Grace : the Absence of (Inessential) Difficulty

Presentation on Grace by Andrew Black and Kim Bruce at Onward! 2012

Transcript

1. Grace : the Absence of (Inessential) Difficulty Andrew P. Black, Kim. B. Bruce, James Noble and Michael Homer
2. Onward! 2012 Why Grace ? • Claim: no existing language is a good choice for a teaching object-oriented programming ° Every one requires that students spend more time mastering the language than learning the fundamentals of object-orientation. • For novices, the “inessential” or “accidental” difficulty of the language often dominates the “essential” difficulty of programming 2
3. Grace : the Absence of (Inessential) Difficulty Andrew P. Black, Kim. B. Bruce, James Noble and Michael Homer The complexity of software is an essential property, not an accidental one Fred Brooks, 1986 Grace is the absence of everything that indicates pain or difficulty, hesitation or incongruity. Wi!iam Hazlitt, 1817
4. Onward! 2012 Goals Integrate proven newer ideas in programming languages into a simple, general-purpose teaching language whose features represent the key concepts of object oriented-programming directly, simply and gracefu!y 4
5. Onward! 2012 Goals • “Simple” ° ~ 50 page spec ° small number of concepts to learn ° each concept has power to “explain” a lot about the language • But not a toy ° Extensible through libraries • Looking towards distributed and parallel implementation ° Support immutable objects 5
6. Onward! 2012 Grace in one slide method average -> Number // reads numbers from this stream and averages { var total := 0 var count := 0 until {atEnd} do { count := count + 1 total := total + readNumber } if (count = 0) then {return 0} total / count } 6 Might appear in a stream object implicit self. implicit self. until()do() is a method Е- Е- nested scope
7. Backward! 2012 What’s new? Not much! • This is by design! ° accessible to instructors, most of whom are not programming language buffs • The Onward! paper focusses on 1. Constructing objects 2. Types 3. “Language Levels” 4. Implementation 7
8. Constructing objects
9. Onward! 2012 Constructing Objects 9 Object Construction Instantiate Class Clone Prototype Class is not an Object Class is an Object
10. Onward! 2012 10 Instantiate Class Class is not an Object Class is an Object meta-level construct instance initialization is messy small number of concepts mind-boggling consequences many extra concepts
11. Onward! 2012 11 Clone Prototype Where do the Prototypes come from? • Prototypes of basic objects are present ab initio • What about user defined objects? ° start with a basic object and mutate it by adding fields and methods 㱺 all objects are mutable not “mainstream”
12. Onward! 2012 Constructing Objects 12 Object Construction Instantiate Class Clone Prototype Class is not an Object Class is an Object
13. Onward! 2012 A Third Way 13 Object Construction Instantiate Class Clone Prototype Class is not an Object Class is an Object Execute object constructor
14. Onward! 2012 Constructing Grace Objects object { ! def name is readable, public = "Wackiki Wabbit" ! def address is readable, public = ! ! ! ! ! ! ! ! ! ! ! "ww@WarnerBros.com" ! method email is public { ! ! EmailApp.openComposerWindowTo(name) ! } } 14 New language feature: the object constructor Defines fields Defines fields Defines methods protocol is name, address, email
15. Onward! 2012 Constructing many Objects • Repeatedly execute an object constructor: set.map { each -> ! object { ! ! def name is readable, public = each.first ! ! def address is readable, public = each.second ! ! method email is public { ! ! ! EmailApp.openComposerWindowTo(name) ! ! } ! } } 15 internal iterator method λ-expression name initialized using each, from surrounding lexical scope each is parameter
16. Onward! 2012 • an object containing a method that constructs an object: def anAddressCard = object { ! method for (aName) at (anAddress) { ! ! object { ! ! ! def name is readable, public = aName ! ! ! def address is readable, public = anAddress ! ! ! method email is public { ! ! ! ! EmailApp.openComposerWindowTo(name) ! ! ! } ! ! } ! } } 16 Factory Object name is initialized to parameter of factory method factory method is called for()at() factory object, has a single method
17. Onward! 2012 • Class syntax: ° defines a class object, its factory method, and the fields and methods of the objects that it creates class anAddressCard.for (aName) at (anAddress) { !! ! ! def name is readable, public = aName ! ! def address is readable, public = anAddress ! ! method email is public { ! ! ! EmailApp.openComposerWindowTo(name) ! ! } } 17 Factory Object = Class
18. Onward! 2012 Inheritance • Any object can inherit from a superobject ° given in the object constructor ° must be newly-created, and manifest ° fields and methods of the superobject are inherited by its heir • Note that the fields of the superobject are already initialized 18
19. Onward! 2012 Example: PhoneCard def wwPhoneCard = ! object { ! ! inherits anAddressCard.for "Wackiki Wabbit" ! ! ! address "ww@WarnerBros.com" ! ! def phoneNumber is readable, public = ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! "866-373-4389" ! ! method call { ! ! ! PhoneDialer.dial(phoneNumber) ! ! } ! } • wwPhoneCard inherits the three methods of its superobject as well as the two new methods 19