CPL 2016, week 12 Clojure large scale design Oleg Batrashev - - PowerPoint PPT Presentation

CPL 2016, week 12

Clojure large scale design Oleg Batrashev

Institute of Computer Science, Tartu, Estonia

April 25, 2016

Overview

Last weeks

◮ Clojure language core ◮ Immutable data structures

This week

◮ Clojure simple state and design

Next weeks

◮ Software transactional memory ◮ Agents in Clojure

Mutable state 56/68

• State, identity and time

From “The Joy of Clojure” book

◮ Time —The relative moments when events occur ◮ State —A snapshot of an entity’s properties at a moment in

time

◮ Identity —The logical entity identified by a common stream of

states occurring over time

Mutable state 57/68

• Clojure’s state and identity

◮ state is often seen as a snapshot of object properties at a given

moment

◮ immutable by definition

◮ identity can have different states in time ◮ sarah = new Person(Sarah, 25)

◮ sarah variable is the identity and object value is her state? ◮ but this state is mutable

◮ Clojure tries to distinguish an identity and its immutable states

◮ preserves historical states of identity ◮ identity is one of Clojure reference types

Mutable state 58/68

• Clojure’s reference types

◮ Refs manage coordinated, synchronous changes to shared

state.

◮ Atoms manage uncoordinated, synchronous changes to shared

state.

◮ Agents manage asynchronous changes to shared state. ◮ Vars manage thread-local state.

Mutable state 59/68

• Vars

◮ def/defn create global variable ◮ by default changable by everyone, no garantees ◮ allows to create thread-local state (def ^:dynamic foo 41)(binding [foo 42] (println foo))(println foo) ◮ get the “var” object itself (var foo) ; -> #'user/foo

◮ user is namespace name

◮ used to store global(namespace)/thread local configuration

and functions

◮ for application data prefer atoms, refs, and agents!

Mutable state 60/68

• Atoms

◮ (atom initialState) ◮ deref/@ – read value of an atom ◮ reset! – set the value of an atom ◮ swap! – atomically modify the value by using function, may be

re-executed in case of conflict

(def a (atom 34)) (swap! a (fn [state] (inc state ))) ; -> 35 (println @a) ; -> 35

Program structure 61/68

• Namespaces

◮ ns – specify namespace

◮ allows to inline the following commands

◮ require – loads namespace into memory, defining its types, etc ◮ refer – imports names from namespace into local scope ◮ use = require + refer (ns suffixtree ) (require '[clojure.pprint :refer [pprint ]]) ◮ import – for Java classes (import '(java.io FileInputStream InputStreamReader ))

Program structure 62/68

• Types and protocols

◮ a way to do “object oriented” programming in Clojure ◮ type/record – collection of data (defrecord Person [name x y])

◮ records are immutable but allow dictionary-like manipulations ◮ types allow mutable fields that are private ◮ both can implement some protocols

◮ protocol – functional interface to types/records ( defprotocol Movable (move [this direc] "Move someone .") (jump [this direc distance ]))

◮ first parameter is special – used for method dispatch ◮ use multimethods for multiparameter dispatch

Program structure 63/68

• Extend types

One type with several protocols

(extend -type AType AProtocol (method -from -AProtocol [this x] (;... implementation for AType )) AnotherProtocol (method1 -from - AnotherProtocol [this x] (;... implementation for AType )) (method2 -from - AnotherProtocol [this x y] (;... implementation for AType ))) ◮ possible to do inline in deftype

Program structure 64/68

• Extend protocols

One protocol for multiple types

(extend -protocol AProtocol AType (method1 -from -AProtocol [this x] (;... implementation for AType )) AnotherType (method1 -from -AProtocol [this x] (;... implementation for AnotherType )) (method2 -from -AProtocol [this x y] (;... implementation for AnotherType ))) ◮ possible to do inline in defprotocol ◮ extend command for reuse of methods

Program structure 65/68

• Multidispatch: defmethod

(defrecord Node [value left right ]) (defmethod print -method Node [node wr] (print -simple (str "<" (. value node) " " "l=" (print -str (. left node )) " " "r=" (print -str (. right node )) ">") wr))

Program structure 66/68

• Java interop

◮ proxy – anonymous class with immediate instance ◮ gen-class – full-featured class extension

Program structure 67/68

• Summary