Quote of the Day. CS Functional Programming. Introductory Notes on Lisp/Clojure. Another Quote. What is Clojure? 1-4

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1 Quote of the Day CS Functional Programming Introductory Notes on Lisp/Clojure Dr. Stephen P. Carl By relieving the brain of all unnecessary work, a good notation sets it free to concentrate on more advanced problems, and, in effect, increases the mental power of the race. - Alfred North Whitehead Another Quote What is Clojure? In 1960, John McCarthy...showed how, given a handful of simple operators and a notation for functions, you can build a whole programming language. He called this language Lisp, for "List Processing". - Paul Graham Clojure is a programming language derived from Lisp, extended for concurrent/distributed computation and simplified web programming. Our job today is to learn the notation of Clojure by comparing it with Arithmetic and Algebra 1-4

2 Arithmetic is Computing Fixed, pre-defined rules for primitive operators: = = 8 cos(0) = 1 Note: the = sign is ambiguous, as we ll see. Let s replace it with the symbol read "evaluates to" Arithmetic is Computing Fixed, pre-defined rules for primitive operators: cos(0) 1 Rules for combining other rules: Evaluate sub-expressions first 4 (2 + 3) Precedence determines subexpressions: Algebra as Computation Algebra as Computation Definition: f(x) = cos(x) + 2 Expression (Function Call): f(0) A function call is an expression, which can then be evaluated. Evaluation: f(0) cos(0) The first step of evaluation is the substitution rule for functions 5-12

3 Notation Our algebraic notation is somewhat ambiguious, so we must answer the following questions: What are the precedence rules? Which parentheses are redundant? How do we know which arguments go with which operators? Why do some primitive operators (like ) go in the middle while others (like cos) go at the front? When does = mean definition and when does it mean a computation step? Simplified Expression Notation All operators come first (prefix notation) Start every operation with an open parenthesis Put a close parenthesis after the last argument Never add extra parentheses Old New (+ 1 2) (+ 4 (* 2 3)) cos(0) + 1 (+ (cos 0) 1) Clojure Syntax Clojure Syntax (operator parameter1 parameter2...) This is called an expression. For example: > ( ) 12.7 > ( ) 75 With very few exceptions, expressions always return a value. Expressions can be nested: > (+ (Math/cos 0) 2) 3 > (* ( ) ( )) > (Math/pow (* 2 (Math/sin 10)) (- x 32)) Compare to calling Java methods as arguments to other methods: Math.pow(2 * Math.sin(10), x - 32); 13-16

4 Clojure Syntax There is in principle no limit to the depth of such nesting and to the overall complexity of the expressions. For example: Simplified Definition Notation Use the keyword defn instead of = Put defn at the front, and group with parentheses Move open parenthesis from after function name to before (+ (* 3 (+ 2 4) (+ 3 5)) (+ (- 10 7) 6)) 57 Old New We use indentation to clarify gnarly expressions: (+ (* 3 (+ 2 4) (+ 3 5)) (+ (- 10 7) 6)) A good editor will enforce this f(x) = cos(x) + 2 (defn f [x] (+ (Math/cos x) 2)) Move open parenthesis in function calls Old New f(0) (f 0) f(2+3) (f (+ 2 3)) Evaluation is the Same as Before Only Values Have Types (defn f [x] (+ (Math/cos x) 2)) But Clojure and Erlang are dynamically typed. This means that values, not variables, have types. Variables take on the type of the value they hold. (f 0) (+ (Math/cos 0) 2) (+ 1 2) 3 In Clojure there are no variable declarations. To use a symbol as a variable, just define it with def: (def george 17) In Clojure (unlike Erlang), variables can be redefined on the fly. However we ll see that a related notion, immutability, is important for concurrent programming

5 Only Values Have Types Built-in Data Types Once a variable is defined, it can refer to any Clojure object. In both languages, variables have bindings rather than assignments. (def george 17) george 17 (def george "not a number!") george "not a number!" Note that rebinding a variable to a new value is not mutation in the sense of a Java assignment. The original value 17 still exists in some memory location. Clojure provides the following basic data types. Conceptually, all Clojure variables are pointers (references) to data in the same way Java object references are, but optimizations are possible. Numbers Java nums + arbitrary-length numbers + rationals Booleans true and false Characters \a, \A, \u00ff, \newline, \space Strings "treated just like in java.lang.string" Built-in Data Types Relational Operators These new types are also built-into Clojure: nil A single object, equivalent to null in Java Symbols Identifiers, evaluate to themselves Keywords Like symbols, used as accessors in collections Regex regular expressions Regular expressions are used in general purpose pattern matching in code, documents, etc. For example, the regex (p h)ail matches the words pail or hail and the regex (\d+)-(\d+) matches any numeric range, such as or The usual relational operators produce boolean values: (< 1 2) true (= 2 3) false (>= 2 2) true == works for numbers, = works for all objects 22-25

6 Collections Acknowledgements Clojure provides its own built-in collection types, plus access to the Java Collections Framework. Pairs/Lists '(a. b) or '(a b c d e) Vectors ['a 'b 12.5] Maps {:name "Stan" :age 25} Sets #{1 2 3} JFC (java.util.arraylist.100) These notes include work adapted from the following sources: Paul Graham - Hackers and Painters Rich Hicky - Matthew Flatt - notes for CS 2010, University of Utah Mark Johnstone - notes for CS 381K, University of Texas 26-27