Pretty-print values in the REPL

Pretty-print values in the REPL by printing each item in a list or
attrset on a separate line. When possible, single-item lists and
attrsets are printed on one line, as long as they don't contain a nested
list, attrset, or thunk.

Before:
```
{ attrs = { a = { b = { c = { }; }; }; }; list = [ 1 ]; list' = [ 1 2 3 ]; }
```

After:
```
{
  attrs = {
    a = {
      b = {
        c = { };
      };
    };
  };
  list = [ 1 ];
  list' = [
    1
    2
    3
  ];
}
```

src/libcmd/repl.cc

@@ -101,7 +101,8 @@ struct NixRepl
             .ansiColors = true,
             .force = true,
             .derivationPaths = true,
-            .maxDepth = maxDepth
+            .maxDepth = maxDepth,
+            .prettyIndent = 2
         });
     }
 };

src/libexpr/print-options.hh

@@ -17,42 +17,64 @@ struct PrintOptions
      * If true, output ANSI color sequences.
      */
     bool ansiColors = false;
+
     /**
      * If true, force values.
      */
     bool force = false;
+
     /**
      * If true and `force` is set, print derivations as
      * `«derivation /nix/store/...»` instead of as attribute sets.
      */
     bool derivationPaths = false;
+
     /**
      * If true, track which values have been printed and skip them on
      * subsequent encounters. Useful for self-referential values.
      */
     bool trackRepeated = true;
+
     /**
      * Maximum depth to evaluate to.
      */
     size_t maxDepth = std::numeric_limits<size_t>::max();
+
     /**
      * Maximum number of attributes in attribute sets to print.
      *
      * Note that this is a limit for the entire print invocation, not for each
      * attribute set encountered.
      */
     size_t maxAttrs = std::numeric_limits<size_t>::max();
+
     /**
      * Maximum number of list items to print.
      *
      * Note that this is a limit for the entire print invocation, not for each
      * list encountered.
      */
     size_t maxListItems = std::numeric_limits<size_t>::max();
+
     /**
      * Maximum string length to print.
      */
     size_t maxStringLength = std::numeric_limits<size_t>::max();
+
+    /**
+     * Indentation width for pretty-printing.
+     *
+     * If set to 0 (the default), values are not pretty-printed.
+     */
+    size_t prettyIndent = 0;
+
+    /**
+     * True if pretty-printing is enabled.
+     */
+    inline bool prettyPrint()
+    {
+        return prettyIndent > 0;
+    }
 };
 
 /**

src/libexpr/print.cc

@@ -153,6 +153,7 @@ struct ImportantFirstAttrNameCmp
 };
 
 typedef std::set<Value *> ValuesSeen;
+typedef std::vector<std::pair<std::string, Value *>> AttrVec;
 
 class Printer
 {
@@ -163,6 +164,21 @@ class Printer
     std::optional<ValuesSeen> seen;
     size_t attrsPrinted = 0;
     size_t listItemsPrinted = 0;
+    std::string indent;
+
+    void increaseIndent()
+    {
+        if (options.prettyPrint()) {
+            indent.append(options.prettyIndent, ' ');
+        }
+    }
+
+    void decreaseIndent()
+    {
+        if (options.prettyPrint()) {
+            indent.resize(indent.size() - options.prettyIndent);
+        }
+    }
 
     void printRepeated()
     {
@@ -260,6 +276,28 @@ class Printer
         }
     }
 
+    bool shouldPrettyPrintAttrs(AttrVec & v)
+    {
+        if (!options.prettyPrint() || v.empty()) {
+            return false;
+        }
+
+        // Pretty-print attrsets with more than one item.
+        if (v.size() > 1) {
+            return true;
+        }
+
+        auto item = v[0].second;
+        if (!item) {
+            return true;
+        }
+
+        // Pretty-print single-item attrsets only if they contain nested
+        // structures.
+        auto itemType = item->type();
+        return itemType == nList || itemType == nAttrs || itemType == nThunk;
+    }
+
     void printAttrs(Value & v, size_t depth)
     {
         if (seen && !seen->insert(&v).second) {
@@ -270,9 +308,10 @@ class Printer
         if (options.force && options.derivationPaths && state.isDerivation(v)) {
             printDerivation(v);
         } else if (depth < options.maxDepth) {
-            output << "{ ";
+            increaseIndent();
+            output << "{";
 
-            std::vector<std::pair<std::string, Value *>> sorted;
+            AttrVec sorted;
             for (auto & i : *v.attrs)
                 sorted.emplace_back(std::pair(state.symbols[i.name], i.value));
 
@@ -281,7 +320,15 @@ class Printer
             else
                 std::sort(sorted.begin(), sorted.end(), ImportantFirstAttrNameCmp());
 
+            auto prettyPrint = shouldPrettyPrintAttrs(sorted);
+
             for (auto & i : sorted) {
+                if (prettyPrint) {
+                    output << "\n" << indent;
+                } else {
+                    output << " ";
+                }
+
                 if (attrsPrinted >= options.maxAttrs) {
                     printElided(sorted.size() - attrsPrinted, "attribute", "attributes");
                     break;
@@ -290,13 +337,42 @@ class Printer
                 printAttributeName(output, i.first);
                 output << " = ";
                 print(*i.second, depth + 1);
-                output << "; ";
+                output << ";";
                 attrsPrinted++;
             }
 
+            decreaseIndent();
+            if (prettyPrint) {
+                output << "\n" << indent;
+            } else {
+                output << " ";
+            }
             output << "}";
-        } else
+        } else {
             output << "{ ... }";
+        }
+    }
+
+    bool shouldPrettyPrintList(std::span<Value * const> list)
+    {
+        if (!options.prettyPrint() || list.empty()) {
+            return false;
+        }
+
+        // Pretty-print lists with more than one item.
+        if (list.size() > 1) {
+            return true;
+        }
+
+        auto item = list[0];
+        if (!item) {
+            return true;
+        }
+
+        // Pretty-print single-item lists only if they contain nested
+        // structures.
+        auto itemType = item->type();
+        return itemType == nList || itemType == nAttrs || itemType == nThunk;
     }
 
     void printList(Value & v, size_t depth)
@@ -306,11 +382,20 @@ class Printer
             return;
         }
 
-        output << "[ ";
         if (depth < options.maxDepth) {
-            for (auto elem : v.listItems()) {
+            increaseIndent();
+            output << "[";
+            auto listItems = v.listItems();
+            auto prettyPrint = shouldPrettyPrintList(listItems);
+            for (auto elem : listItems) {
+                if (prettyPrint) {
+                    output << "\n" << indent;
+                } else {
+                    output << " ";
+                }
+
                 if (listItemsPrinted >= options.maxListItems) {
-                    printElided(v.listSize() - listItemsPrinted, "item", "items");
+                    printElided(listItems.size() - listItemsPrinted, "item", "items");
                     break;
                 }
 
@@ -319,13 +404,19 @@ class Printer
                 } else {
                     printNullptr();
                 }
-                output << " ";
                 listItemsPrinted++;
             }
+
+            decreaseIndent();
+            if (prettyPrint) {
+                output << "\n" << indent;
+            } else {
+                output << " ";
+            }
+            output << "]";
+        } else {
+            output << "[ ... ]";
         }
-        else
-            output << "... ";
-        output << "]";
     }
 
     void printFunction(Value & v)
@@ -488,6 +579,7 @@ class Printer
     {
         attrsPrinted = 0;
         listItemsPrinted = 0;
+        indent.clear();
 
         if (options.trackRepeated) {
             seen.emplace();

tests/functional/lang/eval-fail-bad-string-interpolation-4.err.exp

@@ -6,4 +6,4 @@ error:
              |   ^
            10|
 
-       error: cannot coerce a set to a string: { a = { a = { a = { a = "ha"; b = "ha"; c = "ha"; d = "ha"; e = "ha"; f = "ha"; g = "ha"; h = "ha"; j = "ha"; }; «4294967295 attributes elided»}; «4294967294 attributes elided»}; «4294967293 attributes elided»}
+       error: cannot coerce a set to a string: { a = { a = { a = { a = "ha"; b = "ha"; c = "ha"; d = "ha"; e = "ha"; f = "ha"; g = "ha"; h = "ha"; j = "ha"; }; «4294967295 attributes elided» }; «4294967294 attributes elided» }; «4294967293 attributes elided» }

tests/functional/repl.sh

@@ -146,29 +146,89 @@ echo "$replResult" | grepQuiet -s afterChange
 # Normal output should print attributes in lexicographical order non-recursively
 testReplResponseNoRegex '
 { a = { b = 2; }; l = [ 1 2 3 ]; s = "string"; n = 1234; x = rec { y = { z = { inherit y; }; }; }; }
-' '{ a = { ... }; l = [ ... ]; n = 1234; s = "string"; x = { ... }; }'
+' \
+'{
+  a = { ... };
+  l = [ ... ];
+  n = 1234;
+  s = "string";
+  x = { ... };
+}
+'
 
 # Same for lists, but order is preserved
 testReplResponseNoRegex '
 [ 42 1 "thingy" ({ a = 1; }) ([ 1 2 3 ]) ]
-' '[ 42 1 "thingy" { ... } [ ... ] ]'
+' \
+'[
+  42
+  1
+  "thingy"
+  { ... }
+  [ ... ]
+]
+'
 
 # Same for let expressions
 testReplResponseNoRegex '
 let x = { y = { a = 1; }; inherit x; }; in x
-' '{ x = { ... }; y = { ... }; }'
+' \
+'{
+  x = { ... };
+  y = { ... };
+}
+'
 
 # The :p command should recursively print sets, but prevent infinite recursion
 testReplResponseNoRegex '
 :p { a = { b = 2; }; s = "string"; n = 1234; x = rec { y = { z = { inherit y; }; }; }; }
-' '{ a = { b = 2; }; n = 1234; s = "string"; x = { y = { z = { y = «repeated»; }; }; }; }'
+' \
+'{
+  a = { b = 2; };
+  n = 1234;
+  s = "string";
+  x = {
+    y = {
+      z = {
+        y = «repeated»;
+      };
+    };
+  };
+}
+'
 
 # Same for lists
 testReplResponseNoRegex '
 :p [ 42 1 "thingy" (rec { a = 1; b = { inherit a; inherit b; }; }) ([ 1 2 3 ]) ]
-' '[ 42 1 "thingy" { a = 1; b = { a = 1; b = «repeated»; }; } [ 1 2 3 ] ]'
+' \
+'[
+  42
+  1
+  "thingy"
+  {
+    a = 1;
+    b = {
+      a = 1;
+      b = «repeated»;
+    };
+  }
+  [
+    1
+    2
+    3
+  ]
+]
+'
 
 # Same for let expressions
 testReplResponseNoRegex '
 :p let x = { y = { a = 1; }; inherit x; }; in x
-' '{ x = { x = «repeated»; y = { a = 1; }; }; y = «repeated»; }'
+' \
+'{
+  x = {
+    x = «repeated»;
+    y = { a = 1; };
+  };
+  y = «repeated»;
+}
+'