter :wheels items) ; => ({:name "car", :wheels [:wheel1 :wheel2 :wheel3 :wheel4]} {:name "bike", :wheels [:bicycle-wheel-1 :bicycle-wheel-2]} {:name "tricycle", :wheels [:tricycle-wheel1 :tricycle-wheel2 :tricycle-wheel2]} {:name "unicycle", :wheels [:unicycle-wheel]})
;; or
(keep :wheels items) ; => ([:wheel1 :wheel2 :wheel3 :wheel4] [:bicycle-wheel-1 :bicycle-wheel-2] [:tricycle-wheel1 :tricycle-wheel2 :tricycle-wheel2] [:unicycle-wheel])
;;;; get all the thing WITHOUT wheels:
(remove :wheels items) ; => ({:name "dog", :legs [:dog-leg1 :dog-leg2 :dog-leg3 :dog-leg4]})
;;;;
;;;; get all the wheels as a single seq:
;;;;
(mapcat :wheels items) ; => (:wheel1 :wheel2 :wheel3 :wheel4 :bicycle-wheel-1 :bicycle-wheel-2 :tricycle-wheel1 :tricycle-wheel2 :tricycle-wheel2 :unicycle-wheel)
;;;;
;;;; get everything with more than 2 wheels:
;;;;
(filter #(< 2 (count (:wheels %))) items) ; => ({:name "car", :wheels [:wheel1 :wheel2 :wheel3 :wheel4]} {:name "tricycle", :wheels [:tricycle-wheel1 :tricycle-wheel2 :tricycle-wheel2]})
;;;;
;;;; sort items by number of wheels (ascending)
;;;;
(sort [6 2 8 10]) ; => (2 6 8 10)
(defn wheel-count
[item]
(count (:wheels item)))
(sort-by wheel-count items) ; => ({:name "dog", :legs [:dog-leg1 :dog-leg2 :dog-leg3 :dog-leg4]} {:name "unicycle", :wheels [:unicycle-wheel]} {:name "bike", :wheels [:bicycle-wheel-1 :bicycle-wheel-2]} {:name "tricycle", :wheels [:tricycle-wheel1 :tricycle-wheel2 :tricycle-wheel2]} {:name "car", :wheels [:wheel1 :wheel2 :wheel3 :wheel4]})
;;;;
;;;; sort descending
;;;;
(defn reverse-compare
[a b]
(compare b a))
(sort-by wheel-count reverse-compare items)
;;;;
;;;; some infinite sequences
;;;;
(take 10 (repeatedly rand)) ; => (0.6121773318782444 0.19423236761108165 0.8850590126241661 0.2166941331273856 0.9595494895081473 0.5251935455837357 0.3684493851601529 0.5340716398033004 0.9248327915320852 0.7105046471052063)
(take 10 (repeat 4)) ; => (4 4 4 4 4 4 4 4 4 4)
(take 10 (iterate inc 0)) ; => (0 1 2 3 4 5 6 7 8 9)
(take 10 (cycle [1 2 3 4])) ; => (1 2 3 4 1 2 3 4 1 2)
;; and ranges
(range 10) ; => (0 1 2 3 4 5 6 7 8 9)
(range 10 0 -1) ; => (10 9 8 7 6 5 4 3 2 1)
;;;;
;;;; Summarize a seq:
;;;;
;;;; Turn a seq into a "single value"
;;;;
;;;;
;;;; does every item have legs?
;;;;
(every? :legs items) ; note: predicate names end with "?"
;;;;
;;;; are there things with legs?
;;;;
(some :legs items) ; => [:dog-leg1 :dog-leg2 :dog-leg3 :dog-leg4]
;;;;
;;;; also..
;;;;
(not-every? :legs items) ; => true
(not-any? :antennea items) ; => true
;;;;
;;;; count the wheels. note: (count nil) = 0
;;;;
(reduce + 3 [1 2 3 4]) ; => 13
(reduce conj () (list 1 2 3 4)) ; => (4 3 2 1)
(reduce #(+ %1 (count (:wheels %2))) 0 items)
;; or
(reduce + (map #(count (:wheels %)) items))
;; or
(reduce + (map count (map :wheels items)))
;; or
(reduce + (map wheel-count items))
;;;;
;;;; Some notes
;;;;
;; all of the shown functions are just functions written in pure
;; clojure
;; you can write your own variants and you don't even need macros to
;; do it
;; pmap does map in parallel
;; your performance may vary
;; this presentation is just a clojure source file
;;;;
;;;; _________
;;;; < The End >
;;;; ---------
;;;; \ ^__^
;;;; \ (oo)\_______
;;;; (__)\ )\/\
;;;; ||----w |
;;;; || ||
;;;;
;;;; Functional clojure: sequences
;;;;
;;;; http://joost.zeekat.nl/
;;;; joost@zeekat.nl