'hdevtools' is a backend for text editor plugins, to allow for things such as syntax and type checking of Haskell code, and retrieving type information, all directly from within your text editor.
The advantage that 'hdevtools' has over competitors, is that it runs silently in a persistent background process, and therefore is able to keeps all of your Haskell modules and dependent libraries loaded in memory. This way, when you change only a single source file, only it needs to be reloaded and rechecked, instead of having to reload everything.
This makes 'hdevtools' very fast for checking syntax and type errors (runs just as fast as the ':reload' command in GHCi).
In fact, syntax and type checking is so fast, that you can safely enable auto checking on every save. Even for huge projects, checking is nearly instant.
Once you start using 'hdevtools' and you get used to having your errors shown to you instantly (without having to switch back and forth between GHCi and your editor), and shown directly on your code, in your editor (without having to wait forever for GHC to run) you will wonder how you ever lived without it.
In addition to checking Haskell source code for errors, 'hdevtools' has tools for getting info about identifiers, and getting type information for snippets of code.