diff --git a/nixd/include/nixd/Controller/Controller.h b/nixd/include/nixd/Controller/Controller.h
index 354aca6ed..b33469283 100644
--- a/nixd/include/nixd/Controller/Controller.h
+++ b/nixd/include/nixd/Controller/Controller.h
@@ -106,34 +106,33 @@ class Controller : public lspserver::LSPServer {
       EndWorkDoneProgress(Params);
   }
 
-  std::mutex TUsLock;
+  mutable std::mutex TUsLock;
   llvm::StringMap<std::shared_ptr<NixTU>> TUs;
 
-  template <class T>
-  std::shared_ptr<NixTU> getTU(std::string File,
-                               lspserver::Callback<T> &Reply) {
+  std::shared_ptr<const NixTU> getTU(std::string_view File) const {
     using lspserver::error;
     std::lock_guard G(TUsLock);
     if (!TUs.count(File)) [[unlikely]] {
-      Reply(T{}); // Reply a default constructed response.
       lspserver::elog("cannot get translation unit: {0}", File);
       return nullptr;
     }
-    return TUs[File];
+    return TUs.lookup(File);
   }
 
-  template <class T>
-  std::shared_ptr<nixf::Node> getAST(const NixTU &TU,
-                                     lspserver::Callback<T> &Reply) {
+  static std::shared_ptr<nixf::Node> getAST(const NixTU &TU) {
     using lspserver::error;
     if (!TU.ast()) {
-      Reply(T{});
       lspserver::elog("AST is null on this unit");
       return nullptr;
     }
     return TU.ast();
   }
 
+  std::shared_ptr<const nixf::Node> getAST(std::string_view File) const {
+    auto TU = getTU(File);
+    return TU ? getAST(*TU) : nullptr;
+  }
+
   boost::asio::thread_pool Pool;
 
   /// Action right after a document is added (including updates).
@@ -217,8 +216,9 @@ class Controller : public lspserver::LSPServer {
   void onRename(const lspserver::RenameParams &Params,
                 lspserver::Callback<lspserver::WorkspaceEdit> Reply);
 
-  void onPrepareRename(const lspserver::TextDocumentPositionParams &Params,
-                       lspserver::Callback<lspserver::Range> Reply);
+  void
+  onPrepareRename(const lspserver::TextDocumentPositionParams &Params,
+                  lspserver::Callback<std::optional<lspserver::Range>> Reply);
 
   void onFormat(const lspserver::DocumentFormattingParams &Params,
                 lspserver::Callback<std::vector<lspserver::TextEdit>> Reply);
diff --git a/nixd/lib/Controller/CheckReturn.h b/nixd/lib/Controller/CheckReturn.h
new file mode 100644
index 000000000..011eefc51
--- /dev/null
+++ b/nixd/lib/Controller/CheckReturn.h
@@ -0,0 +1,16 @@
+#pragma once
+
+/// \brief Used for simplify early-returns.
+///
+///   const auto *foo = checkReturn(get(), nullptr)
+#define CheckReturn(x, Ret)                                                    \
+  ({                                                                           \
+    decltype(x) temp = (x);                                                    \
+    if (!temp) {                                                               \
+      return Ret;                                                              \
+    }                                                                          \
+    temp;                                                                      \
+  })
+
+/// \brief Variant of `CheckReturn`, but returns default constructed `CheckTy`
+#define CheckDefault(x) CheckReturn(x, CheckTy{})
diff --git a/nixd/lib/Controller/CodeAction.cpp b/nixd/lib/Controller/CodeAction.cpp
index 7b02fdc7e..ef4baa75c 100644
--- a/nixd/lib/Controller/CodeAction.cpp
+++ b/nixd/lib/Controller/CodeAction.cpp
@@ -3,6 +3,7 @@
 /// [Code Action]:
 /// https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_codeAction
 
+#include "CheckReturn.h"
 #include "Convert.h"
 
 #include "nixd/Controller/Controller.h"
@@ -16,12 +17,15 @@ using namespace lspserver;
 
 void Controller::onCodeAction(const lspserver::CodeActionParams &Params,
                               Callback<std::vector<CodeAction>> Reply) {
+  using CheckTy = std::vector<CodeAction>;
   std::string File(Params.textDocument.uri.file());
   Range Range = Params.range;
   auto Action = [Reply = std::move(Reply), File, Range, this]() mutable {
-    if (auto TU = getTU(File, Reply)) {
-      std::vector<nixf::Diagnostic> Diagnostics = TU->diagnostics();
-      std::vector<CodeAction> Actions;
+    return Reply([&]() -> llvm::Expected<CheckTy> {
+      const auto TU = CheckDefault(getTU(File));
+
+      const auto &Diagnostics = TU->diagnostics();
+      auto Actions = std::vector<CodeAction>();
       Actions.reserve(Diagnostics.size());
       for (const nixf::Diagnostic &D : Diagnostics) {
         auto DRange = toLSPRange(TU->src(), D.range());
@@ -50,8 +54,8 @@ void Controller::onCodeAction(const lspserver::CodeActionParams &Params,
           });
         }
       }
-      Reply(std::move(Actions));
-    }
+      return Actions;
+    }());
   };
   boost::asio::post(Pool, std::move(Action));
 }
diff --git a/nixd/lib/Controller/Completion.cpp b/nixd/lib/Controller/Completion.cpp
index 93dc2343c..f407cb44d 100644
--- a/nixd/lib/Controller/Completion.cpp
+++ b/nixd/lib/Controller/Completion.cpp
@@ -4,6 +4,7 @@
 /// https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_completion
 
 #include "AST.h"
+#include "CheckReturn.h"
 #include "Convert.h"
 
 #include "lspserver/Protocol.h"
@@ -376,72 +377,60 @@ void completeSelect(const nixf::ExprSelect &Select, AttrSetClient &Client,
 
 void Controller::onCompletion(const CompletionParams &Params,
                               Callback<CompletionList> Reply) {
+  using CheckTy = CompletionList;
   auto Action = [Reply = std::move(Reply), URI = Params.textDocument.uri,
                  Pos = toNixfPosition(Params.position), this]() mutable {
-    std::string File(URI.file());
-    if (std::shared_ptr<NixTU> TU = getTU(File, Reply)) [[likely]] {
-      if (std::shared_ptr<nixf::Node> AST = getAST(*TU, Reply)) [[likely]] {
-        const nixf::Node *Desc = AST->descend({Pos, Pos});
-        if (!Desc) {
-          Reply(error("cannot find corresponding node on given position"));
-          return;
-        }
-        if (!Desc->children().empty()) {
-          Reply(CompletionList{});
-          return;
-        }
-        const nixf::Node &N = *Desc;
-        const ParentMapAnalysis &PM = *TU->parentMap();
-        const Node *MaybeUpExpr = PM.upExpr(N);
-        if (!MaybeUpExpr) {
-          // If there is no concrete expression containing the cursor
-          // Reply an empty list.
-          Reply(CompletionList{});
-          return;
-        }
-        // Otherwise, construct the completion list from a set of providers.
-        const Node &UpExpr = *MaybeUpExpr;
-        Reply([&]() -> CompletionList {
-          CompletionList List;
-          const VariableLookupAnalysis &VLA = *TU->variableLookup();
-          try {
-            switch (UpExpr.kind()) {
-            // In these cases, assume the cursor have "variable" scoping.
-            case Node::NK_ExprVar: {
-              completeVarName(VLA, PM,
-                              static_cast<const nixf::ExprVar &>(UpExpr),
-                              *nixpkgsClient(), List.items);
-              break;
-            }
-            // A "select" expression. e.g.
-            // foo.a|
-            // foo.|
-            // foo.a.bar|
-            case Node::NK_ExprSelect: {
-              const auto &Select =
-                  static_cast<const nixf::ExprSelect &>(UpExpr);
-              completeSelect(Select, *nixpkgsClient(), VLA, PM,
-                             N.kind() == Node::NK_Dot, List.items);
-              break;
-            }
-            case Node::NK_ExprAttrs: {
-              completeAttrPath(N, PM, OptionsLock, Options,
-                               ClientCaps.CompletionSnippets, List.items);
-              break;
-            }
-            default:
-              break;
-            }
-          } catch (ExceedSizeError &Err) {
-            List.isIncomplete = true;
+    const auto File = URI.file().str();
+    return Reply([&]() -> llvm::Expected<CompletionList> {
+      const auto TU = CheckDefault(getTU(File));
+      const auto AST = CheckDefault(getAST(*TU));
+
+      const auto *Desc = AST->descend({Pos, Pos});
+      CheckDefault(Desc && Desc->children().empty());
+
+      const auto &N = *Desc;
+      const auto &PM = *TU->parentMap();
+      const auto &UpExpr = *CheckDefault(PM.upExpr(N));
+
+      return [&]() {
+        CompletionList List;
+        const VariableLookupAnalysis &VLA = *TU->variableLookup();
+        try {
+          switch (UpExpr.kind()) {
+          // In these cases, assume the cursor have "variable" scoping.
+          case Node::NK_ExprVar: {
+            completeVarName(VLA, PM, static_cast<const nixf::ExprVar &>(UpExpr),
+                            *nixpkgsClient(), List.items);
+            return List;
+          }
+          // A "select" expression. e.g.
+          // foo.a|
+          // foo.|
+          // foo.a.bar|
+          case Node::NK_ExprSelect: {
+            const auto &Select = static_cast<const nixf::ExprSelect &>(UpExpr);
+            completeSelect(Select, *nixpkgsClient(), VLA, PM,
+                           N.kind() == Node::NK_Dot, List.items);
+            return List;
+          }
+          case Node::NK_ExprAttrs: {
+            completeAttrPath(N, PM, OptionsLock, Options,
+                             ClientCaps.CompletionSnippets, List.items);
+            return List;
           }
+          default:
+            return List;
+          }
+        } catch (ExceedSizeError &Err) {
+          List.isIncomplete = true;
           return List;
-        }());
-      }
-    }
+        }
+      }();
+    }());
   };
   boost::asio::post(Pool, std::move(Action));
 }
+#undef check
 
 void Controller::onCompletionItemResolve(const CompletionItem &Params,
                                          Callback<CompletionItem> Reply) {
diff --git a/nixd/lib/Controller/Definition.cpp b/nixd/lib/Controller/Definition.cpp
index 8e2394812..9d54f8668 100644
--- a/nixd/lib/Controller/Definition.cpp
+++ b/nixd/lib/Controller/Definition.cpp
@@ -5,6 +5,7 @@
 
 #include "Definition.h"
 #include "AST.h"
+#include "CheckReturn.h"
 #include "Convert.h"
 
 #include "nixd/Controller/Controller.h"
@@ -354,52 +355,43 @@ const Definition &nixd::findDefinition(const Node &N,
   return findVarDefinition(*Var, VLA);
 }
 
+#define Check(x) CheckReturn(x, nullptr)
 void Controller::onDefinition(const TextDocumentPositionParams &Params,
                               Callback<llvm::json::Value> Reply) {
   auto Action = [Reply = std::move(Reply), URI = Params.textDocument.uri,
                  Pos = toNixfPosition(Params.position), this]() mutable {
-    std::string File(URI.file());
-    if (std::shared_ptr<NixTU> TU = getTU(File, Reply)) [[likely]] {
-      if (std::shared_ptr<Node> AST = getAST(*TU, Reply)) [[likely]] {
-        const VariableLookupAnalysis &VLA = *TU->variableLookup();
-        const ParentMapAnalysis &PM = *TU->parentMap();
-        const Node *MaybeN = AST->descend({Pos, Pos});
-        if (!MaybeN) [[unlikely]] {
-          Reply(error("cannot find AST node on given position"));
-          return;
+    const auto File = URI.file().str();
+    return Reply([&]() -> llvm::Expected<llvm::json::Value> {
+      const auto TU = Check(getTU(File));
+      const auto AST = Check(getAST(*TU));
+      const auto &VLA = *TU->variableLookup();
+      const auto &PM = *TU->parentMap();
+      const auto &N = *Check(AST->descend({Pos, Pos}));
+      const auto &UpExpr = *Check(PM.upExpr(N));
+
+      return squash([&]() -> llvm::Expected<Locations> {
+        // Special case for inherited names.
+        if (const ExprVar *Var = findInheritVar(N, PM, VLA))
+          return defineVar(*Var, VLA, PM, *nixpkgsClient(), URI, TU->src());
+
+        switch (UpExpr.kind()) {
+        case Node::NK_ExprVar: {
+          const auto &Var = static_cast<const ExprVar &>(UpExpr);
+          return defineVar(Var, VLA, PM, *nixpkgsClient(), URI, TU->src());
         }
-        const Node &N = *MaybeN;
-        const Node *MaybeUpExpr = PM.upExpr(N);
-        if (!MaybeUpExpr) {
-          Reply(nullptr);
-          return;
+        case Node::NK_ExprSelect: {
+          const auto &Sel = static_cast<const ExprSelect &>(UpExpr);
+          return defineSelect(Sel, VLA, PM, *nixpkgsClient());
         }
-
-        const Node &UpExpr = *MaybeUpExpr;
-
-        return Reply(squash([&]() -> llvm::Expected<Locations> {
-          // Special case for inherited names.
-          if (const ExprVar *Var = findInheritVar(N, PM, VLA))
-            return defineVar(*Var, VLA, PM, *nixpkgsClient(), URI, TU->src());
-
-          switch (UpExpr.kind()) {
-          case Node::NK_ExprVar: {
-            const auto &Var = static_cast<const ExprVar &>(UpExpr);
-            return defineVar(Var, VLA, PM, *nixpkgsClient(), URI, TU->src());
-          }
-          case Node::NK_ExprSelect: {
-            const auto &Sel = static_cast<const ExprSelect &>(UpExpr);
-            return defineSelect(Sel, VLA, PM, *nixpkgsClient());
-          }
-          case Node::NK_ExprAttrs:
-            return defineAttrPath(N, PM, OptionsLock, Options);
-          default:
-            break;
-          }
-          return error("unknown node type for definition");
-        }()));
-      }
-    }
+        case Node::NK_ExprAttrs:
+          return defineAttrPath(N, PM, OptionsLock, Options);
+        default:
+          break;
+        }
+        return error("unknown node type for definition");
+      }());
+    }());
   };
   boost::asio::post(Pool, std::move(Action));
 }
+#undef Check
diff --git a/nixd/lib/Controller/DocumentHighlight.cpp b/nixd/lib/Controller/DocumentHighlight.cpp
index 7587faaf0..e7fcdb99d 100644
--- a/nixd/lib/Controller/DocumentHighlight.cpp
+++ b/nixd/lib/Controller/DocumentHighlight.cpp
@@ -3,6 +3,7 @@
 /// [Document Highlight]:
 /// https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_documentHighlight
 
+#include "CheckReturn.h"
 #include "Convert.h"
 #include "Definition.h"
 
@@ -55,26 +56,24 @@ std::vector<DocumentHighlight> highlight(const nixf::Node &Desc,
 void Controller::onDocumentHighlight(
     const TextDocumentPositionParams &Params,
     Callback<std::vector<DocumentHighlight>> Reply) {
+  using CheckTy = std::vector<DocumentHighlight>;
   auto Action = [Reply = std::move(Reply), URI = Params.textDocument.uri,
                  Pos = toNixfPosition(Params.position), this]() mutable {
     std::string File(URI.file());
-    if (std::shared_ptr<NixTU> TU = getTU(File, Reply)) [[likely]] {
-      if (std::shared_ptr<nixf::Node> AST = getAST(*TU, Reply)) [[likely]] {
-        const nixf::Node *Desc = AST->descend({Pos, Pos});
-        if (!Desc) {
-          Reply(error("cannot find corresponding node on given position"));
-          return;
-        }
-        try {
-          const auto &PM = *TU->parentMap();
-          const auto &VLA = *TU->variableLookup();
-          return Reply(highlight(*Desc, PM, VLA, URI, TU->src()));
-        } catch (std::exception &E) {
-          elog("textDocument/documentHighlight failed: {0}", E.what());
-          return Reply(std::vector<DocumentHighlight>{});
-        }
+    return Reply([&]() -> llvm::Expected<CheckTy> {
+      const auto TU = CheckDefault(getTU(File));
+      const auto AST = CheckDefault(getAST(*TU));
+
+      const auto &Desc = *CheckDefault(AST->descend({Pos, Pos}));
+      try {
+        const auto &PM = *TU->parentMap();
+        const auto &VLA = *TU->variableLookup();
+        return highlight(Desc, PM, VLA, URI, TU->src());
+      } catch (std::exception &E) {
+        elog("textDocument/documentHighlight failed: {0}", E.what());
+        return CheckTy{};
       }
-    }
+    }());
   };
   boost::asio::post(Pool, std::move(Action));
 }
diff --git a/nixd/lib/Controller/DocumentLink.cpp b/nixd/lib/Controller/DocumentLink.cpp
index 43fa3511e..e1a9404ab 100644
--- a/nixd/lib/Controller/DocumentLink.cpp
+++ b/nixd/lib/Controller/DocumentLink.cpp
@@ -15,6 +15,7 @@
 ///     FIXME: support flake ref
 ///
 
+#include "CheckReturn.h"
 #include "Convert.h"
 
 #include "nixd/Controller/Controller.h"
@@ -80,16 +81,18 @@ void dfs(const Node *N, const std::string &BasePath,
 void Controller::onDocumentLink(
     const DocumentLinkParams &Params,
     lspserver::Callback<std::vector<DocumentLink>> Reply) {
+  using CheckTy = std::vector<DocumentLink>;
   auto Action = [File = Params.textDocument.uri.file().str(),
                  Reply = std::move(Reply), this]() mutable {
-    if (std::shared_ptr<NixTU> TU = getTU(File, Reply)) [[likely]] {
-      if (std::shared_ptr<nixf::Node> AST = getAST(*TU, Reply)) [[likely]] {
-        // Traverse the AST, provide the links
-        std::vector<DocumentLink> Links;
-        dfs(AST.get(), File, Links, TU->src());
-        Reply(std::move(Links));
-      }
-    }
+    return Reply([&]() -> llvm::Expected<CheckTy> {
+      const auto TU = CheckDefault(getTU(File));
+      const auto AST = CheckDefault(getAST(*TU));
+
+      // Traverse the AST, provide the links
+      std::vector<DocumentLink> Links;
+      dfs(AST.get(), File, Links, TU->src());
+      return Links;
+    }());
   };
   boost::asio::post(Pool, std::move(Action));
 }
diff --git a/nixd/lib/Controller/DocumentSymbol.cpp b/nixd/lib/Controller/DocumentSymbol.cpp
index e0dbd21ad..da23f25fa 100644
--- a/nixd/lib/Controller/DocumentSymbol.cpp
+++ b/nixd/lib/Controller/DocumentSymbol.cpp
@@ -3,6 +3,7 @@
 /// [Document Symbol]:
 /// https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_documentSymbol
 
+#include "CheckReturn.h"
 #include "Convert.h"
 
 #include "nixd/Controller/Controller.h"
@@ -236,15 +237,16 @@ void collect(const Node *AST, std::vector<DocumentSymbol> &Symbols,
 
 void Controller::onDocumentSymbol(const DocumentSymbolParams &Params,
                                   Callback<std::vector<DocumentSymbol>> Reply) {
+  using CheckTy = std::vector<DocumentSymbol>;
   auto Action = [Reply = std::move(Reply), URI = Params.textDocument.uri,
                  this]() mutable {
-    if (std::shared_ptr<NixTU> TU = getTU(URI.file().str(), Reply)) {
-      if (std::shared_ptr<Node> AST = getAST(*TU, Reply)) {
-        std::vector<DocumentSymbol> Symbols;
-        collect(AST.get(), Symbols, *TU->variableLookup(), TU->src());
-        Reply(std::move(Symbols));
-      }
-    }
+    return Reply([&]() -> llvm::Expected<CheckTy> {
+      const auto TU = CheckDefault(getTU(URI.file().str()));
+      const auto AST = CheckDefault(getAST(*TU));
+      auto Symbols = std::vector<DocumentSymbol>();
+      collect(AST.get(), Symbols, *TU->variableLookup(), TU->src());
+      return Symbols;
+    }());
   };
   boost::asio::post(Pool, std::move(Action));
 }
diff --git a/nixd/lib/Controller/FindReferences.cpp b/nixd/lib/Controller/FindReferences.cpp
index f64f4ded2..041388b15 100644
--- a/nixd/lib/Controller/FindReferences.cpp
+++ b/nixd/lib/Controller/FindReferences.cpp
@@ -3,6 +3,7 @@
 /// [Find References]:
 /// https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_references
 
+#include "CheckReturn.h"
 #include "Convert.h"
 #include "Definition.h"
 
@@ -48,25 +49,22 @@ std::vector<Location> findReferences(const nixf::Node &Desc,
 
 void Controller::onReferences(const TextDocumentPositionParams &Params,
                               Callback<std::vector<Location>> Reply) {
+  using CheckTy = std::vector<Location>;
   auto Action = [Reply = std::move(Reply), URI = Params.textDocument.uri,
                  Pos = toNixfPosition(Params.position), this]() mutable {
     std::string File(URI.file());
-    if (std::shared_ptr<NixTU> TU = getTU(File, Reply)) [[likely]] {
-      if (std::shared_ptr<nixf::Node> AST = getAST(*TU, Reply)) [[likely]] {
-        const nixf::Node *Desc = AST->descend({Pos, Pos});
-        if (!Desc) {
-          return Reply(
-              error("cannot find corresponding node on given position"));
-        }
-        const auto &PM = *TU->parentMap();
-        const auto &VLA = *TU->variableLookup();
-        try {
-          return Reply(findReferences(*Desc, PM, VLA, URI, TU->src()));
-        } catch (std::exception &E) {
-          return Reply(error("references: {0}", E.what()));
-        }
+    return Reply([&]() -> llvm::Expected<CheckTy> {
+      const auto TU = CheckDefault(getTU(File));
+      const auto AST = CheckDefault(getAST(*TU));
+      const auto &Desc = CheckDefault(AST->descend({Pos, Pos}));
+      const auto &PM = *TU->parentMap();
+      const auto &VLA = *TU->variableLookup();
+      try {
+        return findReferences(*Desc, PM, VLA, URI, TU->src());
+      } catch (std::exception &E) {
+        return error("references: {0}", E.what());
       }
-    }
+    }());
   };
   boost::asio::post(Pool, std::move(Action));
 }
diff --git a/nixd/lib/Controller/Hover.cpp b/nixd/lib/Controller/Hover.cpp
index 0ffe10f9d..87015d5b0 100644
--- a/nixd/lib/Controller/Hover.cpp
+++ b/nixd/lib/Controller/Hover.cpp
@@ -4,6 +4,7 @@
 /// https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_hover
 
 #include "AST.h"
+#include "CheckReturn.h"
 #include "Convert.h"
 
 #include "nixd/Controller/Controller.h"
@@ -116,84 +117,79 @@ class NixpkgsHoverProvider {
 
 void Controller::onHover(const TextDocumentPositionParams &Params,
                          Callback<std::optional<Hover>> Reply) {
+  using CheckTy = std::optional<Hover>;
   auto Action = [Reply = std::move(Reply),
                  File = std::string(Params.textDocument.uri.file()),
                  RawPos = Params.position, this]() mutable {
-    if (std::shared_ptr<NixTU> TU = getTU(File, Reply)) [[likely]] {
-      if (std::shared_ptr<nixf::Node> AST = getAST(*TU, Reply)) [[likely]] {
-        nixf::Position Pos{RawPos.line, RawPos.character};
-        const nixf::Node *N = AST->descend({Pos, Pos});
-        if (!N) {
-          Reply(std::nullopt);
-          return;
-        }
-        std::string Name = N->name();
-        const VariableLookupAnalysis &VLA = *TU->variableLookup();
-        const ParentMapAnalysis &PM = *TU->parentMap();
-        if (havePackageScope(*N, VLA, PM) && nixpkgsClient()) {
-          // Ask nixpkgs client what's current package documentation.
-          NixpkgsHoverProvider NHP(*nixpkgsClient());
-          auto [Scope, Name] = getScopeAndPrefix(*N, PM);
-          if (std::optional<std::string> Doc =
-                  NHP.resolvePackage(Scope, Name)) {
-            Reply(Hover{
-                .contents =
-                    MarkupContent{
-                        .kind = MarkupKind::Markdown,
-                        .value = std::move(*Doc),
-                    },
-                .range = toLSPRange(TU->src(), N->range()),
-            });
-            return;
-          }
+    return Reply([&]() -> llvm::Expected<CheckTy> {
+      const auto TU = CheckDefault(getTU(File));
+      const auto AST = CheckDefault(getAST(*TU));
+      const auto Pos = nixf::Position{RawPos.line, RawPos.character};
+      const auto &N = *CheckDefault(AST->descend({Pos, Pos}));
+
+      const auto Name = std::string(N.name());
+      const auto &VLA = *TU->variableLookup();
+      const auto &PM = *TU->parentMap();
+      if (havePackageScope(N, VLA, PM) && nixpkgsClient()) {
+        // Ask nixpkgs client what's current package documentation.
+        auto NHP = NixpkgsHoverProvider(*nixpkgsClient());
+        const auto [Scope, Name] = getScopeAndPrefix(N, PM);
+        if (std::optional<std::string> Doc = NHP.resolvePackage(Scope, Name)) {
+          return Hover{
+              .contents =
+                  MarkupContent{
+                      .kind = MarkupKind::Markdown,
+                      .value = std::move(*Doc),
+                  },
+              .range = toLSPRange(TU->src(), N.range()),
+          };
         }
+      }
 
-        std::vector<std::string> Scope;
-        auto R = findAttrPath(*N, PM, Scope);
-        if (R == FindAttrPathResult::OK) {
-          std::lock_guard _(OptionsLock);
-          for (const auto &[_, Client] : Options) {
-            if (AttrSetClient *C = Client->client()) {
-              OptionsHoverProvider OHP(*C);
-              std::optional<OptionDescription> Desc = OHP.resolveHover(Scope);
-              std::string Docs;
-              if (Desc) {
-                if (Desc->Type) {
-                  std::string TypeName = Desc->Type->Name.value_or("");
-                  std::string TypeDesc = Desc->Type->Description.value_or("");
-                  Docs += llvm::formatv("{0} ({1})", TypeName, TypeDesc);
-                } else {
-                  Docs += "? (missing type)";
-                }
-                if (Desc->Description) {
-                  Docs += "\n\n" + Desc->Description.value_or("");
-                }
-                Reply(Hover{
-                    .contents =
-                        MarkupContent{
-                            .kind = MarkupKind::Markdown,
-                            .value = std::move(Docs),
-                        },
-                    .range = toLSPRange(TU->src(), N->range()),
-                });
-                return;
+      auto Scope = std::vector<std::string>();
+      const auto R = findAttrPath(N, PM, Scope);
+      if (R == FindAttrPathResult::OK) {
+        std::lock_guard _(OptionsLock);
+        for (const auto &[_, Client] : Options) {
+          if (AttrSetClient *C = Client->client()) {
+            OptionsHoverProvider OHP(*C);
+            std::optional<OptionDescription> Desc = OHP.resolveHover(Scope);
+            std::string Docs;
+            if (Desc) {
+              if (Desc->Type) {
+                std::string TypeName = Desc->Type->Name.value_or("");
+                std::string TypeDesc = Desc->Type->Description.value_or("");
+                Docs += llvm::formatv("{0} ({1})", TypeName, TypeDesc);
+              } else {
+                Docs += "? (missing type)";
+              }
+              if (Desc->Description) {
+                Docs += "\n\n" + Desc->Description.value_or("");
               }
+              return Hover{
+                  .contents =
+                      MarkupContent{
+                          .kind = MarkupKind::Markdown,
+                          .value = std::move(Docs),
+                      },
+                  .range = toLSPRange(TU->src(), N.range()),
+              };
             }
           }
         }
-
-        // Reply it's kind by static analysis
-        // FIXME: support more.
-        Reply(Hover{
-            .contents =
-                MarkupContent{
-                    .kind = MarkupKind::Markdown,
-                    .value = "`" + Name + "`",
-                },
-            .range = toLSPRange(TU->src(), N->range()),
-        });
       }
-    }
+
+      // Reply it's kind by static analysis
+      // FIXME: support more.
+      return Hover{
+          .contents =
+              MarkupContent{
+                  .kind = MarkupKind::Markdown,
+                  .value = "`" + Name + "`",
+              },
+          .range = toLSPRange(TU->src(), N.range()),
+      };
+    }());
   };
   boost::asio::post(Pool, std::move(Action));
 }
diff --git a/nixd/lib/Controller/InlayHints.cpp b/nixd/lib/Controller/InlayHints.cpp
index bd9d36d47..2938e0af3 100644
--- a/nixd/lib/Controller/InlayHints.cpp
+++ b/nixd/lib/Controller/InlayHints.cpp
@@ -12,7 +12,9 @@
 ///  nix[: 2.19.3]
 ///
 ///
+
 #include "AST.h"
+#include "CheckReturn.h"
 #include "Convert.h"
 
 #include "nixd/CommandLine/Options.h"
@@ -112,24 +114,27 @@ class NixpkgsInlayHintsProvider {
 
 void Controller::onInlayHint(const InlayHintsParams &Params,
                              Callback<std::vector<InlayHint>> Reply) {
+
+  using CheckTy = std::vector<InlayHint>;
+
   // If not enabled, exit early.
   if (!EnableInlayHints)
     return Reply(std::vector<InlayHint>{});
 
   auto Action = [Reply = std::move(Reply), URI = Params.textDocument.uri,
                  Range = Params.range, this]() mutable {
-    std::string File(URI.file());
-    if (std::shared_ptr<NixTU> TU = getTU(File, Reply)) [[likely]] {
-      if (std::shared_ptr<Node> AST = getAST(*TU, Reply)) [[likely]] {
-        // Perform inlay hints computation on the range.
-        std::vector<InlayHint> Response;
-        NixpkgsInlayHintsProvider NP(*nixpkgsClient(), *TU->variableLookup(),
-                                     *TU->parentMap(), Range, Response,
-                                     TU->src());
-        NP.dfs(AST.get());
-        Reply(std::move(Response));
-      }
-    }
+    const auto File = URI.file();
+    return Reply([&]() -> llvm::Expected<CheckTy> {
+      const auto TU = CheckDefault(getTU(File));
+      const auto AST = CheckDefault(getAST(*TU));
+      // Perform inlay hints computation on the range.
+      std::vector<InlayHint> Response;
+      NixpkgsInlayHintsProvider NP(*nixpkgsClient(), *TU->variableLookup(),
+                                   *TU->parentMap(), Range, Response,
+                                   TU->src());
+      NP.dfs(AST.get());
+      return Response;
+    }());
   };
   boost::asio::post(Pool, std::move(Action));
 }
diff --git a/nixd/lib/Controller/Rename.cpp b/nixd/lib/Controller/Rename.cpp
index 41ba232b8..973c9e798 100644
--- a/nixd/lib/Controller/Rename.cpp
+++ b/nixd/lib/Controller/Rename.cpp
@@ -3,6 +3,7 @@
 /// [Rename]:
 /// https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_rename
 
+#include "CheckReturn.h"
 #include "Convert.h"
 #include "Definition.h"
 
@@ -70,53 +71,52 @@ WorkspaceEdit rename(const nixf::Node &Desc, const std::string &NewText,
 
 void Controller::onRename(const RenameParams &Params,
                           Callback<WorkspaceEdit> Reply) {
+  using CheckTy = WorkspaceEdit;
   auto Action = [Reply = std::move(Reply), URI = Params.textDocument.uri,
                  Pos = toNixfPosition(Params.position),
                  NewText = Params.newName, this]() mutable {
-    std::string File(URI.file());
-    if (std::shared_ptr<NixTU> TU = getTU(File, Reply)) [[likely]] {
-      if (std::shared_ptr<nixf::Node> AST = getAST(*TU, Reply)) [[likely]] {
-        const nixf::Node *Desc = AST->descend({Pos, Pos});
-        if (!Desc) {
-          Reply(error("cannot find corresponding node on given position"));
-          return;
-        }
-        const auto &PM = *TU->parentMap();
-        const auto &VLA = *TU->variableLookup();
-        try {
-          return Reply(rename(*Desc, NewText, PM, VLA, URI, TU->src()));
-        } catch (std::exception &E) {
-          return Reply(error(E.what()));
-        }
+    const auto File = URI.file();
+    return Reply([&]() -> llvm::Expected<WorkspaceEdit> {
+      const auto TU = CheckDefault(getTU(File));
+      const auto AST = CheckDefault(getAST(*TU));
+      const auto &Desc = *CheckReturn(
+          AST->descend({Pos, Pos}),
+          error("cannot find corresponding node on given position"));
+
+      const auto &PM = *TU->parentMap();
+      const auto &VLA = *TU->variableLookup();
+      try {
+        return rename(Desc, NewText, PM, VLA, URI, TU->src());
+      } catch (std::exception &E) {
+        return error(E.what());
       }
-    }
+    }());
   };
   boost::asio::post(Pool, std::move(Action));
 }
 
 void Controller::onPrepareRename(
     const lspserver::TextDocumentPositionParams &Params,
-    Callback<lspserver::Range> Reply) {
+    Callback<std::optional<lspserver::Range>> Reply) {
+  using CheckTy = std::optional<lspserver::Range>;
   auto Action = [Reply = std::move(Reply), URI = Params.textDocument.uri,
                  Pos = toNixfPosition(Params.position), this]() mutable {
-    std::string File(URI.file());
-    if (std::shared_ptr<NixTU> TU = getTU(File, Reply)) [[likely]] {
-      if (std::shared_ptr<nixf::Node> AST = getAST(*TU, Reply)) [[likely]] {
-        const nixf::Node *Desc = AST->descend({Pos, Pos});
-        if (!Desc) {
-          return Reply(
-              error("cannot find corresponding node on given position"));
-        }
-        const auto &PM = *TU->parentMap();
-        const auto &VLA = *TU->variableLookup();
-        try {
-          WorkspaceEdit WE = rename(*Desc, "", PM, VLA, URI, TU->src());
-          return Reply(toLSPRange(TU->src(), Desc->range()));
-        } catch (std::exception &E) {
-          return Reply(error(E.what()));
-        }
+    const auto File = URI.file();
+    return Reply([&]() -> llvm::Expected<CheckTy> {
+      const auto TU = CheckDefault(getTU(File));
+      const auto AST = CheckDefault(getAST(*TU));
+
+      const auto &Desc = *CheckDefault(AST->descend({Pos, Pos}));
+
+      const auto &PM = *TU->parentMap();
+      const auto &VLA = *TU->variableLookup();
+      try {
+        WorkspaceEdit WE = rename(Desc, "", PM, VLA, URI, TU->src());
+        return toLSPRange(TU->src(), Desc.range());
+      } catch (std::exception &E) {
+        return error(E.what());
       }
-    }
+    }());
   };
   boost::asio::post(Pool, std::move(Action));
 }
diff --git a/nixd/lib/Controller/SemanticTokens.cpp b/nixd/lib/Controller/SemanticTokens.cpp
index a8a090d4c..42716d90e 100644
--- a/nixd/lib/Controller/SemanticTokens.cpp
+++ b/nixd/lib/Controller/SemanticTokens.cpp
@@ -3,6 +3,7 @@
 /// [Semantic Tokens]:
 /// https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_semanticTokens
 
+#include "CheckReturn.h"
 #include "Convert.h"
 
 #include "nixd/Controller/Controller.h"
@@ -235,15 +236,17 @@ class SemanticTokenBuilder {
 
 void Controller::onSemanticTokens(const SemanticTokensParams &Params,
                                   Callback<SemanticTokens> Reply) {
+  using CheckTy = SemanticTokens;
   auto Action = [Reply = std::move(Reply), URI = Params.textDocument.uri,
                  this]() mutable {
-    if (std::shared_ptr<NixTU> TU = getTU(URI.file().str(), Reply)) {
-      if (std::shared_ptr<Node> AST = getAST(*TU, Reply)) {
-        SemanticTokenBuilder Builder(*TU->variableLookup(), TU->src());
-        Builder.dfs(AST.get());
-        Reply(SemanticTokens{.tokens = Builder.finish()});
-      }
-    }
+    const auto File = URI.file();
+    return Reply([&]() -> llvm::Expected<CheckTy> {
+      const auto TU = CheckDefault(getTU(File));
+      const auto AST = CheckDefault(getAST(*TU));
+      SemanticTokenBuilder Builder(*TU->variableLookup(), TU->src());
+      Builder.dfs(AST.get());
+      return SemanticTokens{.tokens = Builder.finish()};
+    }());
   };
   boost::asio::post(Pool, std::move(Action));
 }