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Magic Literals in Pharo
- 1. Magic Literals In Pharo Julien Delplanque, Stéphane Ducasse, and Oleksandr Zaitsev {firstname}.{lastname}@inria.fr Based on the paper that we published at the International Workshop of Smalltalk Technologies (IWST’19)
- 2. • PhD Student at Inria Lille CIFRE: Inria + Arolla • I live and work in Lille and come to Arolla once every two weeks • My research is about Machine Learning + Software Evolution • I don’t speak French but I’m learning :) About me
- 3. 3
- 4. 4
- 5. Literals in Pharo • true and false • nil • Numbers: 42, -42, 4.2, 4s2, 4e2, … • Individual characters: $a • Strings of characters: ‘foo’ • Symbols: #foo:bar: • Arrays of other literal constants: #(1 2 4) and #[255 0] 5
- 6. Magic Literals 6 — literals of any kind that appear in source code without a clear explanation
- 7. Example of Magic Literals 7
- 8. Example of Magic Literals 8 what is 52? what is 4??
- 9. Same Thing without Magic Literals 9
- 10. Same Thing without Magic Literals 10 aha!
- 11. Why Should We Avoid Magic Literals? 11 1. Readability 2. Logic duplication 3. Modularity
- 12. Closer look at the literals
- 13. 169,133 literals in Pharo 7 We have identified
- 14. 169,133 literals Other Array Boolean Symbol String Integer 14 RQ1: What are the most common data types of literals?
- 15. 169,133 literals Array Pragma Sequence Assignment Return Receiver Argument 15 RQ2: In which part of the AST do those literals appear?
- 16. 16 Tests Non-tests 92 330 20 170 Test Non-test 60 %37 % 40 % 63 % without magic literals with magic literals RQ3: Are there more literals in test methods?
- 18. Literals self-describing their semantics Acceptable literals true false nil #() #[] {} ‘’ 18
- 19. Literals directly assigned to a variable / returned by a method Acceptable literals 19 this is EventTypeNone those are typical file extensions
- 20. Literals located in a test and example methods Acceptable literals 20 acceptable in tests
- 21. Magic Literals
- 22. Any literal that does not fall in one of the acceptable literal category. 22 Magic Literals
- 24. Heuristic implemented as a CodeCritic rule 24
- 25. Evaluation
- 26. Evaluation Strategy Step 1. Select all methods from Pharo 7 Step 2. Run heuristic to find magic literals Step 3. Select a small sample of those methods Step 4. Manually evaluate the results
- 27. 112,500 Evaluation. Step 1 We have collected methods from Pharo 7 image
- 28. Evaluation. Step 2 Our heuristic reported Methods 8 986 160 147 Without magic literals With magic literals
- 29. Evaluation. Step 2 Our heuristic reported Literals 23 292103 514 Non-magic Magic
- 30. Evaluation. Step 3 We randomly selected 100 methods for manual evaluation
- 31. Evaluation. Step 3 According to our heuristic, those methods contain 243 magic literals
- 32. Evaluation. Step 4 32 151 7418 True Positive False Positive ? 243 literals reported as magic
- 33. Evaluation. Step 4 33 True Positive example why 2000?
- 34. Evaluation. Step 4 34 False Positive example the meaning of 1, 2, 3 is clear in this case - they increment the index
- 36. Conclusion
- 37. Conclusion • First heuristic for identifying magic literals • Implemented as code critics (lint) rule • 62% of reported literals were actually magic (and should be fixed)