This package is only useful in combination with a language server for
Haskell. The only implementation so far (Nov '18) is the
haskell-ide-engine (hie):
https://github.com/haskell/haskell-ide-engine

Its packaging is blocked on having it uploaded to hackage first:
https://github.com/haskell/haskell-ide-engine/issues/626

Thus you need to git clone hie and build it yourself.

To troubleshoot problems inspect the buffer *lsp-haskell stderr*.

 -- Thomas Koch <thomas@koch.ro>, Tue, 27 Nov 2018 18:38:07 +0000