When I was learning Haskell, I missed the nice features of IntelliJ IDEA. My first approach was to use the default way of creating an IntelliJ plugin by defining a grammar and a lexer according to Haskell report. That didn't work out because I could not define all the recursion. Then I decided to use grammar and lexer definitions only for tokenizing and parsing Haskell code, and not for syntax checking the code. This is needed for syntax highlighting, all kinds of navigation, and so on....