1. Formal Models for Programming Languages
FJ and FGJ
Alexandra Stefan
11/26/2018

2. Topics of Discussion Programming Languages (PLs) and Formal Models
Case study:
Java - Featherweight Java (FJ)
Extension:
Generic Java (1.5 beta) –
Featherweight Generic Java (FGJ)
Some properties: compositionality ...
Related work
11/26/2018

3. PLs & formal models First was the computer … and then the programs …
and then the programs to write programs …
Numerous programming languages:
imperative: Ada, Pascal, Perl, Java, all of the Cs …
functional: Scheme, SML, Prolog, Haskel, OCaml …
How to choose one? What makes one better than the other?
expressive power?
No.
friendliness? (readability/ modifiability/ debugging)
Yes.
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4. PLs & formal models
Then questions and needs were raised about languages:
How safe is it?
It passed compilation: what next?
Can I trust it to run well?
Will it stab me on the back?
Can it be extended?
Improved efficiency: generic classes
Increased expressivity: XML syntax
How would I design a new language?
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5. PLs & formal models Here we come, the PL people! How do we do this?
check their properties (e.g. type preservation, progress, …)
try to capture the invariants with a type system
How do we do this?
using a formal model
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6. Formal model Design choice: completeness or compactness?
more complex is more unwieldy
too parsimonious is useless
trade-off
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7. Formal model Examples: Lambda calculus
Java: FJ, FGJ, Javas (Javase), Javalight, ClassicJava, …
C: C
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8. Topics of Discussion Programming Languages (PLs) and Formal Models
Case study:
Java - Featherweight Java (FJ)
Extension:
Generic Java (1.5 beta) –
Featherweight Generic Java (FGJ)
Some properties: compositionality ...
Related work
11/26/2018

9. Topics of Discussion Programming Languages (PLs) and Formal Models
Case study:
Java - Featherweight Java (FJ)
11/26/2018

10. Case study: Featherweight Java (FJ)
Proposed by Igarashi, Pierce, and Wadler in 1999.
Models Java’s type system.
Minimal core calculus
“Favors compactness over completeness almost obsessively”
(It hardly resembles Java!)
no assignment -> functional
captures the core features of Java typing.
How close is it to Java?
FJ ~ purely functional core of Java.
(every FJ program is ‘literally’ an executable Java program)
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11. FJ: What can be done with it?
It allows easy proofs for type safety for FJ
FJ’s main application is to model extensions
of Java
FGJ
Featherweight Open Java
Inner classes
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12. FJ What does it include? What does it not include? Object creation (*)
Field access (*)
Method invocation (*)
Cast (*)
Variables (*)
Mutually recursive class definitions
Method override
Method recursion – this
Assignment (final)
Null pointers
Base types
Messages to super
Field shadowing
Method overloading
Access control (public, private, …)
Interfaces
Exceptions
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13. FJ How does an FJ program look like? Sanity conditions for classes
Pair: (class table, expression)
The class table - fixed
Sanity conditions for classes
‘Object’ - not part of the class table
All necessary classes are defined
No cycles in the subtyping relation
induced by the class table
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14. FJ: class declaration examples
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15. FJ: expression examples
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16. FJ: syntax & subtyping rules
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17. FJ: auxiliary functions
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18. FJ: expression typing
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19. FJ: expression typing
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20. FJ: evaluation rules
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21. FJ: results
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22. Topics of Discussion Programming Languages (PLs) and Formal Models
Case study:
Java - Featherweight Java (FJ)
Extension:
Generic Java (1.5 beta) –
Featherweight Generic Java (FGJ)
Some properties: compositionality ...
Related work
11/26/2018

23. Compositional compilation
“Polymorphic Bytecode: Compositional Compilation for Java-like Languages”
D. Anacona
S. Drossopoulou
F. Damiani
E. Zucca
11/26/2018

24. Separate compilation: motivation
Why do we want these properties?
modularity
safety
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25. Separate compilation compile link compile compile fragment
Used fragments
fragment
compile
link
Used fragments
fragment
compile
fragment
compile
fragment
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26. Java-like languages ‘clash of philosophy’
Separate compilation -> byte-code reflects compilation environment
Dynamic linking -> byte-code does not reflect compilation environment
no: (compilation ~ execution) environment
no: (final ~ globally) compiled application
type-safety through runtime verification
Example:
E m(B x){ return x.f1.f2}
CEnv1 = … B {… C f1; …} … C {… E f2 …}
-> mdB1 = E m(B x){ return x[B.f1 C][C.f2 E]}
CEnv2 = … B {… D f1; …} … D {… F f2 …} ->
-> mdB2 = E m(B x){ return x[B.f1 D][D.f2 F]}
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27. Compositionality Compositional analysis: Compositional compilation:
analyze source code fragments separately
put them together
finish analysis of the whole without reanalyzing the fragments
Compositional compilation:
typecheck source code fragments separately
-> generate corresponding binaries
link together fragments whose mutual constraints are satisfied, without reinspecting the code
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28. Compositional compilation: IDEA
Compile fragments separately
Generate
bytecode with type variables
constraints –> relate the type variables
Put the constraints together (particular order)
Solve the constraints –> substitution
Put the bytecode together
Apply the substitution
Done!
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29. Compositional compilation
How good is it?
We are not sure yet, but it looks promising.
My work:
Try to apply it for FGJ.
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30. Topics of Discussion Programming Languages (PLs) and Formal Models
Case study:
Java - Featherweight Java (FJ)
Extension:
Generic Java (1.5 beta) –
Featherweight Generic Java (FGJ)
Some properties: compositionality ...
Related work
11/26/2018

31. Generic Java Version: 1.5 beta Parametric polymorphism
Uses the standard JVM
Backward & Forward compatibility with old code.
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32. FGJ: class declaration examples
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33. FGJ: term examples
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34. Homogeneous vs Heterogeneous polymorphism
Description
One piece of code: handles all instances of a generic class
Stack<T> ---> Stack
Object < ---> T
problem: raw types
The template: rewritten as specialized code for each unique set of parameters
Examples
Generic Java, ML
C++ (templates)
Byte code size
Small
Large:
linear in size of parameter combinations
Type semantics
Clear
Blurred:
the complier does not know the classes derive from the same template
generic class can not be rewritten
Speed
Slow: runtime cast
Fast: no additional casts
11/26/2018

35. Generic Java (GJ) No explicit type arguments to a generic method:
parsing problem
“e.m<A,B>(e’)”  “e.m<A”, “B>(e’)”
Performs parameter inference: has a least type
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36. Featherweight Generic Java (FGJ)
FJ extended with generic classes
Not a subset of GJ: requires type arguments to generic methods
Not concerned with parameter inference
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37. FGJ: syntax & subtyping rules
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38. FGJ: auxiliary functions
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39. FGJ
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40. FGJ
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41. FGJ: expression typing
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42. FGJ: expression typing
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43. FGJ: evaluation rules
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44. FGJ: results
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45. Topics of Discussion Programming Languages (PLs) and Formal Models
Case study:
Java - Featherweight Java (FJ)
Extension:
Generic Java (1.5 beta) –
Featherweight Generic Java (FGJ)
Some properties: compositionality ...
Related work
11/26/2018

46. Related work “Types and Programming Languages” by B.C.Pierce
“Featherweight Java – A Minimal Core Calculus for Java and GJ” by Atsushi Igarashi, Benjamin C. Pierce, Philip Wadler
“GJ Specification” by Gilad Bracha, Martin Odersky, David Stoutamire, Philid Wadler
“Bringing Genericity to Java” by David W. Dunham
“Polymorphic Bytecode: Compositional Compilation for Java-like Languages” by D. Anacona, S. Drossopoulou, F. Damiani, E. Zucca
11/26/2018