At Codacy we donβt only rely on external tools to find code patterns.Β Sometimes we employ other resources like scala.meta.
With Scala we also have a nice repository of homemade patterns. Therefore we need to be able to process Scala source code. And by processing we mainly mean transforming source code into Abstract Syntax Trees (ASTs).
Everything you find here you can try yourself in the repl. Be sure to add the following sbt resolver and dependencies
https://gist.github.com/johannegger/3226587d5ce3e66a8ad1
(Not yet deprecated) scala.reflect
Up until now we used the standard scala.reflect.api and the toolbox it provides. To parse scala code with help of the reflection toolbox we need to run the following:
https://gist.github.com/johannegger/ecc4c5a89f9363764046
The resulting tree variable is of type toolbox.u.Tree and this is the structure we use(d) to find problematic code. That doesnβt look too complicated you might say. Well you are right but the problems start here. Unfortunately those Trees are very complex (after all they represent the complete AST of your program). Also documentation is kinda poorβββas writer of patterns I had to rely mainly on Scaladoc and partly Reflection.
Sidenote: Real lifeΒ .scala files usually reside in packages which means a typical sourcefile will start with package fooΒ β¦ The current implementation of the toolbox however is not able to parse such files. We were forced to implement a workaround that preyed on my mind ever since.
All of this doesnβt really make it easy for someone who doesnβt know the internals of scala.reflect.api to start writing patterns from scratch. But thatβs exactly what we want. We want to make it possible for our users to write custom patterns for their issues.
scala.meta to theΒ rescue
Recently we started a very promising collaboration with Eugene Burmako and Mathieu Demarne from scala.meta. scala.meta was created to simplify metaprogramming in scala and will hopefully be the successor of the current scala.reflect. For us it already is. Letβs try to get a scala.meta.Tree
https://gist.github.com/johannegger/6467eae9b1feb1be0e2b
And thatβs it. (Now try parsing a properΒ .scala file containing a package definitionβββspoiler alertΒ : it works! ) But thatβs not all, in scala.meta.tql we find a neat tree query language that helps us writing patterns. Letβs try to implement a pattern that finds occurrence of try-catch blocks. (On codacy you can read why you shouldnβt use them)
https://gist.github.com/johannegger/051a6c125b00c412951f
We can now pass trees to the tryMatcher, letβs just try a simple test code:
https://gist.github.com/johannegger/c15090d300eea1f49365
Et voilaβββthe repl doesnβt lieββββfound culprit at line 0β
Stay tuned about the future of scala.meta at Codacy.
Edit: We just published an ebook: βThe Ultimate Guide to Code Reviewβ based on a survey of 680+ developers. Enjoy!
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