Modularly Typesafe Interface Dispatch in JPred

1
Modularly Typesafe Interface Dispatch in JPred

• Christopher Frost and Todd Millstein
• University of California, Los Angeles
• frost,todd_at_cs.ucla.edu

2
Background Multimethod Dispatch

• class Spy
• void escape(Vehicle v)
• void escape(Vehicle_at_Driveable v)
• void escape(Vehicle_at_Flyable v)

Vehicle
Vehicle v ... spy.escape(v)
3
Background Multimethod Dispatch

• class Spy
• void escape(Vehicle v)
• void escape(Vehicle_at_Driveable v)
• void escape(Vehicle_at_Flyable v)

Vehicle
void escape(Vehicle v)
?
Flyable
Driveable
?
?
Submersible
void escape(Vehicle_at_Driveable v)

• Multimethod lookup most specific method for
  runtime types of arguments
• Possible message dispatch errors
• Message-not-understood
• Message-ambiguous

4
Background Multimethod Dispatch

• class Spy
• void escape(Vehicle v)
• void escape(Vehicle_at_Driveable v)
• void escape(Vehicle_at_Flyable v)

Vehicle
Flyable
Driveable

• Typechecking ensure methods are exhaustive and
  unambiguous
• Typechecking can be done modularly
• Only use knowledge of modularly available types

5
Dispatch on Java Interfaces
class Spy void escape(Vehicle v) void
escape(Vehicle_at_Driveable v) void
escape(Vehicle_at_Flyable v)
Vehicle
Flyable
Driveable
?
Ultra

• Let Vehicle, Driveable, and Flyable be interfaces

• Method lookup can become ambiguous!
• Modular typechecking must conservatively rule out
• Must rule out multimethods with 2 interface
  dispatches
• Underlying problem multiple inheritance

6
Prior Approaches

• No typechecking
• commonloops, clos, dylan, cmm
• Require whole program analysis
• cecil, tuple, doublecpp, nice
• Forbid interfaces from being dispatched upon
• multijava, jpred (originally)
• Restrict multiple inheritance to only within a
  module
• dubious, half half
• Linearize multiple dispatch or inheritance
  semantics
• polyglot, castagna

7
Our Contributions

• Practical interface dispatch
• Preserves modular typechecking
• Key predicate dispatchs expressiveness Ernst
  et al 98
• Instantiate in the context of JPred Millstein
  04
• Formalize and prove type soundness
• Demonstrate utility in practice through case
  studies
• JPred compiler
• Eclipse

8
Background JPred

• Method predicate guards subsume multimethods
  expressiveness
• Dispatch on class types, fields, linear
  arithmetic, binding, and , , and ! operations
• m1() overrides m2() if m1()s predicate implies
  m2()s

• class Spy
• void escape(Vehicle v)
• void escape(Vehicle v) when v_at_Driveable
• void escape(Vehicle v) when v_at_Flyable

void escape(Vehicle v) when v_at_Driveable
inSwamp boolean inSwamp
9
Fixing Interface Dispatch Modularly
class Spy void escape(Vehicle v) void
escape(Vehicle v) when v_at_Driveable void
escape(Vehicle v) when v_at_Flyable
!v_at_Flyable
void escape(Vehicle v) when v_at_Driveable
v_at_Flyable
void escape(Vehicle v) when v_at_Driveable
v_at_Flyable

• Insight Predicate dispatch allows modular
  resolution of all possible multiple inheritance
  ambiguities

Vehicle

• Example resolution approaches

• Add additional method(s) to cover all possible
  ambiguities

Flyable
Driveable

• Change existing method(s) to cover all possible
  ambiguities

Ultra
10
Ordered Dispatch
class Spy void escape(Vehicle v) void
escape(Vehicle v) when v_at_Flyable void
escape(Vehicle v) when v_at_Driveable
!v_at_Flyable
Vehicle
Flyable
Driveable
P1
Submersible
Submersible
P2
!P1
void escape(Vehicle v) when v_at_Submersible
!v_at_Flyable !v_at_Driveable
P3
!P2
!P1
11
Ordered Dispatch

• class Spy
• void escape(Vehicle v)
• when v_at_Flyable
• when v_at_Driveable
• when v_at_Submersible

Vehicle
Flyable
Driveable
Submersible

• First-match lookup
• Mirrors common instanceof coding styles
• Purely syntactic sugar
• Methods can use combinations
• of ordered and non-ordered dispatch

12
Predicate Satisfiability

• class Spy
• void escape(Vehicle v)
• when v_at_Flyable
• when v_at_Driveable
• when v_at_Ultra

Vehicle
Flyable
Driveable
Ultra
Desugars to
void escape(Vehicle v) when v_at_Ultra
!v_at_Flyable !v_at_Driveable

• Ultras predicate is not satisfiable
• Easy mistake with large interface hierarchies

13
Predicate Satisfiability

• class Spy
• void escape(Vehicle v)
• when v_at_Flyable
• when v_at_Driveable
• when v_at_Ultra

Vehicle
Flyable
Driveable
Ultra
Typechecker unreachable!

• Add static typecheck
• Unsatisfiable predicates

• Also useful for non-ordered dispatch

14
Implementation

• JPred extension to the Polyglot Java compiler
• Nystrom et al 03
• Original JPred disallowed interface dispatch
• JPred compiler changes
• Allowed interface dispatch
• Added ordered dispatch to the parser
• Added predicate satisfiability check
• Code generation unchanged

15
Featherweight JPred

• Extension of Featherweight Java Igarashi et al
  01
• Added interfaces and method predicates
• Formalized syntax and dynamic and static
  semantics
• Proved a type soundness theorem through progress
  and preservation
• Validates sufficiency of modular typechecking
• First provably sound formalization of predicate
  dispatch

16
Case Studies

• JPred Compiler
• Originally written in JPred using class dispatch
• Eclipses Java Development Tooling UI Plugin
• Originally written in Java

17
Case Studies JPred Compiler

• Originally written as a Polyglot extension in
  JPred using class dispatch
• Polyglot structure
• Interface hierarchy representing AST nodes
• Class hierarchy implementing the interfaces
• Compiler passes using visitor design pattern
• instanceof tests to provide specialized AST
  behavior

• Polyglot intends for clients to interact only
  with interfaces

• All AST class dispatches now interface dispatches
• 28 updated messages 14 single, 14 two methods
• Unsatisfiable predicate check caught bug in
  typechecker
• 3 compile time increase

18
Case Studies Eclipse JDT UI

• Goal Evaluate JPreds utility for complex
  programs not designed for predicate dispatch
• Updated
• ITreeContentProvider implementors getChildren()
  hasChildren()
• Eclipses Java Development Tooling UI Plugin
  classes (org.eclipse.jdt.ui)

19
Case Studies Eclipse JDT UI
Eclipse method snippet

• public Object getChildren(Object parentElement)
  
• ...
• if (parentElement instanceof IJavaModel)
• return concatenate(
• super.getChildren(parentElement),
• getNonJavaProjects((IJavaModel)parentElement))
  
• if (parentElement instanceof IProject)
• return ((IProject)parentElement).members()
• return super.getChildren(parentElement)
• ...

20
Case Studies Eclipse JDT UI

• Reduced type casts (all on interfaces) from 30 to
  3
• Errors found by the compiler
• 1 error repeated instanceof tests, second test
  unreachable
• 1 possible error series of instanceof tests
  missing else clause, not exhaustive
• JPred limitations
• Dispatches occur only at top level
• But Eclipse often does applicability tests within
  try/catch
• Method calls are not allowed within predicates
• JPred style targeted at logically independent
  cases
• But Eclipse has instances of fall-through logic

21
Conclusions

• Demonstrated a practical resolution to the
  tension between multiple dispatch and multiple
  inheritance
• While retaining fully modular static typechecking
• Key idea predicate dispatchs expressiveness
  allows programmers to modularly resolve multiple
  inheritance ambiguities
• Validated our approach
• Formalized and proved type soundness
• Demonstrated utility through two case studies

22
Featherweight JPred Details

• Method invocation rule
• unew D() and mbody(m,C,D)(x, t0)
• implies
• (new C(v)).m(u) ? x -gt u, this -gt new C(v)
  t0
• The interesting part of the progress proof
• Showing method lookup always succeeds on
  well-typed programs -- prove sufficiency of the
  exhaustiveness and unambiguity typechecking
• Progress If tT, then either
• t is a value
• t contains a subexpression of the form (S)(new
  C(v)) where TT Clt/S
• there exists some term s such that t?s
• Type Preservation If G tT and t?s, then
• there exists some type S such that G sS and
  TT SltT

23
Case Studies Eclipse JDT UI
Eclipse method

• public Object getChildren(Object parentElement)
  
• try
• if (parentElement instanceof IJavaModel)
• return concatenate(
• super.getChildren(parentElement),
• getNonJavaProjects((IJavaModel)parentElement))
  
• if (parentElement instanceof IProject)
• return ((IProject)parentElement).members()
• return super.getChildren(parentElement)
• catch (CoreException e)
• return NO_CHILDREN

24
Case Studies Eclipse JDT UI
Interface dispatch version of sample eclipse
method

• public Object getChildren(Object parentElement)
  
• try
• return getChildrenHelper(parentElement)
• catch (CoreException e)
• return NO_CHILDREN
• protected Object getChildrenHelper(Object
  parentElement) throws CoreException
• when parentElement_at_IJavaModel
• return concatenate(super.getChildren(parentEleme
  nt),
• getNonJavaProjects(parentElem
  ent))
• when parentElement_at_IProject
• return parentElement.members()
• return super.getChildren(parentElement)