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Lisp and Scheme I 4/10/2012 Versions of LISP Why Study Lisp? • LISP is an acronym for LISt Processing language • It’s a simple, elegant yet powerful language • Lisp (b. 1958) is an old language with many variants • You will learn a lot about PLs from studying it – Fortran is only older language still in wide use • We’ll look at how to implement a Scheme Lisp and – Lisp is alive and well today interpreter in Scheme and Python • Most modern versions are based on Common Lisp • Scheme is one of the major variants • Many features, once unique to Lisp, are now in – We’ll use Scheme, not Lisp, in this class “mainstream” PLs: python, javascript, perl … Scheme I – Scheme is used for CS 101 in some universities • It will expand your notion of what a PL can be • The essentials haven’t changed much • Lisp is considered hip and esoteric among computer scientists LISP Features • S-expression as the universal data type – either at atom (e.g., number, symbol) or a list of atoms or sublists • Functional Programming Style – computation done by applying functions to arguments, functions are first class objects, minimal use of side-effects • Uniform Representation of Data & Code – (A B C D) can be interpreted as data (i.e., a list of four elements) or code (calling function ‘A’ to the three parameters B, C, and D) • Reliance on Recursion – iteration is provided too, but We lost the documentation on quantum mechanics. You'll have to decode recursion is considered more natural and elegant the regexes yourself. • Garbage Collection – frees programmer’s explicit memory management How Programming Language Fanboys See Each Others’ Languages 1 4/10/2012 What’s Functional Programming? Pure Lisp and Common Lisp Scheme • The FP paradigm: computation is applying • Lisp has a small and elegant conceptual core • Scheme is a dialect of Lisp that is favored by functions to data that has not changed much in almost 50 years. people who teach and study programming languages • Imperative or procedural programming: a • McCarthy’s original Lisp paper defined all of program is a set of steps to be done in order Lisp using just seven primitive functions • Why? – ’ • FP eliminates or minimizes side effects and • Common Lisp, developed in the 1980s as an It s simpler and more elegant than Lisp mutable objects that create/modify state ANSI standard, is large (>800 builtin functions), –It’s pioneered many new programming language ideas (e.g., continuations, call/cc) –E.g., consider f1( f2(a), f2(b)) has most modern data-types, good program- –It’s influenced Lisp (e.g., lexical scoping of variables) • FP treats functions as objects that can stored, ming environments, and good compilers – ’ ’ passed as arguments, composed, etc. It s still evolving, so it s a good vehicle for new ideas But I want to learn Lisp! But I want to learn Clojure! DrScheme and MzScheme • We’ll use the PLT Scheme system developed by • Lisp is used in many practical systems, but • Clojure is a new Lisp dialect that compiles to a group of academics (Brown, Northeastern, Scheme is not the Java Virtual Machine Chicago, Utah) • Learning Scheme is a good introduction to Lisp • It offers advantages of both Lisp (dynamic • It’s most used for teaching introductory CS typing, functional programming, closures, etc.) • We can only give you a brief introduction to courses either language, and at the core, Scheme and and Java (multi-threading, fast execution) • MzScheme is the basic scheme engine and can Lisp are the same • We’ll look at Clojure briefly later be called from the command line and assumes • We’ll point out some differences along the way a terminal style interface • DrScheme is a graphical programming environ- ment for Scheme 2 4/10/2012 Mzscheme DrScheme on gl.umbc.edu Informal Scheme/Lisp Syntax Hello World DrRacket • An atom can be an integer, or an identifier, or a string, or… (define (helloWorld) • A list is a left parenthesis, followed by ;; prints and returns the message. zero or more S-expressions, followed by a right parenthesis (printf "Hello World\n")) • An S-expression is an atom or a list • Example: () • (A (B 3) (C) ( ( ) ) ) 3 4/10/2012 Square REPL What is evaluation? • We evaluate an expression producing a value • Lisp and Scheme are interactive and use what –Evaluating “2 + sqrt(100)” produces 12 > (define (square n) is known as the “read, eval, print loop” • Scheme has a set of rules specifying how to –While true ;; returns square of a numeric argument evaluate an s-expression •Read one expression from the open input (* n n)) • We will get to these very soon •Evaluate the expression –There are only a few rules > (square 10) •Print its returned value –Creating an interpreter for scheme means writing 100 • (define (repl) (print (eval (read))) (repl)) a program to • read scheme expressions, • apply the evaluation rules, and • print the result Built-in Scheme Datatypes Lisp: T and NIL Scheme: #t, #f, and ‘() • Scheme cleaned this up a bit Basic Datatypes The Rest • Since 1958, Lisp has used two special symbols: • Booleans • Bytes & Byte Strings NIL and T • Scheme’s boolean datatype includes #t and #f • Keywords • Numbers • NIL is the name of the empty list, ( ) • #t is a special symbol that represents true • Characters • • #f represents false Strings • Vectors • As a boolean, NIL means “false” • Procedures • Hash Tables • T is usually used to mean “true,” but… • In practice, anything that’s not #f is true • Symbols • Boxes • …anything that isn’t NIL is “true” • Booleans evaluate to themselves • Pairs and Lists • Void and Undefined • NIL is both an atom and a list • Scheme represents empty lists as the literal ( ) which is also the value of the symbol null – it’s defined this way, so just accept it –(define null ‘()) 4 4/10/2012 Numbers Strings Predicates • Numbers evaluate to themselves • Strings are fixed length arrays of characters • A predicate (in any computer language) is a function that returns a boolean value • Scheme has a rich collection of number –"foo" types including the following • In Lisp and Scheme predicates returns –"foo bar\n" –Integers (42) either #f or often something else that –Floats (3.14) –"foo \"bar\"” might be useful as a true value –Rationals: (/ 1 3) => 1/3 • Strings are immutable – The member function returns true iff it’s –Complex numbers: (* 2+2i -2-2i) => 0-8i • Strings evaluate to themselves 1st argument is in the list that is it’s 2nd –Infinite precision integers: (expt 99 99) => 369…99 – (member 3 (list 1 2 3 4 5 6)) => (3 4 5 6)) (contains 198 digits!) –And more… Function calls and data Simple evaluation rules Example • Numbers evaluate to themselves (define a 100) • define is a special form • A function call is written as a list > a that doesn’t follow the – the first element is the name of the function • #t and #f evaluate to themselves regular evaluation rules 100 – remaining elements are the arguments • Any other atoms (e.g., foo) represents • Scheme only has a few of > add1 these • Example: (F A B) variables and evaluate to their values #<procedure:add1> • Define doesn’t evaluate its – F A B • A list of n elements represents a function call calls function with arguments and first argument –e.g., (add1 a) > (add1 (add1 a)) • Data is written as atoms or lists • if is another special form –Evaluate each of the n elements (e.g., add1->a 102 • (F A B) Example: is a list of three elements procedure, a->100) > (if (> a 0) (+ a 1)(- a 1)) • What do you think is – special about if? Do you see a problem here? –Apply function to arguments and return value 103 5 4/10/2012 Quoting Symbols Basic Functions • Is (F A B) a call to F, or is it just data? • Symbols are atomic names • car returns the head of a list • All literal data must be quoted (atoms, too) > ’foo (car ‘(1 2 3)) => 1 • (QUOTE (F A B)) is the list (F A B) foo (first ‘(1 2 3)) => 1 ;; for people who don’t like car – QUOTE is not a function, but a special form > (symbol? ‘foo) • cdr returns the tail of a list – Arguments to a special form aren’t evaluated #t (cdr ‘(1 2 3)) => (2 3) or are evaluated in some special manner • Symbols are used as names of variables and (rest ‘(1 2 3)) => (2 3) ;; for people who don’t like cdr • '(F A B) is another way to quote data procedures • cons constructs a new listbeginning with it’s first arg ’ – There is just one single quote at the beginning –(define foo 100) and continuing with it s second (cons 1 ‘(2 3)) => (1 2 3) – It quotes one S-expression –(define (fact x) (if (= x 1) 1 (* x (fact (- x 1))))) CAR, CDR and CONS More Basic Functions Comment on Names • These names date back to 1958 • eq? compares two atoms for equality • Lisp used the convention (inconsistently) of ending predicate functions with a P –Before lower case characters were invented (eq? ‘foo ‘foo) => #t –E.g., MEMBERP, EVENP • CONS = CONStruct (eq? ‘foo ‘bar) => #f • Scheme uses the more sensible convention to • CAR and CDR were each implemented by a Note: eq? is just a pointer test, like Java’s ‘=‘ use ? at the end such functions single hardware instruction on the IBM 704 • equal? tests two list structures –e.g., eq?, even? –CAR: Contents of Address Register (equal? ‘(a b c) ‘(a b c)) =#t • –CDR: Contents of Decrement Register Even Scheme is not completely consistent in (equal? ‘(a b) ‘((a b))) => #f using this convention Note: equal? compares two complex objects, –E.g., the test for list membership is member and not like a Java object’s equal method member? 6 4/10/2012 Other useful Functions More useful Functions More useful Functions • list makes a list of its arguments • append concatenates two lists • (null? S) tests if S is the empty list – (list 'A '(B C) 'D) => (A (B C) D) – (append ‘(1 2) ‘(3 4)) => (1 2 3 4) – (null? ‘(1 2 3) => #f – (list (cdr '(A B)) 'C) => ((B) C) – (append '(A B) '((X) Y)) => (A B (X) Y) – (null? ‘()) => #t • Note that the parenthesized prefix notation makes it – (append ‘( ) ‘(1 2 3)) => (1 2 3) • (list? S) tests if S is a list easy to define functions that take a varying number of arguments.
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