Scala Abide: A lint tool for Scala

Feb 18, 20158 likes2,740 views

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.

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

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

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

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

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

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")

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

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

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`

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

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

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

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

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

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

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

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Scala Abide: A lint tool for Scala

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

Scala Abide: A lint tool for Scala

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