An Introduction to Clojure

ClojureI had the pleasure this week to sit in on a 2 day Introduction to Clojure workshop being run by one of Clojure’s core developers, Stuart Sierra. YOW! and Relevance, Inc. are running these workshops on the east coast of Australia throughout May 2013 and for someone still very new to functional concepts it was a great introduction to functional programming as well as the Clojure language itself.

The key to a workshop like this is the slides with the code examples and the labs to practice the learnings, Nonetheless, here are my notes from the course on some of the concepts that I picked up.

Introduction to Clojure

  • designed to fix shared state concurrency, designed to run on the host platform
  • ClojureScript designed to target devices that cannot run a full JVM like mobile platforms, browsers, etc..
  • most of this course is applicable to ClojureScript, just the interrops to the host environment are different
  • BigInt (trailing N), BigDecimal (trailing M)
  • Strings in double quotes, characters in “”, reg ex are started with a #
  • literal true and false, nil is the same as Java null
  • symbols are words in your program you want to reference from somewhere else, always resolve to something else
  • keywords are a special type of symbol, start with a colon :, fixed labels that have no meaning, used for names of fields often, close to enum in Java
  • REPL – read, eval, print, loop – from LISP, all the steps are separate which is different to interactive terminal
  • every expression starts with an expression, everything returns an expression which is why you get nil when doing a println
  • REPL *1 will get you most recently used expression
  • print and println will give you human readable, prn and pr will give you the machine readable output
  • list are written in parens, usually represent function calls
  • vectors usually reflect collections, in []
  • maps are key value collections, in {}
  • set starts with a hash and curly brackets
  • commas are whitespace in Clojure
  • first is the function position, usually starts wuth a function or an expression that returns a function
  • quote (‘) is syntactic sugar for quote, prevents evaluation, common issue when starting Clojure when dealing with lists
  • doc – built in documentation
  • Leiningen – built on Maven, hides complexity but gives you its goodness
  • project.clj – dependency file

Functions

  • functions are first class and values, most of the core code are pure functions (no side effects)
  • defn defines a named function (defn greet [name] <body>) is a definition called greet, passes a name and runs some functions, then call as (greet )
  • can have multiple aritys, [] is no arguments, others are the named arguments, useful way to provide defaults
  • & in argument vector says next arguments gets all the remaining arguments in a list (0 or more)
  • fn is anonymous function with no name (fn [parms] body), to call it you can stick it at the head of a list, use this if you need to do something one time
  • shorter synatx is #(), and use % sign for the arguments, syntactic convenience
  • usually invoke a function by putting it at the head of the list
  • apply takes a function and list or collection or arguments, same as invoking a function on those arguments
  • let used for local variables, uilt into the compiler, takes a pair set of bindings (symbol and an evaluated expression) and then a body of any number of expressions
  • fn creates a closure, locals are stored and kept with the function
  • invoking Java – instantiation (Class. argument)
  • read Clojure inside out, read the inner most function and then read out

Expressions and Loops

  • in Clojure everything is an expression, always return a value even if that is nil, multiple expressions always return the last value
  • flow controls always return a value as well
  • logical false is false or nil, everything else is true, similar to the semantics in Ruby and Python
  • when is a macro – check and do a buch of things if evaluates true
  • cond – tree of if then else without lots of nesting, test – expression pair, stops when it hits the first true test, can end with an :else default by convention only
  • case – occasionally useful, like switch in Java, takes constants and returns value, if you don’t define a default you get an error, odd number of expressions, the last one is the default
  • dotimes – basic loop (dotimes [i 3]), only useful for side effects as it never returns a value
  • doseq – iterates over a collection, like for-each
  • for – not for looping, generates data, returns a sequence
  • recur – for recursion, rarely used, loop defines bindings which you recur
  • exceptions – don’t use them much in Clojure, no checked exceptions, try-catch-finally as per Java, throw to throw an exception
  • ex-info and ex-data that allows you to pass back a map
  • with-open – convenience wrapper, works with any method that needs to be closed

Namespaces

  • Clojure runtime is a singleton within the JVM
  • namespaces allow you thave the same name mean different things in different places
  • ns – use macro at the top of the source file to create a namespace, :require keyword to load the dependent libraries
  • implicit mapping between namespaces and files, dots into slashes and hypens into underscores
  • ns :use : only is deprecated form of :require from 1.4 up
  • ns :import for importing Java classes, every namespace imports java.lang
  • in-ns to change namespaces
  • equivalent macros for when using the REPL instead of a file
  • a bunch of functions for returning namespace information, but would rarely use these
  • private vars are just a metdata symbol, use ^:private, not truly private or hidden

Collections

  • a class or a struct becomes a map in Clojure
  • clojure.set – mathematical set operations
  • vectors are functions, can call using the indices
  • mostly use vectors, lists are useful when you want to work at the head of the list as it is more efficient, like simulating a stack
  • seq – returns a sequential view of something, like an iterator but it is not an iterator, can get first or rest
  • range – infinite lazy sequence of integers
  • into – puts a sequence into a collection
  • take / drop – take the first n in the sequence, drop the first n in the sequence, also filter and remove
  • map – the essence of functional programming – calls a function on a sequence and returns a sequence, the functional equivalent of for/each
  • reduce – powerful, uses first element if not initiated
  • some – to return the first logical true in the sequence

Concurrency

  • concurrency – multiple things happening concurrently that are sharing the same state
  • deref – returns current state, shortcut is @<ref>
  • atom – basic container with mutable state, changes are atomic, synchronous
  • ref – coordinate and share identities, synchronous, must be iolated in a transaction, uses locking under the hood
  • ensure – ensure the state is the same when the transaction is completed
  • alter – change the value of a reference in a transaction, if another transaction tries to change it gets aborted
  • commute – same behaviour as alter but allows concurent updates in a transaction, use for counters, adding elements to a map, if you are not sure use alter
  • agent – asynchronous communication, atomic updates, ensure only one thing done at a time, every agent has its own queue
  • send – send an action (function) to an agent, execute in the order that you send them, can’t guarantee that other threads are not sending action, fixed size thread pool
  • send-off – variable size thread pool
  • vars – thread safe, global identity, alter-var-root to modify global state created by a def, by convention dynamic var is surrounded by * (called earmuffs)
  • swap! and reset!, convention to put a bang at the end of functions that have side effects or shouldn’t be put inside a transation
  • watches – experimental, takes 4 arguments – key, reference, old state and new state, you can then add-watch
  • future – background process, if you de-reference the future it will block (@<ref>)
  • promise – like a future but no running process, like a container, can wait for it to deliver that promise to another thread, can only be delivered once
  • realized? – to ask if a promise has been delivered

Polymorphism

  • polymorphism – two types protocols and multi-methods
  • type – what is the type of class using class or type
  • maps do not have a notion of type, record allow you to give a type to a map, defrecord creates a record, by convention the name has a capital, records are maps so we can call all the map functions
  • record automatically creates the constructor function called -><function>, map-><function> takes a map
  •  when you create a record you are generating a Java class, useful when creating a lot of maps with the same structure or to implement protocols
  • protocol – like a Java interface, group of functions that do not provide an implementation, called based on the first argument, defprotocol to create the functions
  • convention _ is an argument I don’t care about
  • reify – creates anonymous types
  • extend-type and extend-protocol – allows you to create new interfaces to existing types, solves the monkey patching issue
  • multimethods – generic methods, much more flexible than protocols, uses defmulti and defmethod, an extensible switch

Macros

  • macro – pure functions – defmacro – code is input and output, define the symbols to reflect the code that we want, macroexpand-1 is used for testing and debugging to see what that macro does
  • syntax quote `
  • unquote ~ and unquote splicing ~@
  • most of the core language where not functions are macros, write macros for syntactic convenience

Clojure and the JVM

  • Clojure is tightly integrated to the JVM, is unlikely to be the bottleneck performance wise
  • arrays can use aset and aget to access, use Clojure Collections but maybe needed for interoperability, can use to-array and into-array
  • types and collections are all mapped over to Clojure
  • built in benchmarking in Clojure using time function, time a loop to get a better indication rather than just time on its own
  • by default everything is boxed in Clojure, but Clojure has limited support for long and double primitives
  • compile – pass your namespace, will give you ahead of time compile which gives a modest (30% – 40%) speed increase amongst other benefits, usually will us leinengen or another tool (lein compile)

ClojureScript

  • ClojureScript is designed to deliver optimised JavaScript
  • two pieces – ClojureScrpt compiler written in Clojure on the JVM, compiles Clojure to JavaScript, then the Google Closure compiler optimises for space for performance
  • lein-cljsbuild makes ClojureScript tolerable
  • ClojureScript One but is superceded by Pedestal now
  • testing frameworks – Clojure.test, Midje testing framework written by Brian Marick and Simulant
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