New CLI flag --replace-eval-errors

Co-authored-by: Farid Zakaria <farid.m.zakaria@gmail.com>

Author: mightyiam

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»"}
+```

src/libexpr/include/nix/expr/eval-settings.hh

@@ -53,6 +53,13 @@ struct EvalSettings : Config
 
     std::vector<PrimOp> extraPrimOps;
 
+    Setting<bool> replaceEvalErrors{
+        this, false, "replace-eval-errors",
+        R"(
+          If set to `true`, the Nix evaluator will replace evaluation errors
+          with a fixed value.
+        )"};
+
     Setting<bool> enableNativeCode{this, false, "allow-unsafe-native-code-during-evaluation", R"(
         Enable built-in functions that allow executing native code.
 

src/libexpr/value-to-json.cc

@@ -61,6 +61,15 @@ json printValueAsJSON(EvalState & state, bool strict,
                     try {
                         out.emplace(state.symbols[a->name], printValueAsJSON(state, strict, *a->value, a->pos, context, copyToStore));
                     } catch (Error & e) {
+
+                        bool isEvalError = dynamic_cast<EvalError *>(&e);
+                        bool isFileNotFoundError = dynamic_cast<FileNotFound *>(&e);
+                        // Restrict replaceEvalErrors only only evaluation errors
+                        if (state.settings.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;

src/nix-instantiate/nix-instantiate.cc

@@ -147,6 +147,8 @@ static int main_nix_instantiate(int argc, char * * argv)
                 xmlOutputSourceLocation = false;
             else if (*arg == "--strict")
                 strict = true;
+            else if (*arg == "--replace-eval-errors")
+                evalSettings.replaceEvalErrors = true;
             else if (*arg == "--dry-run")
                 settings.readOnlyMode = true;
             else if (*arg != "" && arg->at(0) == '-')

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.

tests/functional/eval.sh

@@ -35,6 +35,16 @@ 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="$(echo '{
+"missingAttr":"«evaluation error»"
+}' | tr -d '\n')"
+actual="$(nix-instantiate --eval --json --strict --replace-eval-errors "./replace-eval-errors.nix")" || true
+[[ $actual == "$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")

tests/functional/replace-eval-errors.nix

@@ -0,0 +1,7 @@
+{
+  missingAttr =
+    let
+      bar = { };
+    in
+    bar.notExist;
+}