FLEXIBO Language Design

1
FLEXIBO Language Design

• Work-in-Progress
• Yifeng Chen
• University of Leicester

7 May 2004 UNU/IIST, Macau
2
Language Design

• Programming languages form an interface between
  computer science and computer engineering.
• Good software engineering practice eventually
  goes into language design.

3
Design Philosophy of FLEXIBO

• Open source with different levels of trust in
  decentralized multi-user environment.
• Developing Testing Running
• Limiting the effective scope of bugs.
• Specifications with optional controls.
• True flexibility flexibility inflexibility.
• Compilation as execution.

4
Main Features

• Everything is a value.
• Classes, types, methods and specifications of
  different shapes.
• Variables of different colors.
• Correctness, ownership, resources control and
  smooth exception handling.
• Run-time type checking for compilation.
• Full Java access

5
Example Program Hello World!

• print "Hello world!"

6
Imperative Programming

• var x 12, "A string in an array"
• x0 x0 1
• if x0gt12 print "yes\n" else print "no\n"
• var x"inside a block\n" print x
• var x"inside another block\n" print x
• while xgt0 print x x x - 1
• while true

7
Class, Method and Invocation

• var c class Value
• var dynamic x "static value in class\n"
• var static n method x var x"value of
  local variable\n" return x
• var static m method x return x
• var o new c print o.n "value from
  argument\n"
• print o.m ! "value from argument\n"
• print o. (1 m ! 2)

8
Specification and implementation

• var spec specification x,y pre true post
  true
• spec.implementbody (x "I implement by adding
  a prefix to "x return x)
• print spec"the argument.\n",2
• spec specification x,y pre true post true
  
• print spec"the argument.\n",2

9
Ownership Control

• print project // show the shared project
• project.spec "I have modified changed this
  specification."
• print project // show its change
• project.spec "I have reset this
  specification."
• project "I want to delete the whole project."
  
• print project // This point is not reachable.

10
Exception Handling

• try print 1true
• catch "Cannot add" print "I have caught it.\n"
  
• var timer new tclass
• try timer.start2000 while true
• catch "" print "I have caught my own timeout
  error.\n"
• timernew tclass
• try timer.start5000 while true
• catch "" print "I cannot catch the server's
  timeout.\n"

11
Overloading Method Invocation

• var c class Value var m method a
  return a
• var n class Value var invoke method a
  return "Overloading method invocation is "a
• print (new c).m "invoked in a usual way\n" //
  x-gtm-gta
• var xnew c print ((x.m).invoke "Invoked as a
  pre-defined method\n")
• print (new c).n "invoked in a usual way\n" //
  x-gtm-gta
• print ((x.n).invoke "invoked as a normal
  method\n")

12
Side Effect and Evaluation Order

• var a "Evaluated as an argument"
• var c class Value
• var dynamic y
• var m method z print z
• var env class Value var x new c
• env . ( (print "Evaluate x in env\n" x). (print
  "Evaluate m in x\n" m) (print "Evaluate a in
  the defined environment.\n" a) )

13
Language Reflection

• print quote (1 "A quoted exp is not
  evaluated.")
• var c class Value
• var v"\nA value in a class\n"
• var b body (print v)
• print (c.b).eval

14
Methods in Different Shapes

• var c class Method var dynamic msg "together
  with a document of the method.\n"
• var init method x print msg return x
• var m new cquote (y), body (print y)
• print m"Invoking a user-defined kind of
  method.\n"

15
Colored Variables (attvar)

• var P class Value var type "Type of
  Persons\n"
• var returnType method return atttype
  
• var S class P var type "Type of Staff\n"
  
• print (new P).returnType // or (new
  P).attreturnType
• print (new S).returnType

16
Colored Variables (defvar)

• var SimpleAccount class Value
• var dynamic private deposit200
• var dynamic private withdrawal100
• var getTotal method return (deposit -
  withdrawal)
• var payable method payment return (paymentlt
  defgetTotal)
• var Account class SimpleAccount
• var dynamic private savings 400
• var getTotal method return
  (super.getTotal savings)
• var sa new SimpleAccount
• print sa.payable200 // not payable
• var a new Account
• print a.payable200

17
The Default Rule

• y . ( x . m ! a )
• y --- def variable (evaluated in defined env)
• x --- att variable (evaluated in y)
• m --- att variable (evaluated in x)
• a --- def variable (evaluated in defined env)
• The default rule implies dynamic binding.

18
Colored Variables (gen/rawvar)

• var Account class Value
• var dynamic deposit200
• var dynamic withdrawal100
• var genbalance((deposit - withdrawal),
• method b deposit b
  withdrawal)
• var account new Account
• print account.balance
• account.balance 300 // updation
• print account.balance
• print account.rawbalance // return the
  expressions

19
Types and Run-time Checking

• print "This must be a string according to the
  type constraint.\n"String
• print Int Float - Int // This is used for
  user-defined type checking algorithm.
• print "This string is wrongly typed!" Int

20
User-defined Types

• var YType class Value var dynamic type
• var init method t type t return
  this
• var check method t return type lt
  t.getType
• var Persons class Value var msg "I am a
  person.\n"
• var Staff class Persons var msg "I am a
  staff.\n"
• var YStaff new YTypeStaff
• var sYStaff // Upwards-closure type
• s new Persons print s.msg
• var pStaff // Normal type requires exact match.
• p new Persons print s.msg

21
Future Work

• Client-side HCI (using Java RMI).
• From FlexibO to C.
• Exploring the new mechanisms.
• Integration of UML.
• Predicative semantics and proof of development
  non-monotonicity.