New CLI flag --replace-eval-errors

mightyiam · fzakaria · mightyiam · commit 79256341a5a1 · 2025-05-12T18:44:07.000+07:00
Co-authored-by: Farid Zakaria &lt;farid.m.zakaria@gmail.com&gt;
diff --git a/doc/manual/source/command-ref/nix-instantiate.md b/doc/manual/source/command-ref/nix-instantiate.md
@@ -112,6 +112,11 @@ standard input.
   When used with `--eval`, print the resulting value as an JSON
   representation of the abstract syntax tree rather than as a Nix expression.
 
+- `--replace-eval-errors`
+
+  When used with `--eval` and `--json`, replace any evaluation errors with the string
+  `"«evaluation error»"`.
+
 - `--xml`
 
   When used with `--eval`, print the resulting value as an XML
@@ -205,3 +210,10 @@ $ nix-instantiate --eval --xml --strict --expr '{ x = {}; }'
   </attrs>
 </expr>
 ```
+
+Replacing evaluation errors:
+
+```console
+$ nix-instantiate --eval --json --replace-eval-errors --expr '{ a = throw "fail"; }'
+{"a":"«evaluation error»"}
+```
diff --git a/src/libexpr-tests/json.cc b/src/libexpr-tests/json.cc
@@ -9,7 +9,7 @@ namespace nix {
             std::string getJSONValue(Value& value) {
                 std::stringstream ss;
                 NixStringContext ps;
-                printValueAsJSON(state, true, value, noPos, ss, ps);
+                printValueAsJSON(state, true, false, value, noPos, ss, ps);
                 return ss.str();
             }
     };
diff --git a/src/libexpr/include/nix/expr/value-to-json.hh b/src/libexpr/include/nix/expr/value-to-json.hh
@@ -10,10 +10,10 @@
 
 namespace nix {
 
-nlohmann::json printValueAsJSON(EvalState & state, bool strict,
+nlohmann::json printValueAsJSON(EvalState & state, bool strict, bool replaceEvalErrors,
     Value & v, const PosIdx pos, NixStringContext & context, bool copyToStore = true);
 
-void printValueAsJSON(EvalState & state, bool strict,
+void printValueAsJSON(EvalState & state, bool strict, bool replaceEvalErrors,
     Value & v, const PosIdx pos, std::ostream & str, NixStringContext & context, bool copyToStore = true);
 
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
@@ -1388,7 +1388,7 @@ static void derivationStrictInternal(
 
                     if (i->name == state.sStructuredAttrs) continue;
 
-                    jsonObject->emplace(key, printValueAsJSON(state, true, *i->value, pos, context));
+                    jsonObject->emplace(key, printValueAsJSON(state, true, false, *i->value, pos, context));
 
                     if (i->name == state.sBuilder)
                         drv.builder = state.forceString(*i->value, context, pos, context_below);
@@ -2328,7 +2328,7 @@ static void prim_toJSON(EvalState & state, const PosIdx pos, Value * * args, Val
 {
     std::ostringstream out;
     NixStringContext context;
-    printValueAsJSON(state, true, *args[0], pos, out, context);
+    printValueAsJSON(state, true, false, *args[0], pos, out, context);
     v.mkString(toView(out), context);
 }
 
diff --git a/src/libexpr/primops/fetchTree.cc b/src/libexpr/primops/fetchTree.cc
@@ -135,7 +135,7 @@ static void fetchTree(
                 attrs.emplace(state.symbols[attr.name], uint64_t(intValue));
             } else if (state.symbols[attr.name] == "publicKeys") {
                 experimentalFeatureSettings.require(Xp::VerifiedFetches);
-                attrs.emplace(state.symbols[attr.name], printValueAsJSON(state, true, *attr.value, pos, context).dump());
+                attrs.emplace(state.symbols[attr.name], printValueAsJSON(state, true, false, *attr.value, pos, context).dump());
             }
             else
                 state.error<TypeError>("argument '%s' to '%s' is %s while a string, Boolean or integer is expected",
diff --git a/src/libexpr/value-to-json.cc b/src/libexpr/value-to-json.cc
@@ -6,12 +6,13 @@
 #include <cstdlib>
 #include <iomanip>
 #include <nlohmann/json.hpp>
+#include <typeinfo>
 
 
 namespace nix {
 using json = nlohmann::json;
 // TODO: rename. It doesn't print.
-json printValueAsJSON(EvalState & state, bool strict,
+json printValueAsJSON(EvalState & state, bool strict, bool replaceEvalErrors,
     Value & v, const PosIdx pos, NixStringContext & context, bool copyToStore)
 {
     checkInterrupt();
@@ -54,13 +55,27 @@ json printValueAsJSON(EvalState & state, bool strict,
                 break;
             }
             if (auto i = v.attrs()->get(state.sOutPath))
-                return printValueAsJSON(state, strict, *i->value, i->pos, context, copyToStore);
+                return printValueAsJSON(state, strict, replaceEvalErrors, *i->value, i->pos, context, copyToStore);
             else {
                 out = json::object();
                 for (auto & a : v.attrs()->lexicographicOrder(state.symbols)) {
                     try {
-                        out.emplace(state.symbols[a->name], printValueAsJSON(state, strict, *a->value, a->pos, context, copyToStore));
+                        out.emplace(state.symbols[a->name], printValueAsJSON(state, strict, replaceEvalErrors, *a->value, a->pos, context, copyToStore));
                     } catch (Error & e) {
+                        std::cerr << "Caught an Error of type: " << typeid(e).name() << std::endl;
+                        // std::cerr << "Caught an Error of type: " << e.message() << std::endl;
+                        // std::cerr << "Caught an Error of type: " << e.what() << std::endl;
+
+                        // TODO: Figure out what Error is here?
+                        // We seem to be not catching FileNotFoundError.
+                        bool isEvalError = dynamic_cast<EvalError *>(&e);
+                        bool isFileNotFoundError = dynamic_cast<FileNotFound *>(&e);
+                        // Restrict replaceEvalErrors only only evaluation errors
+                        if (replaceEvalErrors && (isEvalError || isFileNotFoundError)) {
+                            out.emplace(state.symbols[a->name], "«evaluation error»");
+                            continue;
+                        }
+
                         e.addTrace(state.positions[a->pos],
                             HintFmt("while evaluating attribute '%1%'", state.symbols[a->name]));
                         throw;
@@ -75,8 +90,9 @@ json printValueAsJSON(EvalState & state, bool strict,
             int i = 0;
             for (auto elem : v.listItems()) {
                 try {
-                    out.push_back(printValueAsJSON(state, strict, *elem, pos, context, copyToStore));
+                    out.push_back(printValueAsJSON(state, strict, replaceEvalErrors, *elem, pos, context, copyToStore));
                 } catch (Error & e) {
+                    // TODO: Missing catch
                     e.addTrace(state.positions[pos],
                         HintFmt("while evaluating list element at index %1%", i));
                     throw;
@@ -106,11 +122,11 @@ json printValueAsJSON(EvalState & state, bool strict,
     return out;
 }
 
-void printValueAsJSON(EvalState & state, bool strict,
+void printValueAsJSON(EvalState & state, bool strict, bool replaceEvalErrors,
     Value & v, const PosIdx pos, std::ostream & str, NixStringContext & context, bool copyToStore)
 {
     try {
-        str << printValueAsJSON(state, strict, v, pos, context, copyToStore);
+        str << printValueAsJSON(state, strict, replaceEvalErrors, v, pos, context, copyToStore);
     } catch (nlohmann::json::exception & e) {
         throw JSONSerializationError("JSON serialization error: %s", e.what());
     }
diff --git a/src/libflake/flake.cc b/src/libflake/flake.cc
@@ -91,7 +91,7 @@ static void parseFlakeInputAttr(
             if (attr.name == state.symbols.create("publicKeys")) {
                 experimentalFeatureSettings.require(Xp::VerifiedFetches);
                 NixStringContext emptyContext = {};
-                attrs.emplace(state.symbols[attr.name], printValueAsJSON(state, true, *attr.value, attr.pos, emptyContext).dump());
+                attrs.emplace(state.symbols[attr.name], printValueAsJSON(state, true, false, *attr.value, attr.pos, emptyContext).dump());
             } else
                 state.error<TypeError>("flake input attribute '%s' is %s while a string, Boolean, or integer is expected",
                     state.symbols[attr.name], showType(*attr.value)).debugThrow();
diff --git a/src/libutil/posix-source-accessor.cc b/src/libutil/posix-source-accessor.cc
@@ -54,7 +54,7 @@ void PosixSourceAccessor::readFile(
     #endif
         ));
     if (!fd)
-        throw SysError("opening file '%1%'", ap.string());
+        throw SysError("opening file9 '%1%'", ap.string());
 
     struct stat st;
     if (fstat(fromDescriptorReadOnly(fd.get()), &st) == -1)
diff --git a/src/nix-env/nix-env.cc b/src/nix-env/nix-env.cc
@@ -984,7 +984,7 @@ static void queryJSON(Globals & globals, std::vector<PackageInfo> & elems, bool
                         metaObj[j] = nullptr;
                     } else {
                         NixStringContext context;
-                        metaObj[j] = printValueAsJSON(*globals.state, true, *v, noPos, context);
+                        metaObj[j] = printValueAsJSON(*globals.state, true, false, *v, noPos, context);
                     }
                 }
             }
diff --git a/src/nix-instantiate/nix-instantiate.cc b/src/nix-instantiate/nix-instantiate.cc
@@ -28,7 +28,7 @@ static int rootNr = 0;
 enum OutputKind { okPlain, okRaw, okXML, okJSON };
 
 void processExpr(EvalState & state, const Strings & attrPaths,
-    bool parseOnly, bool strict, Bindings & autoArgs,
+    bool parseOnly, bool strict, bool replaceEvalErrors, Bindings & autoArgs,
     bool evalOnly, OutputKind output, bool location, Expr * e)
 {
     if (parseOnly) {
@@ -58,7 +58,7 @@ void processExpr(EvalState & state, const Strings & attrPaths,
             else if (output == okXML)
                 printValueAsXML(state, strict, location, vRes, std::cout, context, noPos);
             else if (output == okJSON) {
-                printValueAsJSON(state, strict, vRes, v.determinePos(noPos), std::cout, context);
+                printValueAsJSON(state, strict, replaceEvalErrors, vRes, v.determinePos(noPos), std::cout, context);
                 std::cout << std::endl;
             } else {
                 if (strict) state.forceValueDeep(vRes);
@@ -106,6 +106,7 @@ static int main_nix_instantiate(int argc, char * * argv)
         OutputKind outputKind = okPlain;
         bool xmlOutputSourceLocation = true;
         bool strict = false;
+        bool replaceEvalErrors = false;
         Strings attrPaths;
         bool wantsReadWrite = false;
 
@@ -147,6 +148,8 @@ static int main_nix_instantiate(int argc, char * * argv)
                 xmlOutputSourceLocation = false;
             else if (*arg == "--strict")
                 strict = true;
+            else if (*arg == "--replace-eval-errors")
+                replaceEvalErrors = true;
             else if (*arg == "--dry-run")
                 settings.readOnlyMode = true;
             else if (*arg != "" && arg->at(0) == '-')
@@ -184,7 +187,7 @@ static int main_nix_instantiate(int argc, char * * argv)
 
         if (readStdin) {
             Expr * e = state->parseStdin();
-            processExpr(*state, attrPaths, parseOnly, strict, autoArgs,
+            processExpr(*state, attrPaths, parseOnly, strict, replaceEvalErrors, autoArgs,
                 evalOnly, outputKind, xmlOutputSourceLocation, e);
         } else if (files.empty() && !fromArgs)
             files.push_back("./default.nix");
@@ -193,7 +196,7 @@ static int main_nix_instantiate(int argc, char * * argv)
             Expr * e = fromArgs
                 ? state->parseExprFromString(i, state->rootPath("."))
                 : state->parseExprFromFile(resolveExprPath(lookupFileArg(*state, i)));
-            processExpr(*state, attrPaths, parseOnly, strict, autoArgs,
+            processExpr(*state, attrPaths, parseOnly, strict, replaceEvalErrors, autoArgs,
                 evalOnly, outputKind, xmlOutputSourceLocation, e);
         }
 
diff --git a/src/nix/eval.cc b/src/nix/eval.cc
@@ -15,6 +15,7 @@ namespace nix::fs { using namespace std::filesystem; }
 struct CmdEval : MixJSON, InstallableValueCommand, MixReadOnlyOption
 {
     bool raw = false;
+    bool replaceEvalErrors = false;
     std::optional<std::string> apply;
     std::optional<fs::path> writeTo;
 
@@ -39,6 +40,12 @@ struct CmdEval : MixJSON, InstallableValueCommand, MixReadOnlyOption
             .labels = {"path"},
             .handler = {&writeTo},
         });
+
+        addFlag({
+            .longName = "replace-eval-errors",
+            .description = "When used with `--json` the Nix evaluator will replace evaluation errors with a fixed value.",
+            .handler = {&replaceEvalErrors, true},
+        });
     }
 
     std::string description() override
@@ -118,7 +125,7 @@ struct CmdEval : MixJSON, InstallableValueCommand, MixReadOnlyOption
         }
 
         else if (json) {
-            printJSON(printValueAsJSON(*state, true, *v, pos, context, false));
+            printJSON(printValueAsJSON(*state, true, replaceEvalErrors, *v, pos, context, false));
         }
 
         else {
diff --git a/src/nix/eval.md b/src/nix/eval.md
@@ -48,6 +48,13 @@ R""(
   # cat ./out/subdir/bla
   123
 
+* Replace evaluation errors:
+
+  ```console
+  $ nix eval --json --replace-eval-errors --expr '{ a = throw "fail"; }'
+  {"a":"«evaluation error»"}
+  ```
+
 # Description
 
 This command evaluates the given Nix expression, and prints the result on standard output.
diff --git a/tests/functional/eval.sh b/tests/functional/eval.sh
@@ -35,6 +35,23 @@ nix-instantiate --eval -E 'assert 1 + 2 == 3; true'
 [[ $(nix-instantiate -A int --eval - < "./eval.nix") == 123 ]]
 [[ "$(nix-instantiate --eval -E '{"assert"=1;bar=2;}')" == '{ "assert" = 1; bar = 2; }' ]]
 
+expected="$(echo '{
+"missingAttr":"«evaluation error»",
+"insideAList":["«evaluation error»"],
+"deeper":{v:"«evaluation error»"},
+"failedAssertion":"«evaluation error»",
+"missingFile":"«evaluation error»",
+"missingImport":"«evaluation error»",
+"outOfBounds":"«evaluation error»",
+"failedCoersion":"«evaluation error»",
+"failedAddition":"«evaluation error»"
+}' | tr -d '\n')"
+actual="$(nix-instantiate --eval --json --strict --replace-eval-errors "./replace-eval-errors.nix")"
+[[ $actual == "$expected" ]] || diff --unified <(echo "$actual") <(echo "$expected") >&2
+
+actual="$(nix eval --json --replace-eval-errors -f "./replace-eval-errors.nix")"
+[[ $actual == "$expected" ]] || diff --unified <(echo "$actual") <(echo "$expected") >&2
+
 # Check that symlink cycles don't cause a hang.
 ln -sfn cycle.nix "$TEST_ROOT/cycle.nix"
 (! nix eval --file "$TEST_ROOT/cycle.nix")
diff --git a/tests/functional/replace-eval-errors.nix b/tests/functional/replace-eval-errors.nix
@@ -0,0 +1,11 @@
+{
+  missingAttr = let bar = { }; in bar.notExist;
+  insideAList = [ (throw "a throw") ];
+  deeper = { v = throw "v"; };
+  failedAssertion = assert true; assert false; null;
+  missingFile = builtins.readFile ./missing-file.txt;
+  missingImport = import ./missing-import.nix;
+  outOfBounds = builtins.elemAt [ 1 2 3 ] 100;
+  failedCoersion = "${1}";
+  failedAddition = 1.0 + "a string";
+}

Original file line number	Diff line number	Diff line change
`@@ -9,7 +9,7 @@ namespace nix {`
`9`	`9`	`std::string getJSONValue(Value& value) {`
`10`	`10`	`std::stringstream ss;`
`11`	`11`	`NixStringContext ps;`
`12`		`- printValueAsJSON(state, true, value, noPos, ss, ps);`
	`12`	`+ printValueAsJSON(state, true, false, value, noPos, ss, ps);`
`13`	`13`	`return ss.str();`
`14`	`14`	`}`
`15`	`15`	`};`
Original file line number	Diff line number	Diff line change
`@@ -1388,7 +1388,7 @@ static void derivationStrictInternal(`
`1388`	`1388`
`1389`	`1389`	`if (i->name == state.sStructuredAttrs) continue;`
`1390`	`1390`
`1391`		`- jsonObject->emplace(key, printValueAsJSON(state, true, *i->value, pos, context));`
	`1391`	`+ jsonObject->emplace(key, printValueAsJSON(state, true, false, *i->value, pos, context));`
`1392`	`1392`
`1393`	`1393`	`if (i->name == state.sBuilder)`
`1394`	`1394`	`drv.builder = state.forceString(*i->value, context, pos, context_below);`
`@@ -2328,7 +2328,7 @@ static void prim_toJSON(EvalState & state, const PosIdx pos, Value * * args, Val`
`2328`	`2328`	`{`
`2329`	`2329`	`std::ostringstream out;`
`2330`	`2330`	`NixStringContext context;`
`2331`		`- printValueAsJSON(state, true, *args[0], pos, out, context);`
	`2331`	`+ printValueAsJSON(state, true, false, *args[0], pos, out, context);`
`2332`	`2332`	`v.mkString(toView(out), context);`
`2333`	`2333`	`}`
`2334`	`2334`
Original file line number	Diff line number	Diff line change
`@@ -135,7 +135,7 @@ static void fetchTree(`
`135`	`135`	`attrs.emplace(state.symbols[attr.name], uint64_t(intValue));`
`136`	`136`	`} else if (state.symbols[attr.name] == "publicKeys") {`
`137`	`137`	`experimentalFeatureSettings.require(Xp::VerifiedFetches);`
`138`		`- attrs.emplace(state.symbols[attr.name], printValueAsJSON(state, true, *attr.value, pos, context).dump());`
	`138`	`+ attrs.emplace(state.symbols[attr.name], printValueAsJSON(state, true, false, *attr.value, pos, context).dump());`
`139`	`139`	`}`
`140`	`140`	`else`
`141`	`141`	`state.error<TypeError>("argument '%s' to '%s' is %s while a string, Boolean or integer is expected",`
Original file line number	Diff line number	Diff line change
`@@ -984,7 +984,7 @@ static void queryJSON(Globals & globals, std::vector<PackageInfo> & elems, bool`
`984`	`984`	`metaObj[j] = nullptr;`
`985`	`985`	`} else {`
`986`	`986`	`NixStringContext context;`
`987`		`- metaObj[j] = printValueAsJSON(globals.state, true, v, noPos, context);`
	`987`	`+ metaObj[j] = printValueAsJSON(globals.state, true, false, v, noPos, context);`
`988`	`988`	`}`
`989`	`989`	`}`
`990`	`990`	`}`