Merge pull request #10209 from 9999years/rethrow-for-top-level-errors-in-repl

Print top-level errors normally in `nix repl`

src/libcmd/repl.cc

@@ -123,7 +123,8 @@ struct NixRepl
             .force = true,
             .derivationPaths = true,
             .maxDepth = maxDepth,
-            .prettyIndent = 2
+            .prettyIndent = 2,
+            .errors = ErrorPrintBehavior::ThrowTopLevel,
         });
     }
 };

src/libexpr/print-options.hh

@@ -8,6 +8,29 @@
 
 namespace nix {
 
+/**
+ * How errors should be handled when printing values.
+ */
+enum class ErrorPrintBehavior {
+    /**
+     * Print the first line of the error in brackets: `«error: oh no!»`
+     */
+    Print,
+    /**
+     * Throw the error to the code that attempted to print the value, instead
+     * of suppressing it it.
+     */
+    Throw,
+    /**
+     * Only throw the error if encountered at the top level of the expression.
+     *
+     * This will cause expressions like `builtins.throw "uh oh!"` to throw
+     * errors, but will print attribute sets and other nested structures
+     * containing values that error (like `nixpkgs`) normally.
+     */
+    ThrowTopLevel,
+};
+
 /**
  * Options for printing Nix values.
  */
@@ -68,6 +91,11 @@ struct PrintOptions
      */
     size_t prettyIndent = 0;
 
+    /**
+     * How to handle errors encountered while printing values.
+     */
+    ErrorPrintBehavior errors = ErrorPrintBehavior::Print;
+
     /**
      * True if pretty-printing is enabled.
      */
@@ -86,7 +114,7 @@ static PrintOptions errorPrintOptions = PrintOptions {
     .maxDepth = 10,
     .maxAttrs = 10,
     .maxListItems = 10,
-    .maxStringLength = 1024
+    .maxStringLength = 1024,
 };
 
 }

src/libexpr/print.cc

@@ -271,7 +271,6 @@ private:
 
     void printDerivation(Value & v)
     {
-        try {
         Bindings::iterator i = v.attrs->find(state.sDrvPath);
         NixStringContext context;
         std::string storePath;
@@ -287,9 +286,6 @@ private:
         output << "»";
         if (options.ansiColors)
             output << ANSI_NORMAL;
-        } catch (Error & e) {
-            printError_(e);
-        }
     }
 
     bool shouldPrettyPrintAttrs(AttrVec & v)
@@ -510,13 +506,9 @@ private:
         output.flush();
         checkInterrupt();
 
-        if (options.force) {
         try {
+            if (options.force) {
                 state.forceValue(v, v.determinePos(noPos));
-            } catch (Error & e) {
-                printError_(e);
-                return;
-            }
             }
 
             switch (v.type()) {
@@ -569,6 +561,14 @@ private:
                 printUnknown();
                 break;
             }
+        } catch (Error & e) {
+            if (options.errors == ErrorPrintBehavior::Throw
+                || (options.errors == ErrorPrintBehavior::ThrowTopLevel
+                    && depth == 0)) {
+                throw;
+            }
+            printError_(e);
+        }
     }
 
 public: