Project Url: cretz/kastree
Introduction: Simple Kotlin Source AST and Syntax Parsing, Editing, and Writing
More: Author   ReportBugs   

Kastree is a simple library to manipulate Kotlin source code as a set of AST objects. Features:

  • Simple, immutable, hierarchical set of data classes representing Kotlin AST
  • Simple writer implementation (some advanced features not yet supported)
  • Support for regular and mutable visitors
  • Basic support for blank-line and comment map (some advanced use cases not yet supported)
  • Support for parsing (via Kotlin compiler's parser) and converting to the AST

The project is a simple one and probably will not support a lot of features. It was created to facilitate advanced Kotlin code generation beyond the string-based versions that exist.


Kastree is currently not being actively developed.

Another kotlin AST parsing library is kotlinx.ast



This project has three libraries that are deployed to Maven Central. To simply build the AST from a Kotlin JVM or Java project, add the following dependency in Gradle:

compile 'com.github.cretz.kastree:kastree-ast-jvm:0.4.0'

That does not include the parser. To include the parser (which transitively includes the entire Kotlin compiler), instead use:

compile 'com.github.cretz.kastree:kastree-ast-psi:0.4.0'

While the parser only works from JVM projects, the AST itself (and writers/visitors) can be used from other multiplatform projects. In the shared/common project, include the common lib:

compile 'com.github.cretz.kastree:kastree-ast-common:0.4.0'


Examples below are simple Kotlin scripts.

Parse code

In this example, we use the wrapper around the Kotlin compiler's parser:

import kastree.ast.psi.Parser

val code = """
    package foo

    fun bar() {
        // Print hello
        println("Hello, World!")

    fun baz() = println("Hello, again!")
// Call the parser with the code
val file = Parser.parseFile(code)
// The file var is now a kastree.ast.Node.File that is used in future examples...

The file variable has the full AST. Note, if you want to parse with blank line and comment information, you can create a converter that holds the extras:

// ...

val extrasMap = Converter.WithExtras()
val file = Parser(extrasMap).parseFile(code)
// extrasMap is an instance of kastree.ast.ExtrasMap

Write code

To write the code created above, simply use the writer

import kastree.ast.Writer
// ...


This outputs a string of the written code from the AST file object. To include the extra blank line and comment info from the previous parse example, pass in the extras map:

// ...

println(Writer.write(file, extrasMap))

This outputs the code with the comments.

View nodes of a type

This will get all strings:

import kastree.ast.Node
import kastree.ast.Visitor
// ...

var strings = emptyList<String>()
Visitor.visit(file) { v, _ ->
    if (v is Node.Expr.StringTmpl.Elem.Regular) strings += v.str
// Prints [Hello, World!, Hello, again!]

The first parameter of the lambda is the nullable node and the second parameter is the parent. There is a tag var on each node that can be used to store per-node state if desired.

Modify nodes

This will change "Hello, World!" and "Hello, again!" to "Howdy, World!" and "Howdy, again":

import kastree.ast.MutableVisitor
// ...

val newFile = MutableVisitor.preVisit(file) { v, _ ->
    if (v !is Node.Expr.StringTmpl.Elem.Regular) v
    else v.copy(str = v.str.replace("Hello", "Howdy"))

Now newFile is a transformed version of file. As before, the first parameter of the lambda is the nullable node and the second parameter is the parent. The difference between preVisit (as used here) and postVisit is that preVisit is called before children are traversed/mutated whereas postVisit is called afterwards.

Note, since extraMap support relies on object identities and this creates entirely new objects in the immutable tree, the extra map becomes invalid on this step. This is not solved in the library yet, but could be done fairly easily.

Running tests

The tests rely on a checked out version of the Kotlin source repository since it uses the test data there to build a corpus to test against. The path to the base of the repo needs to be set as the KOTLIN_REPO environment variable. Once set, run:

path/to/gradle :ast-psi:test

This will ignore all Kotlin files with expected parse errors and only test against the ones that are valid (178 as of this writing). The test parses the Kotlin code into this AST, then re-writes this AST, then re-parses what was just written and confirms it matches the original AST field for field.

About Me
GitHub: Trinea
Facebook: Dev Tools