coding with style: the scalastyle style checker
DESCRIPTION
Coding with style: The Scalastyle style checkerScalastyle is a static code analysis tool that helps programmers write Scala code according to the coding standards their project.It is heavily influenced by Checkstyle, PMD and FindBugs for Java. These tools are used by java developers everywhere, and Scalastyle will help Scala gain more acceptance within companies.Scalastyle can check many aspects of your code. It has as goals:- to check the style of your code according to house style (usage of tabs, correct whitespace usage),- coding conventions (too many parameters to a method, usage of disallowed libraries, usage of return)- and code which exhibits bug patterns (equals and hashCode not implemented in the same class).Currently, scalastyle works as standalone, and there is an Eclipse plugin, and plugins for sbt and maven are planned.In this talk, we discuss why we're creating Scalastyle, what it currently does (the rules that are checked), and how it does it (using the wonderful Scalariform), along with the current state of third part integration (Eclipse, Maven).TRANSCRIPT
The style checker for Scala
Scala Days 2012
Matthew Farwell @matthewfarwell
@scalastyle
Java technical expert @ 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!
Matthew Farwell
Why
What
Demo
How
What next?
The plan
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?
Why an automated style checker?
Scalastyle is 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.
What is Scalastyle?
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
What does it check? Format 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
What does it check? Style rules
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
What does it check? Known sources of bugs
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
What can’t it do?
Eclipse plugin – configuration & usage
Command line tool
// scalastyle:off magic.number
Demo
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.
How
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
Rules which are currently 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
Rules which are currently implemented
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.
How do I get involved?
Questions?