refactor: specify commands independently, not via lspconfig (#737)

This is not a desirable solution, but it's better than as-is.

There's a couple of reasons for this:
1) lspconfig only enables these commands upon successful attachment of
   servers, which doesn't really fit their use case.
2) We gain direct access to new Lua APIs for defining user commands,
   allowing us to leverage Lua callbacks and functions instead of
   intermediary global variables.

4 files changed, 89 insertions, 76 deletions

diff --git a/lua/nvim-lsp-installer/servers/pylsp/README.md b/lua/nvim-lsp-installer/servers/pylsp/README.md
index 426cf7fd..2434bb4b 100644
--- a/lua/nvim-lsp-installer/servers/pylsp/README.md
+++ b/lua/nvim-lsp-installer/servers/pylsp/README.md
@@ -10,8 +10,7 @@ In order for these plugins to work with the `pylsp` server managed by this plugi
 :PylspInstall pyls-flake8 pylsp-mypy pyls-isort
 ```
 
-The `:PylspInstall` command will only be available once the `pylsp` server is started (this happens once you've opened a
-Python file, for example).
+The `:PylspInstall` command will only be available once the `pylsp` server has been set up.
 
 **Note that these extra pylsp plugins will not be reinstalled if you update/reinstall the `pylsp` server, you will have to manage
 them manually.**
diff --git a/lua/nvim-lsp-installer/servers/pylsp/init.lua b/lua/nvim-lsp-installer/servers/pylsp/init.lua
index 54e6861d..4f0a01d1 100644
--- a/lua/nvim-lsp-installer/servers/pylsp/init.lua
+++ b/lua/nvim-lsp-installer/servers/pylsp/init.lua
@@ -1,21 +1,55 @@
+local a = require "nvim-lsp-installer.core.async"
+local _ = require "nvim-lsp-installer.core.functional"
 local server = require "nvim-lsp-installer.server"
 local pip3 = require "nvim-lsp-installer.core.managers.pip3"
 local process = require "nvim-lsp-installer.core.process"
 local notify = require "nvim-lsp-installer.notify"
-
-function _G.lsp_installer_pylsp_install_completion()
-    return table.concat({
-        "pyls-flake8",
-        "pylsp-mypy",
-        "pyls-spyder",
-        "pyls-isort",
-        "python-lsp-black",
-        "pyls-memestra",
-        "pylsp-rope",
-    }, "\n")
-end
+local middleware = require "nvim-lsp-installer.middleware"
+local spawn = require "nvim-lsp-installer.core.spawn"
 
 return function(name, root_dir)
+    middleware.register_server_hook(name, function()
+        vim.api.nvim_create_user_command(
+            "PylspInstall",
+            a.scope(function(opts)
+                local plugins = opts.fargs
+                local plugins_str = table.concat(plugins, ", ")
+                notify(("Installing %s..."):format(plugins_str))
+                local result = spawn.pip {
+                    "install",
+                    "-U",
+                    "--disable-pip-version-check",
+                    plugins,
+                    stdio_sink = process.simple_sink(),
+                    with_paths = { pip3.venv_path(root_dir) },
+                }
+                if vim.in_fast_event() then
+                    a.scheduler()
+                end
+                result
+                    :on_success(function()
+                        notify(("Successfully installed pylsp plugins %s"):format(plugins_str))
+                    end)
+                    :on_failure(function()
+                        notify("Failed to install requested pylsp plugins.", vim.log.levels.ERROR)
+                    end)
+            end),
+            {
+                desc = "[nvim-lsp-installer] Installs the provided packages in the same venv as pylsp.",
+                nargs = "+",
+                complete = _.always {
+                    "pyls-flake8",
+                    "pylsp-mypy",
+                    "pyls-spyder",
+                    "pyls-isort",
+                    "python-lsp-black",
+                    "pyls-memestra",
+                    "pylsp-rope",
+                },
+            }
+        )
+    end)
+
     return server.Server:new {
         name = name,
         root_dir = root_dir,
@@ -24,34 +58,6 @@ return function(name, root_dir)
         installer = pip3.packages { "python-lsp-server[all]" },
         default_options = {
             cmd_env = pip3.env(root_dir),
-            commands = {
-                PylspInstall = {
-                    function(...)
-                        -- `nargs+` requires at least one argument -> no empty table
-                        local plugins = { ... }
-                        local plugins_str = table.concat(plugins, ", ")
-                        notify(("Installing %q..."):format(plugins_str))
-                        process.spawn(
-                            "pip",
-                            {
-                                args = vim.list_extend({ "install", "-U", "--disable-pip-version-check" }, plugins),
-                                stdio_sink = process.simple_sink(),
-                                env = process.graft_env(pip3.env(root_dir)),
-                            },
-                            vim.schedule_wrap(function(success)
-                                if success then
-                                    notify(("Successfully installed %q"):format(plugins_str))
-                                else
-                                    notify("Failed to install requested plugins.", vim.log.levels.ERROR)
-                                end
-                            end)
-                        )
-                    end,
-                    description = "Installs the provided packages in the same venv as pylsp.",
-                    ["nargs=+"] = true,
-                    ["complete=custom,v:lua.lsp_installer_pylsp_install_completion"] = true,
-                },
-            },
         },
     }
 end
diff --git a/lua/nvim-lsp-installer/servers/tflint/README.md b/lua/nvim-lsp-installer/servers/tflint/README.md
index 359a6165..51298d69 100644
--- a/lua/nvim-lsp-installer/servers/tflint/README.md
+++ b/lua/nvim-lsp-installer/servers/tflint/README.md
@@ -2,8 +2,13 @@
 
 ## Installing TFLint plugins
 
-To install TFLint plugins, using the same installation of TFLint as nvim-lsp-installer, you may run the `:TFLintInit`
-command. This command assumes that:
+TFLint has [third party plugins](https://github.com/terraform-linters/tflint/blob/master/docs/user-guide/plugins.md) which are not installed by default.
 
-1. there exists a `.tflint.hcl` file in neovim's current working directory.
-1. you've called `server:setup(opts)` for `tflint`
+To install TFLint plugins, there's a convenient `:TFLintInit` command that does this for you. It will use Neovim's
+current working directory to locate the plugins to install (according to `tflint --init`):
+
+```
+:TFLintInit
+```
+
+The `:TFLintInit` command will only be available once the `tflint` server has been set up.
diff --git a/lua/nvim-lsp-installer/servers/tflint/init.lua b/lua/nvim-lsp-installer/servers/tflint/init.lua
index 421333c5..bf370fe3 100644
--- a/lua/nvim-lsp-installer/servers/tflint/init.lua
+++ b/lua/nvim-lsp-installer/servers/tflint/init.lua
@@ -1,14 +1,44 @@
+local a = require "nvim-lsp-installer.core.async"
+local notify = require "nvim-lsp-installer.notify"
+local spawn = require "nvim-lsp-installer.core.spawn"
+local process = require "nvim-lsp-installer.core.process"
 local server = require "nvim-lsp-installer.server"
 local functional = require "nvim-lsp-installer.core.functional"
-local a = require "nvim-lsp-installer.core.async"
 local platform = require "nvim-lsp-installer.core.platform"
 local github = require "nvim-lsp-installer.core.managers.github"
-local spawn = require "nvim-lsp-installer.core.spawn"
-local process = require "nvim-lsp-installer.core.process"
+local middleware = require "nvim-lsp-installer.middleware"
 
 local coalesce, when = functional.coalesce, functional.when
 
 return function(name, root_dir)
+    middleware.register_server_hook(name, function()
+        vim.api.nvim_create_user_command(
+            "TFLintInit",
+            a.scope(function()
+                notify "Installing TFLint plugins…"
+                local result = spawn.tflint {
+                    "--init",
+                    cwd = vim.loop.cwd(),
+                    stdio_sink = process.simple_sink(),
+                    with_paths = { root_dir },
+                }
+                if vim.in_fast_event() then
+                    a.scheduler()
+                end
+                result
+                    :on_success(function()
+                        notify "Successfully installed TFLint plugins."
+                    end)
+                    :on_failure(function()
+                        notify("Failed to install TFLint plugins.", vim.log.levels.ERROR)
+                    end)
+            end),
+            {
+                desc = "[nvim-lsp-installer] Runs tflint --init in the current directory.",
+            }
+        )
+    end)
+
     return server.Server:new {
         name = name,
         root_dir = root_dir,
@@ -31,33 +61,6 @@ return function(name, root_dir)
             cmd_env = {
                 PATH = process.extend_path { root_dir },
             },
-            commands = {
-                TFLintInit = {
-                    a.scope(function()
-                        local notify = require "nvim-lsp-installer.notify"
-
-                        notify "Installing TFLint plugins…"
-                        spawn.tflint({
-                            "--init",
-                            cwd = vim.loop.getcwd(),
-                            with_paths = { root_dir },
-                        })
-                            :on_success(function()
-                                if vim.in_fast_event() then
-                                    a.scheduler()
-                                end
-                                notify "Successfully installed TFLint plugins."
-                            end)
-                            :on_failure(function()
-                                if vim.in_fast_event() then
-                                    a.scheduler()
-                                end
-                                notify "Failed to install TFLint plugins."
-                            end)
-                    end),
-                    description = "Runs tflint --init in the current working directory.",
-                },
-            },
         },
     }
 end