'&$%CSE 341:Programming LanguagesDan GrossmanSpring 2008Lecture 20— Introduction to RubyDan Grossman CSE341 Spring 2008, Lecture 20 1'&$%TodayWhy Ruby?Some basics of Ruby programs• Syntax• Classes, Methods• Variables, fields, scope• Dynamic typing• The rep-loop, the main class, etc.Note: Read Thomas book chapters 1–9 (or free first edition 1–8)• Skip/skim regexps and ranges• Not every detail: focus on OO, dynamic typing, blocks, mixinsDan Grossman CSE341 Spring 2008, Lecture 20 2'&$%Ruby• Pure object-oriented: all values are objects• Class-based• Dynamically typed• Convenient reflectionA good starting point for discussing what each of these means andwhat other languages look like.dynamically typed statically typedfunctional Scheme MLobject-oriented Ruby JavaDan Grossman CSE341 Spring 2008, Lecture 20 3'&$%Ruby vs. SmalltalkSmalltalk, unchanged since 1980, is also pure OO, class-based,dynamically-typed.• Smalltalk: tiny language (smaller than Scheme), elegant, regular,can learn whole thing• Smalltalk: integrated into cool, malleable GUI environment• Ruby: large language with a “why not?” attitude• Ruby: scripting language (light syntax, some “odd” scope rules)• Ruby: very popular, massive library support especially for strings,regular expressions, “Ruby on Rails”– Won’t be our focus at all• Ruby: mixins (a cool, advanced OO modularity feature)• Ruby: blocks, libraries encourage lots of FP idiomsDan Grossman CSE341 Spring 2008, Lecture 20 4'&$%Really key ideas• Really, everything is an object (with constructor, fields, methods)• Every object has a class, which determines how the objectresponds to messages.• Dynamic typing (everything is an object)• Dynamic dispatch (focus of next lecture)• Sends to self (a special identifier; Java’s this)• Everything is “dynamic” – evaluation can add/remove classes,add/remove methods, add/remove fields, etc.• Blocks are almost first-class anonymous functions (later)– Can convert to/from real lambdas (class Proc)(Also has some more Java/C like features – loops, return, etc.)Dan Grossman CSE341 Spring 2008, Lecture 20 5'&$%Lack of variable declarationsIf you assign to a variable in scope, it’s mutation.If the variable is not in scope, it gets created (!)• Scope is the method you are inSame with fields: an object has a field if you assign to it• So different objects of the same class can have different fields (!)This “cuts down on typing” but catches fewer bugs (misspellings)• A hallmark of “scripting languages” (an informal term)Dan Grossman CSE341 Spring 2008, Lecture 20 6'&$%Protection?• Fields are inaccessible outside of instance– Define accessor/mutator methods as desired∗ Use attr_read and attr_writer– Good OO design: subclasses can override accessors/mutators• Methods are public, protected, or private– protected: only callable from class or subclass object– private: only callable from self• Later: namespace management, but no hidingDan Grossman CSE341 Spring 2008, Lecture 20 7'&$%Unusual syntaxJust a few random things (keep your own mental list):• Variables and fields are written differently (@ for fields)– @@ for class fields (Java’s static fields)• Newlines often matter — need extra semicolons, colons, etc. toput things on one line• Message sends do not need parentheses (especially with 0arguments)• Operators like + are just message sends• Class names must be capitalized• self is Java’s this• ...Dan Grossman CSE341 Spring 2008, Lecture 20
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