Treat undefined and null initialization options the same way (#605)

* Overhaul Nix configuration

- allow usage of Nix Flakes
- remove `niv` code (may be a controversial change)
- redirect `nix-shell` to `flake.nix`

* Fix minor typo in LSP configuration note

* Treat `undefined` and `null` initialization options the same way

Treat non-existing "initializationOptions", and `"initializationOptions": null`
the same way.

Fixes #604

.gitignore

@@ -21,3 +21,4 @@ hie
 hie.yaml
 .envrc
 **/.golden/*/actual
+/.direnv/

default.nix

@@ -0,0 +1,12 @@
+(import (
+  let
+    lock = builtins.fromJSON (builtins.readFile ./flake.lock);
+    nodeName = lock.nodes.root.inputs.flake-compat;
+  in
+  fetchTarball {
+    url =
+      lock.nodes.${nodeName}.locked.url
+        or "https://github.com/edolstra/flake-compat/archive/${lock.nodes.${nodeName}.locked.rev}.tar.gz";
+    sha256 = lock.nodes.${nodeName}.locked.narHash;
+  }
+) { src = ./.; }).defaultNix

flake.nix

@@ -0,0 +1,39 @@
+{
+  description = "Haskell development environment";
+
+  inputs.flake-compat.url = "https://flakehub.com/f/edolstra/flake-compat/1.tar.gz";
+
+  inputs.flake-utils.url = "github:numtide/flake-utils";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+
+  outputs =
+    {
+      flake-utils,
+      nixpkgs,
+      ...
+    }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+        hpkgs = pkgs.haskellPackages;
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          packages = [
+            # Haskell toolchain.
+            hpkgs.cabal-fmt
+            hpkgs.cabal-install
+            hpkgs.fourmolu
+            hpkgs.ghc
+            hpkgs.haskell-language-server
+
+            # Misc.
+            pkgs.zlib
+            pkgs.pkg-config
+          ];
+        };
+      }
+    );
+}

lsp/src/Language/LSP/Server/Core.hs

@@ -775,13 +775,13 @@ not have to support `workspace/configuration`! In practice,
 many clients seem to follow the sensible approach laid out here:
 https://github.com/microsoft/language-server-protocol/issues/972#issuecomment-626668243
 
-To make this work, we try to be tolerant by using the following strategy.
-When we receive a configuration object from any of the sources above, we first
-check to see if it has a field corresponding to our configuration section. If it
-does, then we assume that it our config and try to parse it. If it does not, we
-try to parse the entire config object. This hopefully lets us handle a variety
-of sensible cases where the client sends us mostly our config, either wrapped
-in our section or not.
+To make this work, we try to be tolerant by using the following strategy. When
+we receive a configuration object from any of the sources above, we first check
+to see if it has a field corresponding to our configuration section. If it does,
+then we assume that it is our config and try to parse it. If it does not parse,
+we try to parse the entire config object. This hopefully lets us handle a
+variety of sensible cases where the client sends us mostly our config, either
+wrapped in our section or not.
 -}
 
 {- Note [Client- versus server-initiated progress]

lsp/src/Language/LSP/Server/Processing.hs

@@ -33,6 +33,7 @@ import Data.Aeson hiding (
   Null,
   Options,
  )
+import Data.Aeson qualified as J
 import Data.Aeson.Lens ()
 import Data.Aeson.Types hiding (
   Error,
@@ -145,6 +146,9 @@ initializeRequestHandler logger ServerDefinition{..} vfs sendFunc req = do
         configObject = lookForConfigSection configSection <$> (p ^. L.initializationOptions)
 
     initialConfig <- case configObject of
+      -- Treat non-existing "initializationOptions" and `"initializationOptions": null` the same way.
+      Nothing -> pure defaultConfig
+      Just J.Null -> pure defaultConfig
       Just o -> case parseConfig defaultConfig o of
         Right newConfig -> do
           liftIO $ logger <& LspCore (NewConfig o) `WithSeverity` Debug
@@ -153,7 +157,6 @@ initializeRequestHandler logger ServerDefinition{..} vfs sendFunc req = do
           -- Warn not error here, since initializationOptions is pretty unspecified
           liftIO $ logger <& LspCore (ConfigurationParseError o err) `WithSeverity` Warning
           pure defaultConfig
-      Nothing -> pure defaultConfig
 
     stateVars <- liftIO $ do
       resVFS <- newTVarIO (VFSData vfs mempty)