Why a New Language? Seeking Grace: a New Object-Oriented Language for Novices. OO Teaching Frustrations. Java Problems. Java is 20 years old in 2015

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1 Seeking Grace: a New Object-Oriented Language for Novices Andrew Black, Kim Bruce, Michael Homer, James Noble, Amy Ruskin, & Richard Yannow Why a New Language? Java is 20 years old in 2015 Showing its age Additions becoming more visible Never designed as teaching language Some trying Python Almost 25 years old Not good support for OO principles OO Teaching Frustrations Want to teach objects first but must teach classes first. Why not just define objects directly? Too many mysterious keywords and generally noisy syntax Java Problems public static void main(string [] args) Primitives versus objects, == versus equals Flawed implementation of generics Static versus instance on variables & methods float versus double versus int versus long 3 4

2 Goal for Grace Hello World in Grace: Integrate current ideas in programming languages into a simple, general-purpose language aimed at novices. print "hello world" 5 6 Objects def mysquare = object { var side := 10 method area { side * side method stretchby(n) { side := side + n Types Types are optional and can be added gradually Types are structural Don t have to be declared if it quacks like a duck, it is a duck Classes are not types Defaults:!instance variables and constants are private, methods are public 7 8

3 Typed Objects one numeric type type Square = { area -> Number stretchby(n:number) -> Done like Void def mysquare:square = object { var side:number := 10 method area -> Number { side * side method stretchby(n:number) -> Done { side := side + n 9 Classes A class is an object with a factory method: class asquare.withside(s:number) -> Square { var side:number := s method area -> Number { side * side method stretchby(n:number) -> Done { interpolated side := side + n values print "Created square with side {s" def mysquare = asquare.withside(10) Type annotations can be omitted or included 10 Demos: UpDown, Laundry, BoxBall Blocks Syntax for anonymous functions def squareit = {n -> n * n squareit.apply(7) // returns 49 def nums = alist.from(1)to(100) def squares = nums.map {n -> n * n function 12

4 Blocks Blocks make it simple define new control structures as methods block, evaluated repeatedly while {boolexp do { somestuff boolean expression, evaluated squares.foreach {n -> once if (n.iseven) then {print n Pattern Matching Provides type-safe switch/case match(myval) case{ n: Number -> "The number {n seen" case{ s: String -> "The string "++s++" seen" case{ true: Boolean -> "This is true!" Avoid Hoare s Billion Dollar Mistake No built-in null Accessing uninitialized variable is error Replace null by: sentinel objects, or error actions Sentinel Objects A real object, tailored for the situation, (like we did with recursion) e.g.: def emptylist = object { method asstring {"<emptylist>" method do(action) { method map(function) {self method size {0 15

5 Sentinel Objects Simplifies code, eliminates testing for null class alist.cons(value, taillist) { method asstring {"({head:{tail)" method head {value method tail {taillist method do(action) { action.apply(head) tail.do(action) method map(function) { alist.cons(function.apply(head), tail.map(function)) method size {1 + tail.size boolean expression, no conditional evaluated code once Variant Types def absent = object {... type OptionNumber = Number singleton(absent) var x: OptionNumber := table.lookup(key) match(x) case {x':number ->... x'... case {(absent) -> return unknown(key) Static guarantee that x wi" always be matched val: A B iff val:a or val:b Allows elimination of null 18 Error Actions Grace encourages the use of blocks to specify error actions or default values: var x := table.at(key)ifabsent{ return unknown(key) Current Status Language specification at version 0.4 minigrace prototype implementation with 2 backends: On web via Javascript Command line via C DrRacket implementation under way

6 Conclusion Grace is a small yet powerful language with simple conceptual foundations Starting with objects simplifies teaching Classes and patterns can be introduced later Separating classes from types is conceptually important Grace Experimental section of CS 51 (hopefully) coming in fall of Information at gracelang.org Gradual typing provides flexibility for instructors add types once students have seen the need Final Exam Office Hours Comprehensive But most focus on material since midterm Do problems from text!! This week: ThF 2:45-3:45 p.m. Next week: MWThF 2-3 p.m. T 3-4 p.m. or by appt.