Coding with style: The Scalastyle style checker

Coding with style: The Scalastyle style checker

• 1.
  The style checkerfor Scala Scala Days 2012 Matthew Farwell @matthewfarwell @scalastyle
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  Matthew Farwell Java technicalexpert @ SQLI Suisse in Lausanne > 20 years development in various languages from Fortran to Java/Scala Contributor to various OSS projects, scala-ide, Junit, and of course Scalastyle Enjoying Scala Days 2012!
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  The plan Why What Demo How What next?
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  Why an automatedstyle checker? Douglas Crockford: "All languages should have a lint". Problem: too many people Problem: too many lines of code Solution: code reviews? Solution: style guides? Solution: unit tests? Solution: integration tests? Solution: fewer lines of code? Solution: Scalastyle?
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  What is Scalastyle? Scalastyleis a style checker, similar to Checkstyle, including bits of Findbugs & PMD. There is a continuum of checks that we need to make on our code, but which shouldn’t belong in the compiler. Projects are different, so everything needs to be configurable. They are just guidelines, they aren’t rules. But they can rules be for your project.
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  What does itcheck? Format rules Heavily inspired by Checkstyle Format rules: Files do not contain tabs Files contain correct headers, license and disclaimer etc. General format rules: def foo[T](t: T) = fn(t) + 3 not def foo [ T ] (t : T ) = fn(t)+3
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  What does itcheck? Style rules Style rules (house style, things which make the code harder to understand or more complex): Methods do not have more than x parameters Number of classes in a file Null not used Return not used Use of structural types
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  What does itcheck? Known sources of bugs Known sources of bugs, things to avoid: A class/object implements equals() but not hashCode() or vice versa A class/object implements a covariant equals(x) but not equals(Any) Missing case default Structural types
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  What can’t itdo? Anything which you don’t specify in the scala file. Inferred types Mismatches between types Implicit conversions Multi-file checks. Scalastyle only does one file at a time
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  Demo Eclipse plugin –configuration & usage Command line tool // scalastyle:off magic.number
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  How The excellent Scalariform.This gives us an AST and/or list of tokens. Lines: (list of lines) We don’t use compiler plugins for these (main) reasons: Speed (we can check scala compiler & library (1500 files) in ~80s) Comments (they get swallowed by the compiler) for comprehensions get desugared The Compiler Plugin API isn’t fully nailed down.
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  Rules which arecurrently implemented FileLengthChecker - Check the number of lines in a file FileLineLengthChecker - Check the number of characters in a line FileTabChecker - Check that there are no tabs in a file HeaderMatchesChecker - Check the first lines of each file matches the text WhitespaceEndOfLineChecker - Check that there is no trailing whitespace at the end of lines ClassNamesChecker - Check that class names match a regular expression CovariantEqualsChecker - Check that classes and objects do not define equals without overriding equals(java.lang.Object). EqualsHashCodeChecker - Check that if a class implements either equals or hashCode, it should implement the other IllegalImportsChecker - Check that a class does not import certain classes
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  Rules which arecurrently implemented IllegalImportsChecker - Check that a class does not import certain classes MagicNumberChecker - Checks for use of magic numbers NoCloneChecker - Check that classes and objects do not define the clone() method NoFinalizeChecker - Check that classes and objects do not define the finalize() method NoWhitespaceAfterLeftBracketChecker - No whitespace after left bracket [ NoWhitespaceBeforeLeftBracketChecker - No whitespace before left bracket [ NullChecker - Check that null is not used ObjectNamesChecker - Check that object names match a regular expression ParameterNumberChecker - Maximum number of Parameters for a method ReturnChecker - Check that return is not used SpacesAfterPlusChecker - Check that the plus sign is followed by a space SpacesBeforePlusChecker - Check that the plus sign is preceded by a space StructuralTypeChecker - Check that structural types are not used RegexChecker – Check that no line matches a defined regular expression
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  How do Iget involved? Me: Matthew Farwell @matthewfarwell Fork us on github: https://github.com/scalastyle/scalastyle List of rules to implement: https://github.com/scalastyle/scalastyle/wiki User mailing list: [email protected] Twitter: @scalastyle Major pieces still needed: Maven plugin, Intellij plugin, sbt plugin. As always, documentation.
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  Questions?