Print top-level errors normally in nix repl

Previously, errors while printing values in `nix repl` would be printed
in `«error: ...»` brackets rather than displayed normally:

```
nix-repl> legacyPackages.aarch64-darwin.pythonPackages.APScheduler
«error: Package ‘python-2.7.18.7’ in /nix/store/6s0m1qc31zw3l3kq0q4wd5cp3lqpkq0q-source/pkgs/development/interpreters/python/cpython/2.7/default.nix:335 is marked as insecure, refusing to evaluate.»
```

Now, errors will be displayed normally if they're emitted at the
top-level of an expression:

```
nix-repl> legacyPackages.aarch64-darwin.pythonPackages.APScheduler
error:
       … in the condition of the assert statement
         at /nix/store/6s0m1qc31zw3l3kq0q4wd5cp3lqpkq0q-source/lib/customisation.nix:268:17:
          267|     in commonAttrs // {
          268|       drvPath = assert condition; drv.drvPath;
             |                 ^
          269|       outPath = assert condition; drv.outPath;

       … in the left operand of the OR (||) operator
         at /nix/store/6s0m1qc31zw3l3kq0q4wd5cp3lqpkq0q-source/pkgs/development/interpreters/python/passthrufun.nix:28:45:
           27|         if lib.isDerivation value then
           28|           lib.extendDerivation (valid value || throw "${name} should use `buildPythonPackage` or `toPythonModule` if it is to be part of the Python packages set.") {} value
             |                                             ^
           29|         else

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: Package ‘python-2.7.18.7’ in /nix/store/6s0m1qc31zw3l3kq0q4wd5cp3lqpkq0q-source/pkgs/development/interpreters/python/cpython/2.7/default.nix:335 is marked as insecure, refusing to evaluate.
```

Errors emitted in nested structures (like e.g. when printing `nixpkgs`)
will still be printed in brackets.

Change-Id: I25aeddf08c017582718cb9772a677bf51b9fc2ad

doc/manual/rl-next/better-errors-in-nix-repl.md

@@ -1,6 +1,7 @@
 ---
 synopsis: Concise error printing in `nix repl`
 prs: 9928
+cls: 811
 ---
 
 Previously, if an element of a list or attribute set threw an error while

src/libcmd/repl.cc

@@ -167,7 +167,8 @@ 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

@@ -229,7 +229,6 @@ private:
 
     void printDerivation(Value & v)
     {
-        try {
         Bindings::iterator i = v.attrs->find(state.sDrvPath);
         NixStringContext context;
         std::string storePath;
@@ -245,9 +244,6 @@ private:
         output << "»";
         if (options.ansiColors)
             output << ANSI_NORMAL;
-        } catch (Error & e) {
-            printError_(e);
-        }
     }
 
     bool shouldPrettyPrintAttrs(AttrVec & v)
@@ -472,13 +468,9 @@ 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()) {
@@ -531,6 +523,14 @@ private:
                 printUnknown();
                 break;
             }
+        } catch (Error & e) {
+            if (options.errors == ErrorPrintBehavior::Throw
+                || (options.errors == ErrorPrintBehavior::ThrowTopLevel
+                    && depth == 0)) {
+                throw;
+            }
+            printError_(e);
+        }
     }
 
 public:

tests/functional/repl_characterization/data/errors.test

@@ -0,0 +1,27 @@
+Errors at the top of an expression are printed normally:
+
+  nix-repl> builtins.throw "Evil puppy detected!!!"
+  error:
+         … while calling the 'throw' builtin
+           at «string»:1:1:
+              1| builtins.throw "Evil puppy detected!!!"
+               | ^
+
+         error: Evil puppy detected!!!
+
+Errors in attribute values are printed inline, to make it easier to explore
+values like nixpkgs where some parts of the value fail to evaluate:
+
+  nix-repl> { puppy = builtins.throw "This puppy is EVIL!!!"; puppy2 = "This puppy is GOOD :)"; }
+  {
+    puppy = «error: This puppy is EVIL!!!»;
+    puppy2 = "This puppy is GOOD :)";
+  }
+
+Same for list values:
+
+  nix-repl> [ (builtins.throw "This puppy is EVIL!!!") ("This puppy is GOOD :)") ]
+  [
+    «error: This puppy is EVIL!!!»
+    "This puppy is GOOD :)"
+  ]

tests/functional/repl_characterization/repl_characterization.cc

@@ -186,5 +186,6 @@ REPL_TEST(repl_overlays_destructure_without_formals_ok);
 REPL_TEST(repl_overlays_error);
 REPL_TEST(repl_printing);
 REPL_TEST(stack_vars);
+REPL_TEST(errors);
 
 }; // namespace nix