Conventional debuggers require programmers to work on multiple levels of abstraction at once when inspecting call stacks or data. This demands considerable cognitive overhead and deep system knowledge of all implementation technologies involved. When developing an interpreter, programmers often create a dedicated debugger to have a higher-level perspective on the client-language; the resulting use of multiple debuggers at once leads to mental context switches and needs an elaborated method. We present an integrated debugging tool in which interpreter developers define and select the levels of abstraction on which they focus. Our debugger provides them with an abstraction-specialized view. We consider both host-language and guest-language levels, since either may be levels of interest in a debugging session. We show how this separation into host-language levels can ease the debugging of applications through filtering call stacks and specializing call stack representation on levels.

1. Multi-level Debugging for Interpreter Developers
Bastian Kruck, Stefan Lehmann, Christoph Kessler, Jakob
Reschke, Tim Felgentreff, Jens Lincke, Robert Hirschfeld
Software Architecture Group, HPI Potsdam
15.03.2016, LaMOD16, Málaga

2. 1. Debugging across Languages
2. Debugging within Languages across Abstraction Barriers
3. Unification of such levels
Three Phases
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck Chart 2

3. How does debugging across languages look like?
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck Chart 3

4. How does debugging across languages look like?
Chart 4
Bastian Kruck
HPI Potsdam
Multi-level
Debugging for
Interpreter
Developers

5. Scheme embedded in Qoppa
Chart 5
Scheme
Qoppa
Embedded in
(define if (vau (b t f) env
(eval env (bool (eval env b) t f))))
(define quote (vau (x) _ x))
■ Qoppa is a Scheme without special forms: all arguments are
evaluated before entering the method
■ Vau is the only explicitly evaluating primitive (no automatic
evaluation)
■ Vau is used to implement all other special forms
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

6. Scheme embedded in Qoppa
Chart 6
Scheme
Qoppa
(define if (vau (b t f) env
(eval env (bool (eval env b) t f))))
(define quote (vau (x) _ x))
Smalltalk
Embedded in
Interpreted by
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

7. How to teach language barriers
to a debugger?

8. The common symbolic debugger
Chart 8Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

9. The Minimal Feature Set of a Debugger
Chart 9
Step buttons
Call Stack
Helps to zoom from
behavioral
overview into detailOverview
Detail
Source View
Helps to look at
the past and
future
Future
Past
Object Inspector
Helps to interact with
state of the current
Level of details
• Detail State
• Detail State
• Detail State
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

10. eval:in:
Call stack of QoppaS
Chart 10Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

11. Call stack of QoppaS
Chart 11
operate:on:in:
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

12. Call stack of QoppaS
Chart 12
Operative>>in:operateOn:
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

13. Call stack of QoppaS
Chart 13
wrapPrim:countingArgs:
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

14. Call stack of QoppaS
Chart 14
buildGlobalEnv
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

15. Call stack of QoppaS
Chart 15
• Recurrence Patterns
in call stacks signal
an unnoticed
abstraction level
• The varying barrier-
width makes it hard
to read
• Debugger users
should be able to:
• Filter stack frames
• Fold open levels on
demand
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

16. Higher-level information are encoded in all those
Chart 16
Step buttons
Call Stack
Various barrier-
widths are present
in the call chainOverview
Detail
Source View
Source code
implements
abstract concepts
such as events or
state machines
Future
Past
Object Inspector
Data contain
implementation specifica
such as storage
strategies
• Detail State
• Detail State
• Detail State
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

17. How to teach language abstraction barriers
to a debugger?

18. ■ Data abstraction enables modularity
□ Isolate math domain implementation
□ Isolate abstract type
□ Isolate representation details
■ Behavioral black-box abstractions to
enable modularity
■ Not present in current debugging tools
■ Interpreters form a new layer of abstractions
that is loosely coupled to the lower one
□ because of that very explicit interface
□ A program is the interpreters data
Abstraction barriers should be respected in
debuggers
Chart 18
Structure and
Interpretation
of Computer
Programs
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

19. Current debuggers that respect abstraction barriers:
■ Rails BacktraceCleaner
■ Visual studio with Show only my code
■ Eclipse with Step filters
□ AOP & COP
□ Code metrics
■ Squeak UnhandledException
■ Squeak Step through
There is no general abstraction-aware debugger so far
(Current state of abstraction barriers in
debuggers)
Chart 19Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

20. (define (fact-r n acc)
(if (<= n 1)
(begin (halt) acc)
(fact-r (- n 1) (* acc n))))
(fact (- 5 2))
Example of a Multi-level Debugger
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck Chart 20

21. Chart 21Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

22. Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck Chart 22

23. Chart 23
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

24. Chart 24
Grouping frames by
interpreter frames
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

25. Chart 25
Specialized
Representation
Grouping frames by
interpreter frames
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

26. Call stack of QoppaS
Chart 26Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

27. Chart 27
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

28. Chart 28
Level icons show
level affiliation

29. Chart 29
Filter levels to
focus
e.g. Smalltalk Packages,
or
Authoring sessions
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

30. ■ By package
■ By class
■ By author
■ By creation time
■ By JIT-Optimization-State
(Identifying levels)
Chart 30Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

31. Implementation of both
Language barriers
Abstraction barriers

32. „A mediator is a software module that exploits encoded knowledge about
certain sets or subsets of data to create information for a higher layer of
applications“ [Wie92]
Mediator-Wrapper-Architecture
Chart 32
Mediator
Higher level
information
Encoded Knowledge
Data
Data
Gio Wiederhold: “Mediators in the Architecture of Future Information Systems”
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

33. Mediators enrich call frames
Chart 33
Title
Icon
Group
Source
Data State
Multi-level
Debugger
INOUT
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

34. Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck
Mediator-Wrapper-Architecture
Chart 34
Squeak VM
Reflection:
Squeak
contexts
Vivide
QoppaS
Interpreter
Squeak
Primitives
Qoppa
Language
…
Multi-
level
Debugger
…
WIP

35. Future Work: Debugger Primitives
Chart 35
Multi-level
Debugger
Scheme
Language
Squeak as
implementation language
Qoppa as
implementation language
Qoppa
Language
Debugger
Primitive
■ Reuse low-level debugger
for higher levels
■ Cross level functionality
comes for free
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

36. Multi-level Debugging for Interpreter Developers
Thanks for your frameworks and support:
Marcel Taeumel (VIVIDE),
Patrick Rein (OhmS)
Thanks to Mod16 and SWA Group for funding
Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

37. [1] Charles, P. et al. 2009. Accelerating the creation of customized, language-Specific IDEs in Eclipse. ACM Sigplan Notices.
44, 10 (2009), 191.
[2] Chiş, A. et al. 2015. Practical domain-specific debuggers using the Moldable Debugger framework. Computer
Languages, Systems and Structures. 44, (Dec. 2015), 89–113.
[3] Chiş, A. et al. 2015. The moldable inspector. 2015 ACM - Onward! 2015 (New York, New York, USA, Oct. 2015), 44–60.
[4] Erdweg, S. et al. 2012. Language Composition Untangled. LDTA ’12. (2012).
[5] Freudenberg, B. et al. 2014. SqueakJS A Modern and Practical Smalltalk that Runs in Any Browser. DLS ’14. (2014),
57–66.
[6] Pavletic, D. et al. 2014. Extensible Debuggers for Extensible Languages. Softwaretechnik-Trends. 33, 2 (2014), 51–52.
[7] Pavletic, D. and Raza, S.A. 2015. Multi-Level Debugging for Extensible Languages. Workshop Software-Reengineering
und -Evolution. 17 (2015), 21–23.
[8] Renggli, L. et al. 2010. Embedding Languages Without Breaking Tools. ECOOP. (2010), 380–404.
[9] Wu, H. et al. 2004. Debugging Domain-Specific Languages In Eclipse. Eclipse Technology Exchange Poster. (2004), 1–
5.
Related Work
Chart 37Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck

38. Debugger Primitives
■ How to bake them into the known reflective capabilities of languages?
Challenges
Chart 38Multi-level Debugging for Intrepreter Developers, LaMOD’16, 15.03.2016, Bastian Kruck