I'm asking this from the perspective of a custom language plugin author. For something like Java, we have a standard directory that is home to the language installation, the JDK. For other languages we have something similar. But in the case of something like Haskell, you may or may not. You may be using a specific build tool (Stack or Cabal) or just plain GHC (the Haskell compiler). If you are using Stack, it manages your GHC installation for you, so you might not even have it installed yet. So where is your SDK home? The path to the build tool or GHC? In this case, it seems easier to just not even have an SDK and let the user configure how to build the project using the Build and Compiler settings.
Does an SDK serve some other purpose? Am I missing something obvious?