Use default config on missing configuration section

On serving initialize request, the deserialization of HIE configuration
embedded in InitializeParam passed by client will result in an error if
during the deserialization process the server cannot find HIE specific
configuration key under initializationOptions.

This commit changes the initializationOptions deserialization to return
the default configuration if configuration key cannot be found under
initializationOptions. Here, setting the key with a value of null will
also be considered as part of not found condition to accommodate clients
that fills missing user options as null.

Author: aufarg

hie-plugin-api/Haskell/Ide/Engine/Config.hs

@@ -55,16 +55,18 @@ instance Default Config where
 -- TODO: Add API for plugins to expose their own LSP config options
 instance FromJSON Config where
   parseJSON = withObject "Config" $ \v -> do
-    s <- v .: "haskell" <|> v.: "languageServerHaskell"
-    flip (withObject "Config.settings") s $ \o -> Config
-      <$> o .:? "hlintOn"                     .!= hlintOn def
-      <*> o .:? "diagnosticsOnChange"         .!= diagnosticsOnChange def
-      <*> o .:? "maxNumberOfProblems"         .!= maxNumberOfProblems def
-      <*> o .:? "diagnosticsDebounceDuration" .!= diagnosticsDebounceDuration def
-      <*> o .:? "liquidOn"                    .!= liquidOn def
-      <*> o .:? "completionSnippetsOn"        .!= completionSnippetsOn def
-      <*> o .:? "formatOnImportOn"            .!= formatOnImportOn def
-      <*> o .:? "formattingProvider"          .!= formattingProvider def
+    c <- v .:! "haskell" <|> v.:! "languageServerHaskell"
+    case c of
+      Nothing -> return def
+      Just s -> flip (withObject "Config.settings") s $ \o -> Config
+        <$> o .:? "hlintOn"                     .!= hlintOn def
+        <*> o .:? "diagnosticsOnChange"         .!= diagnosticsOnChange def
+        <*> o .:? "maxNumberOfProblems"         .!= maxNumberOfProblems def
+        <*> o .:? "diagnosticsDebounceDuration" .!= diagnosticsDebounceDuration def
+        <*> o .:? "liquidOn"                    .!= liquidOn def
+        <*> o .:? "completionSnippetsOn"        .!= completionSnippetsOn def
+        <*> o .:? "formatOnImportOn"            .!= formatOnImportOn def
+        <*> o .:? "formattingProvider"          .!= formattingProvider def
 
 -- 2017-10-09 23:22:00.710515298 [ThreadId 11] - ---> {"jsonrpc":"2.0","method":"workspace/didChangeConfiguration","params":{"settings":{"haskell":{"maxNumberOfProblems":100,"hlintOn":true}}}}
 -- 2017-10-09 23:22:00.710667381 [ThreadId 15] - reactor:got didChangeConfiguration notification: