This document provides an overview of Abide, a static checker tool for Scala based on type-checked ASTs. It has the goals of unifying existing rule-based approaches, making it easy to write and use rules, and providing reasonable performance and good integrations. Rules are decoupled from the tool and can come from core, extra, or project-specific packages. The document gives examples of core rules and rules for Akka and describes Abide's architecture, use, anatomy including traversers, directives, and how rules are coded. It encourages contributing rules to help the value of the tool.

1. Scala
Iulian Dragos

2. Overview
• Static checker tool
• Based on type-checked ASTs
• Easy to use and write rules

3. Goals
• Unify existing approaches in one platform
• Easy to write rules
• Easy to use other people’s rules
• Reasonable performance
• Good integrations

4. Abide
• Rules are decoupled from the tool
• rule packages
• project-specific rules
• Integrates with Sbt
• planned: Maven, Eclipse

5. Examples
• Core package
• Unused members
• Vars that are never re-assigned
• Renamed arguments with defaults
• Akka
• sender method called inside future

6. Architecture
Abide
abide-plugin
scalac
core rules
extra rules
project-specific
...

7. Use
addSbtPlugin("com.typesafe" % "sbt-abide" % "0.1.0")
libraryDependencies += "com.typesafe" %% "abide-core" % “0.1.0” % "abide"
libraryDependencies += "com.typesafe" %% "abide-extra" % “0.1.0” % "abide"
Project(...).dependsOn(rules % "abide")

8. Anatomy

9. Traversers
• Fused: one-pass for all rules
• Optimised: rules are filtered on what they match

10. Directives
• Shared context between rules
• compiler universe
• user-defined mix-ins

11. Rules
val step = {
case varDef @ q"$mods var $name : $tpt = $value" =>
nok(varDef.symbol, Warning(varDef))
case q"$rcv = $expr" =>
ok(rcv.symbol)
}
quasi-quote
state helpers
Replace `var` with `val`

12. Rules
• partial function Tree => Unit
• collect keys that can do one transition
• nok (not ok)
• ok (stable state)

13. Rules
• existential rules (what we’ve seen)
• scoping rules
• helpers to maintain nesting
• enter (no need for removing, handled by the library)
• path rules
• like scoping, but richer state checks

14. Coding a rule

15. For example..

16. Mutation inside iteration
• Let’s forbid mutating the underlying collection while
iterating
for (x <- xs)
xs -= x xs map { x => xs -= x }
ys foreach { case (k, v) =>
ys remove 1
}
for (p <- icodes; x <- xs) {
icodes -= p._1
xs remove x
}

17. Simplifying assumptions
• purely syntactical matching (no points-to analysis)
• name-based
xs map { x => xs -= x }

18. Let’s code

19. Future

20. Next
• SupressWarning support
• Integrations: maven and IDEs
• More rules

21. Contribute!
• The value is in rules
• Contribute to scala/scala-abide
• ..or write your own rule package

22. Acknowledgements
Nicolas Voirol (@samarion) did all the work during
his internship at Typesafe Switzerland

23. Thank you