How to build project

Clone this project;
Go to root of project;
Start sbt from the shell which will download automatically the IntelliJ Community SDK with sources;
Install/enable the following plugins in IntelliJ: Plugin Devkit, Grammar-Kit and PsiViewer;
Import this project as an sbt project in IntelliJ;
Select Build>Build Project;

How to prepare plugin for deployment

Right-click on top of intellij-haskell.iml inside intellij-haskell folder;
Select Import module;
Be sure unmanaged-jars dependency is set to provided inside Project structure>Project settings>Modules>Dependencies (btw, setting provided inside sbt file gives an error);
Right-click on top of intellij-haskell plugin module and select Prepare Plugin Module 'intellij-haskell' for deployment;

In SDKs create Jetbrains JDK by choosing Add JDK... and selecting the path to the JDK which is included in the IntelliJ application;
Create Project SDK in Project structure>Project settings>Project by using the Add SDK option and selecting Intellij Platform Plugin SDK and setting the path to the IntelliJ folder. Select as JDK the just created JDK of the previous step;
Set Plugin SDK settings right inside Project structure>Platform settings>SDKs. For example, set SDK home path to idea/142.5239.7 inside project root folder;
Set Module-SDK right for intellij-haskell plugin module inside Project structure>Project structure>Project settings>Modules;
To run plugin inside IntelliJ, the first-run configuration has to be created. Navigate to Run>Edit configurations and create plugin configuration for intellij-haskell;

After making changes to _HaskellLexer.flex, run Run Flex Generator. This will generate _HaskellLexer.java;
After making changes to haskell.bnf, run Generate Parser Code. This will generate parser Java files in gen directory;
Never touch the generated code. They should be committed as-is.
Add sources.zip "inside" idea>[idea build #] to Project structure>Project settings>Modules>Dependencies>unmanaged-jars to see IntelliJ sources;