Close #830 - Implement LSP connections via TCP sockets

autoload/ale/lsp.vim

@@ -3,7 +3,7 @@
 
 " A List of connections, used for tracking servers which have been connected
 " to, and programs which are run.
-let s:connections = []
+let s:connections = get(s:, 'connections', [])
 let g:ale_lsp_next_message_id = 1
 
 " Exposed only so tests can get at it.
@@ -36,7 +36,7 @@ endfunction
 
 function! s:FindConnection(key, value) abort
     for l:conn in s:connections
-        if has_key(l:conn, a:key) && get(l:conn, a:key) == a:value
+        if has_key(l:conn, a:key) && get(l:conn, a:key) is# a:value
             return l:conn
         endif
     endfor
@@ -319,13 +319,13 @@ function! ale#lsp#ConnectToAddress(address, project_root, callback, initializati
     let l:conn = !empty(l:conn) ? l:conn : ale#lsp#NewConnection(a:initialization_options)
 
     if !has_key(l:conn, 'channel_id') || !ale#socket#IsOpen(l:conn.channel_id)
-        let l:conn.channnel_id = ale#socket#Open(a:address, {
+        let l:conn.channel_id = ale#socket#Open(a:address, {
         \   'callback': function('s:HandleChannelMessage'),
         \})
     endif
 
-    if l:conn.channnel_id < 0
+    if l:conn.channel_id < 0
-        return 0
+        return ''
     endif
 
     let l:conn.id = a:address
@@ -333,7 +333,7 @@ function! ale#lsp#ConnectToAddress(address, project_root, callback, initializati
     call uniq(sort(add(l:conn.callback_list, a:callback)))
     call ale#lsp#RegisterProject(l:conn, a:project_root)
 
-    return 1
+    return a:address
 endfunction
 
 " Stop all LSP connections, closing all jobs and channels, and removing any

autoload/ale/lsp_linter.vim

@@ -166,7 +166,7 @@ function! ale#lsp_linter#StartLSP(buffer, linter, callback) abort
 
     let l:language_id = ale#util#GetFunction(a:linter.language_callback)(a:buffer)
 
-    if !l:conn_id
+    if empty(l:conn_id)
         if g:ale_history_enabled && !empty(l:command)
             call ale#history#Add(a:buffer, 'failed', l:conn_id, l:command)
         endif

doc/ale.txt

@@ -2371,8 +2371,16 @@ ale#linter#Define(filetype, linter)                       *ale#linter#Define()*
 
                          When this argument is set to `'stdio'`, then the
                          linter will be defined as an LSP linter which keeps a
-                         process for a language server runnning, and
+                         process for a language server running, and
                          communicates with it directly via a |channel|.
+                         `executable` or `executable_callback` must be set,
+                         and `command` or `command_callback` must be set.
+
+                         When this argument is set to `'socket'`, then the
+                         linter will be defined as an LSP linter via a TCP
+                         socket connection. `address_callback` must be set
+                         with a callback returning an address to connect to.
+                         ALE will not start a server automatically.
 
                          When this argument is not empty, only one of either
                          `language` or `language_callback` must be defined,
@@ -2388,6 +2396,13 @@ ale#linter#Define(filetype, linter)                       *ale#linter#Define()*
                          `initialization_options_callback` may be defined to
                          pass initialization options to the LSP.
 
+  `address_callback`       A |String| or |Funcref| for a callback function
+                         accepting a buffer number. A |String| should be
+                         returned with an address to connect to.
+
+                         This argument must only be set if the `lsp` argument
+                         is set to `'socket'`.
+
   `project_root_callback`  A |String| or |Funcref| for a callback function
                          accepting a buffer number. A |String| should be
                          returned representing the path to the project for the