repl: allow :p to print derivation attrs.

Change-Id: I6a6a6964a58c3340acca9272c616c079c5218d6e

doc/manual/rl-next/print-derivations.md

@@ -0,0 +1,20 @@
+---
+synopsis: Derivations can now be printed in detail in `nix repl`
+cls: [3842]
+category: Improvements
+credits: [Lunaphied]
+---
+
+Traditionally derivations printed in the REPL would only print a formatted object
+representing the path of the derivation file it refers to. This makes inspecting
+the enhanced derivation attribute sets encountered from `mkDerivation` or similar
+wrappers more difficult. Even the `:p`/`:print` command would not elaborate attribute sets
+tagged as a derivation.
+
+With this change you can now use `:p`/`:print` to directly inspect a derivation
+by providing one as the top-level object. Derivation attribute sets will only be
+printed two levels deep and internal derivation attrsets will remain in unexpanded
+path form as before. `drvAttrs` will also be elided as these attributes are already
+present in the top-level attribute set of the derivation. These heuristics provide
+a balance between readability and functionality. When the `:p`/`:print` is omitted,
+a bare derivation is printed in the path format as before.

lix/libcmd/repl.cc

@@ -209,12 +209,14 @@ struct NixRepl
 
     void printValue(std::ostream & str,
                               Value & v,
-                              unsigned int maxDepth = std::numeric_limits<unsigned int>::max())
+                              unsigned int maxDepth = std::numeric_limits<unsigned int>::max(),
+                              bool replDerivation = false)
     {
         ::nix::printValue(state, str, v, PrintOptions {
             .ansiColors = true,
             .force = true,
-            .derivationPaths = true,
+            .derivationPaths = !replDerivation,
+            .replDerivation = replDerivation,
             .maxDepth = maxDepth,
             .prettyIndent = 2,
             .errors = ErrorPrintBehavior::ThrowTopLevel,
@@ -793,8 +795,10 @@ ProcessLineResult NixRepl::processLine(std::string line)
         evalString(arg, v);
         if (v.type() == nString) {
             std::cout << v.str();
+        } else if (v.type() == nAttrs && state.isDerivation(v)) {
+            printValue(std::cout, v, 2, true);
         } else {
-            printValue(std::cout, v);
+            printValue(std::cout, v, std::numeric_limits<unsigned int>::max());
         }
         std::cout << std::endl;
     }

lix/libexpr/print-options.hh

@@ -53,6 +53,12 @@ struct PrintOptions
      */
     bool derivationPaths = false;
 
+    /**
+     * If true, we are interactively printing a full derivation and should
+     * switch back to printing paths as desired normally after the first entry.
+     */
+    bool replDerivation = false;
+
     /**
      * If true, track which values have been printed and skip them on
      * subsequent encounters. Useful for self-referential values.

lix/libexpr/print.cc

@@ -286,6 +286,11 @@ private:
         } else if (seen && !v.attrs()->empty() && !seen->insert(v.attrs()).second) {
             printRepeated();
         } else if (depth < options.maxDepth || v.attrs()->empty()) {
+            bool isPrintingReplDerivation = depth == 0 && options.replDerivation && state.isDerivation(v);
+            if (isPrintingReplDerivation) {
+                // Switch back to eliding paths if it was initially off.
+                options.derivationPaths = true;
+            }
             increaseIndent();
             output << "{";
 
@@ -304,7 +309,7 @@ private:
             for (auto & i : sorted) {
                 printSpace(prettyPrint);
 
-                if (attrsPrinted >= options.maxAttrs) {
+                if (attrsPrinted >= options.maxAttrs ) {
                     printElided(sorted.size() - printedHere, "attribute", "attributes");
                     break;
                 }
@@ -324,6 +329,21 @@ private:
                 }
 
                 output << " = ";
+
+                // Elide repeated drvAttrs attribute.
+                if (isPrintingReplDerivation && i.first == "drvAttrs") {
+                    state.forceValue(i.second->value, noPos);
+                    if (i.second->value.type() == ValueType::nAttrs) {
+                        printElided(i.second->value.attrs()->size(), "attribute", "attributes");
+                    } else {
+                        print(i.second->value, depth + 1);
+                    }
+                    output << ";";
+                    attrsPrinted++;
+                    printedHere++;
+                    continue;
+                }
+
                 print(i.second->value, depth + 1);
                 output << ";";
                 attrsPrinted++;
@@ -582,7 +602,6 @@ public:
             seen.reset();
         }
 
-        ValuesSeen seen;
         print(v, 0);
     }
 };

tests/functional/repl_characterization/data/repl_printing.test

@@ -100,3 +100,15 @@ Printing an environment with :env after adding a variable to the scope
 
   Env level 1
   abort baseNameOf break builtins derivation derivationStrict dirOf false fetchGit fetchMercurial fetchTarball fetchTree fromTOML import isNull map null placeholder removeAttrs scopedImport throw toString true
+
+Printing a derivation or something
+
+  nix-repl> fakeDrv = let drvAttrs = { builder = "meow"; system = "meower"; name = "mrowmrow"; }; in { inherit (drvAttrs) builder system name; inherit drvAttrs; type = "derivation"; }
+  nix-repl> :p fakeDrv
+  {
+   builder = "meow";
+   drvAttrs = «3 attributes elided»;
+   name = "mrowmrow";
+   system = "meower";
+   type = "derivation";
+  }