Add vim-lsp for Haskell goodness

pkgs/vim/default.nix

@@ -107,6 +107,7 @@ let
         vim-hcl
         vim-ledger
         vim-lilypond
+        vim-lsp
         vim-nix
         vim-orgmode
         vim-selinux
@@ -255,6 +256,46 @@ let
         au BufRead,BufNewFile Vagrantfile set filetype=ruby
       augroup END
 
+      " Haskell Language Server
+      if executable('dhall-lsp-server')
+        au User lsp_setup call lsp#register_server({
+            \ 'name': 'dhall-lsp',
+            \ 'cmd': {server_info->['dhall-lsp-server']},
+            \ 'allowlist': ['dhall'],
+            \ })
+      endif
+      if executable('haskell-language-server-wrapper')
+        au User lsp_setup call lsp#register_server({
+            \ 'name': 'haskell-lsp',
+            \ 'cmd': {server_info->['haskell-language-server-wrapper', '--lsp']},
+            \ 'allowlist': ['haskell'],
+            \ })
+      endif
+
+      function! s:on_lsp_buffer_enabled() abort
+        setlocal omnifunc=lsp#complete
+        setlocal signcolumn=yes
+        if exists('+tagfunc') | setlocal tagfunc=lsp#tagfunc | endif
+        nmap <buffer> gd <plug>(lsp-definition)
+        nmap <buffer> gr <plug>(lsp-references)
+        nmap <buffer> gi <plug>(lsp-implementation)
+        nmap <buffer> gt <plug>(lsp-type-definition)
+        nmap <buffer> <leader>rn <plug>(lsp-rename)
+        nmap <buffer> [g <Plug>(lsp-previous-diagnostic)
+        nmap <buffer> ]g <Plug>(lsp-next-diagnostic)
+        nmap <buffer> K <plug>(lsp-hover)
+        nmap <buffer> <leader>a <plug>(lsp-code-action)
+        nmap <buffer> <leader>dd <plug>(lsp-document-diagnostics)
+
+        " refer to doc to add more commands
+      endfunction
+
+      augroup lsp_install
+        au!
+        " call s:on_lsp_buffer_enabled only for languages that has the server registered.
+        autocmd User lsp_buffer_enabled call s:on_lsp_buffer_enabled()
+      augroup END
+
       " Extra options
       ${extraOptions}
     '';