4. Lexer and Parser Definition

The lexical analyzer defines how the contents of a file are broken into tokens, which is the basis for supporting custom language features. The easiest way to create a lexer is to use JFlex.

Define a Lexer

Define a Simple.flex file with rules for the Simple Language lexer in package org.intellij.sdk.language.

Generate a Lexer Class

Now generate a lexer class via from the context menu on Simple.flex file.

The Grammar-Kit plugin uses the JFlex lexer generation. When running for the first time, JFlex prompts for a destination folder to download the JFlex library and skeleton. Choose the project root directory, for example code_samples/simple_language_plugin.

After that, the IDE generates the lexer under the gen directory, for example in simple_language_plugin/src/main/gen/org/intellij/sdk/language/SimpleLexer.

Define a Lexer Adapter

The JFlex lexer needs to be adapted to the IntelliJ Platform Lexer API. Implement SimpleLexerAdapter by subclassing FlexAdapter.

Define a Root File

The SimpleFile implementation is the top-level node of the tree of PsiElements for a Simple Language file.

Define SimpleTokenSets

Define all sets of related token types from SimpleTypes in SimpleTokenSets.

Define a Parser

The Simple Language parser is defined in SimpleParserDefinition by subclassing ParserDefinition. To avoid unnecessary classloading when initializing the extension point implementation, all TokenSet return values should use constants from dedicated $Language$TokenSets class.

Register the Parser Definition

Registering the parser definition in the plugin.xml file makes it available to the IntelliJ Platform. Use the com.intellij.lang.parserDefinition extension point for registration. For example, see simple_language_plugin/src/main/resources/META-INF/plugin.xml.

<extensions defaultExtensionNs="com.intellij">
  <lang.parserDefinition
      language="Simple"
      implementationClass="org.intellij.sdk.language.SimpleParserDefinition"/>
</extensions>

Run the Project

Run the plugin by using the Gradle runIde task.

Create a test.simple file with the following content:

# You are reading the ".properties" entry.
! The exclamation mark can also mark text as comments.
website = https://en.wikipedia.org/
language = English
# The backslash below tells the application to continue reading
# the value onto the next line.
message = Welcome to \
          Wikipedia!
# Add spaces to the key
key\ with\ spaces = This is the value that could be looked up with the key "key with spaces".
# Unicode
tab : \u0009

Now open the PsiViewer tool window and check how the lexer breaks the content of the file into tokens, and the parser transforms the tokens into PSI elements.

Last modified: 29 九月 2022