Unify and refactor value printing

Previously, there were two mostly-identical value printers -- one in
`libexpr/eval.cc` (which didn't force values) and one in
`libcmd/repl.cc` (which did force values and also printed ANSI color
codes).

This PR unifies both of these printers into `print.cc` and provides a
`PrintOptions` struct for controlling the output, which allows for
toggling whether values are forced, whether repeated values are tracked,
and whether ANSI color codes are displayed.

Additionally, `PrintOptions` allows tuning the maximum number of
attributes, list items, and bytes in a string that will be displayed;
this makes it ideal for contexts where printing too much output (e.g.
all of Nixpkgs) is distracting. (As requested by @roberth in
https://github.com/NixOS/nix/pull/9554#issuecomment-1845095735)

Please read the tests for example output.

Future work:
- It would be nice to provide this function as a builtin, perhaps
  `builtins.toStringDebug` -- a printing function that never fails would
  be useful when debugging Nix code.
- It would be nice to support customizing `PrintOptions` members on the
  command line, e.g. `--option to-string-max-attrs 1000`.

(cherry picked from commit 0fa08b451682fb3311fe58112ff05c4fe5bee3a4, )

===

Restore ambiguous value printer for `nix-instantiate`

The Nix team has requested that this output format remain unchanged.
I've added a warning to the man page explaining that `nix-instantiate
--eval` output will not parse correctly in many situations.

(cherry picked from commit df84dd4d8dd3fd6381ac2ca3064432ab31a16b79)

Change-Id: I7cca6b4b53cd0642f2d49af657d5676a8554c9f8

doc/manual/src/command-ref/nix-instantiate.md

@@ -35,13 +35,50 @@ standard input.
 
   - `--parse`\
     Just parse the input files, and print their abstract syntax trees on
-    standard output in ATerm format.
+    standard output as a Nix expression.
 
   - `--eval`\
     Just parse and evaluate the input files, and print the resulting
     values on standard output. No instantiation of store derivations
     takes place.
 
+    > **Warning**
+    >
+    > This option produces ambiguous output which is not suitable for machine
+    > consumption. For example, these two Nix expressions print the same result
+    > despite having different types:
+    >
+    > ```console
+    > $ nix-instantiate --eval --expr '{ a = {}; }'
+    > { a = <CODE>; }
+    > $ nix-instantiate --eval --expr '{ a = <CODE>; }'
+    > { a = <CODE>; }
+    > ```
+    >
+    > For human-readable output, `nix eval` (experimental) is more informative:
+    >
+    > ```console
+    > $ nix-instantiate --eval --expr 'a: a'
+    > <LAMBDA>
+    > $ nix eval --expr 'a: a'
+    > «lambda @ «string»:1:1»
+    > ```
+    >
+    > For machine-readable output, the `--xml` option produces unambiguous
+    > output:
+    >
+    > ```console
+    > $ nix-instantiate --eval --xml --expr '{ foo = <CODE>; }'
+    > <?xml version='1.0' encoding='utf-8'?>
+    > <expr>
+    >   <attrs>
+    >     <attr column="3" line="1" name="foo">
+    >       <unevaluated />
+    >     </attr>
+    >   </attrs>
+    > </expr>
+    > ```
+
   - `--find-file`\
     Look up the given files in Nix’s search path (as specified by the
     `NIX_PATH` environment variable). If found, print the corresponding
@@ -61,11 +98,11 @@ standard input.
 
   - `--json`\
     When used with `--eval`, print the resulting value as an JSON
-    representation of the abstract syntax tree rather than as an ATerm.
+    representation of the abstract syntax tree rather than as a Nix expression.
 
   - `--xml`\
     When used with `--eval`, print the resulting value as an XML
-    representation of the abstract syntax tree rather than as an ATerm.
+    representation of the abstract syntax tree rather than as a Nix expression.
     The schema is the same as that used by the [`toXML`
     built-in](../language/builtins.md).
 
@@ -133,28 +170,29 @@ $ nix-instantiate --eval --xml --expr '1 + 2'
 The difference between non-strict and strict evaluation:
 
 ```console
-$ nix-instantiate --eval --xml --expr 'rec { x = "foo"; y = x; }'
+$ nix-instantiate --eval --xml --expr '{ x = {}; }'
-...
+<?xml version='1.0' encoding='utf-8'?>
-  <attr name="x">
+<expr>
-    <string value="foo" />
+  <attrs>
-  </attr>
+    <attr column="3" line="1" name="x">
-  <attr name="y">
+      <unevaluated />
-    <unevaluated />
+    </attr>
-  </attr>
+  </attrs>
-...
+</expr>
 ```
 
 Note that `y` is left unevaluated (the XML representation doesn’t
 attempt to show non-normal forms).
 
 ```console
-$ nix-instantiate --eval --xml --strict --expr 'rec { x = "foo"; y = x; }'
+$ nix-instantiate --eval --xml --strict --expr '{ x = {}; }'
-...
+<?xml version='1.0' encoding='utf-8'?>
-  <attr name="x">
+<expr>
-    <string value="foo" />
+  <attrs>
-  </attr>
+    <attr column="3" line="1" name="x">
-  <attr name="y">
+      <attrs>
-    <string value="foo" />
+      </attrs>
-  </attr>
+    </attr>
-...
+  </attrs>
+</expr>
 ```

src/libcmd/repl.cc

@@ -90,9 +90,17 @@ struct NixRepl
     void evalString(std::string s, Value & v);
     void loadDebugTraceEnv(DebugTrace & dt);
 
-    typedef std::set<Value *> ValuesSeen;
+    void printValue(std::ostream & str,
-    std::ostream & printValue(std::ostream & str, Value & v, unsigned int maxDepth);
+                              Value & v,
-    std::ostream & printValue(std::ostream & str, Value & v, unsigned int maxDepth, ValuesSeen & seen);
+                              unsigned int maxDepth = std::numeric_limits<unsigned int>::max())
+    {
+        ::nix::printValue(*state, str, v, PrintOptions {
+            .ansiColors = true,
+            .force = true,
+            .derivationPaths = true,
+            .maxDepth = maxDepth
+        });
+    }
 };
 
 std::string removeWhitespace(std::string s)
@@ -713,7 +721,8 @@ bool NixRepl::processLine(std::string line)
     else if (command == ":p" || command == ":print") {
         Value v;
         evalString(arg, v);
-        printValue(std::cout, v, 1000000000) << std::endl;
+        printValue(std::cout, v);
+        std::cout << std::endl;
     }
 
     else if (command == ":q" || command == ":quit") {
@@ -775,7 +784,8 @@ bool NixRepl::processLine(std::string line)
         } else {
             Value v;
             evalString(line, v);
-            printValue(std::cout, v, 1) << std::endl;
+            printValue(std::cout, v, 1);
+            std::cout << std::endl;
         }
     }
 
@@ -897,144 +907,6 @@ void NixRepl::evalString(std::string s, Value & v)
 }
 
 
-std::ostream & NixRepl::printValue(std::ostream & str, Value & v, unsigned int maxDepth)
-{
-    ValuesSeen seen;
-    return printValue(str, v, maxDepth, seen);
-}
-
-
-
-
-// FIXME: lot of cut&paste from Nix's eval.cc.
-std::ostream & NixRepl::printValue(std::ostream & str, Value & v, unsigned int maxDepth, ValuesSeen & seen)
-{
-    str.flush();
-    checkInterrupt();
-
-    state->forceValue(v, v.determinePos(noPos));
-
-    switch (v.type()) {
-
-    case nInt:
-        str << ANSI_CYAN << v.integer << ANSI_NORMAL;
-        break;
-
-    case nBool:
-        str << ANSI_CYAN;
-        printLiteralBool(str, v.boolean);
-        str << ANSI_NORMAL;
-        break;
-
-    case nString:
-        str << ANSI_WARNING;
-        printLiteralString(str, v.string.s);
-        str << ANSI_NORMAL;
-        break;
-
-    case nPath:
-        str << ANSI_GREEN << v.path().to_string() << ANSI_NORMAL; // !!! escaping?
-        break;
-
-    case nNull:
-        str << ANSI_CYAN "null" ANSI_NORMAL;
-        break;
-
-    case nAttrs: {
-        seen.insert(&v);
-
-        bool isDrv = state->isDerivation(v);
-
-        if (isDrv) {
-            str << "«derivation ";
-            Bindings::iterator i = v.attrs->find(state->sDrvPath);
-            NixStringContext context;
-            if (i != v.attrs->end())
-                str << state->store->printStorePath(state->coerceToStorePath(i->pos, *i->value, context, "while evaluating the drvPath of a derivation"));
-            else
-                str << "???";
-            str << "»";
-        }
-
-        else if (maxDepth > 0) {
-            str << "{ ";
-
-            typedef std::map<std::string, Value *> Sorted;
-            Sorted sorted;
-            for (auto & i : *v.attrs)
-                sorted.emplace(state->symbols[i.name], i.value);
-
-            for (auto & i : sorted) {
-                printAttributeName(str, i.first);
-                str << " = ";
-                if (seen.count(i.second))
-                    str << "«repeated»";
-                else
-                    try {
-                        printValue(str, *i.second, maxDepth - 1, seen);
-                    } catch (AssertionError & e) {
-                        str << ANSI_RED "«error: " << e.msg() << "»" ANSI_NORMAL;
-                    }
-                str << "; ";
-            }
-
-            str << "}";
-        } else
-            str << "{ ... }";
-
-        break;
-    }
-
-    case nList:
-        seen.insert(&v);
-
-        str << "[ ";
-        if (maxDepth > 0)
-            for (auto elem : v.listItems()) {
-                if (seen.count(elem))
-                    str << "«repeated»";
-                else
-                    try {
-                        printValue(str, *elem, maxDepth - 1, seen);
-                    } catch (AssertionError & e) {
-                        str << ANSI_RED "«error: " << e.msg() << "»" ANSI_NORMAL;
-                    }
-                str << " ";
-            }
-        else
-            str << "... ";
-        str << "]";
-        break;
-
-    case nFunction:
-        if (v.isLambda()) {
-            std::ostringstream s;
-            s << state->positions[v.lambda.fun->pos];
-            str << ANSI_BLUE "«lambda @ " << filterANSIEscapes(s.str()) << "»" ANSI_NORMAL;
-        } else if (v.isPrimOp()) {
-            str << ANSI_MAGENTA "«primop»" ANSI_NORMAL;
-        } else if (v.isPrimOpApp()) {
-            str << ANSI_BLUE "«primop-app»" ANSI_NORMAL;
-        } else {
-            abort();
-        }
-        break;
-
-    case nFloat:
-        str << v.fpoint;
-        break;
-
-    case nThunk:
-    case nExternal:
-    default:
-        str << ANSI_RED "«unknown»" ANSI_NORMAL;
-        break;
-    }
-
-    return str;
-}
-
-
 std::unique_ptr<AbstractNixRepl> AbstractNixRepl::create(
    const SearchPath & searchPath, nix::ref<Store> store, ref<EvalState> state,
    std::function<AnnotatedValues()> getValues)

src/libexpr/eval.cc

@@ -100,117 +100,23 @@ RootValue allocRootValue(Value * v)
 #endif
 }
 
-void Value::print(const SymbolTable &symbols, std::ostream &str,
-                  std::set<const void *> *seen, int depth) const
-
-{
-    checkInterrupt();
-
-    if (depth <= 0) {
-        str << "«too deep»";
-        return;
-    }
-    switch (internalType) {
-    case tInt:
-        str << integer;
-        break;
-    case tBool:
-        printLiteralBool(str, boolean);
-        break;
-    case tString:
-        printLiteralString(str, string.s);
-        break;
-    case tPath:
-        str << path().to_string(); // !!! escaping?
-        break;
-    case tNull:
-        str << "null";
-        break;
-    case tAttrs: {
-        if (seen && !attrs->empty() && !seen->insert(attrs).second)
-            str << "«repeated»";
-        else {
-            str << "{ ";
-            for (auto & i : attrs->lexicographicOrder(symbols)) {
-                str << symbols[i->name] << " = ";
-                i->value->print(symbols, str, seen, depth - 1);
-                str << "; ";
-            }
-            str << "}";
-        }
-        break;
-    }
-    case tList1:
-    case tList2:
-    case tListN:
-        if (seen && listSize() && !seen->insert(listElems()).second)
-            str << "«repeated»";
-        else {
-            str << "[ ";
-            for (auto v2 : listItems()) {
-                if (v2)
-                    v2->print(symbols, str, seen, depth - 1);
-                else
-                    str << "(nullptr)";
-                str << " ";
-            }
-            str << "]";
-        }
-        break;
-    case tThunk:
-    case tApp:
-        if (!isBlackhole()) {
-            str << "<CODE>";
-        } else {
-            // Although we know for sure that it's going to be an infinite recursion
-            // when this value is accessed _in the current context_, it's likely
-            // that the user will misinterpret a simpler «infinite recursion» output
-            // as a definitive statement about the value, while in fact it may be
-            // a valid value after `builtins.trace` and perhaps some other steps
-            // have completed.
-            str << "«potential infinite recursion»";
-        }
-        break;
-    case tLambda:
-        str << "<LAMBDA>";
-        break;
-    case tPrimOp:
-        str << "<PRIMOP>";
-        break;
-    case tPrimOpApp:
-        str << "<PRIMOP-APP>";
-        break;
-    case tExternal:
-        str << *external;
-        break;
-    case tFloat:
-        str << fpoint;
-        break;
-    default:
-        printError("Nix evaluator internal error: Value::print(): invalid value type %1%", internalType);
-        abort();
-    }
-}
-
-void Value::print(const SymbolTable &symbols, std::ostream &str,
-                  bool showRepeated, int depth) const {
-    std::set<const void *> seen;
-    print(symbols, str, showRepeated ? nullptr : &seen, depth);
-}
-
 // Pretty print types for assertion errors
 std::ostream & operator << (std::ostream & os, const ValueType t) {
     os << showType(t);
     return os;
 }
 
-std::string printValue(const EvalState & state, const Value & v)
+std::string printValue(EvalState & state, Value & v)
 {
     std::ostringstream out;
-    v.print(state.symbols, out);
+    v.print(state, out);
     return out.str();
 }
 
+void Value::print(EvalState & state, std::ostream & str, PrintOptions options)
+{
+    printValue(state, str, *this, options);
+}
 
 const Value * getPrimOp(const Value &v) {
     const Value * primOp = &v;
@@ -717,6 +623,26 @@ void PrimOp::check()
 }
 
 
+std::ostream & operator<<(std::ostream & output, PrimOp & primOp)
+{
+    output << "primop " << primOp.name;
+    return output;
+}
+
+
+PrimOp * Value::primOpAppPrimOp() const
+{
+    Value * left = primOpApp.left;
+    while (left && !left->isPrimOp()) {
+        left = left->primOpApp.left;
+    }
+
+    if (!left)
+        return nullptr;
+    return left->primOp;
+}
+
+
 void Value::mkPrimOp(PrimOp * p)
 {
     p->check();

src/libexpr/eval.hh

@@ -83,6 +83,8 @@ struct PrimOp
     void check();
 };
 
+std::ostream & operator<<(std::ostream & output, PrimOp & primOp);
+
 /**
  * Info about a constant
  */
@@ -126,7 +128,7 @@ std::unique_ptr<ValMap> mapStaticEnvBindings(const SymbolTable & st, const Stati
 void copyContext(const Value & v, NixStringContext & context);
 
 
-std::string printValue(const EvalState & state, const Value & v);
+std::string printValue(EvalState & state, Value & v);
 std::ostream & operator << (std::ostream & os, const ValueType t);
 
 

src/libexpr/print-ambiguous.cc

@@ -0,0 +1,99 @@
+#include "print-ambiguous.hh"
+#include "print.hh"
+
+namespace nix {
+
+// See: https://github.com/NixOS/nix/issues/9730
+void printAmbiguous(
+    Value &v,
+    const SymbolTable &symbols,
+    std::ostream &str,
+    std::set<const void *> *seen,
+    int depth)
+{
+    checkInterrupt();
+
+    if (depth <= 0) {
+        str << "«too deep»";
+        return;
+    }
+    switch (v.type()) {
+    case nInt:
+        str << v.integer;
+        break;
+    case nBool:
+        printLiteralBool(str, v.boolean);
+        break;
+    case nString:
+        printLiteralString(str, v.string.s);
+        break;
+    case nPath:
+        str << v.path().to_string(); // !!! escaping?
+        break;
+    case nNull:
+        str << "null";
+        break;
+    case nAttrs: {
+        if (seen && !v.attrs->empty() && !seen->insert(v.attrs).second)
+            str << "«repeated»";
+        else {
+            str << "{ ";
+            for (auto & i : v.attrs->lexicographicOrder(symbols)) {
+                str << symbols[i->name] << " = ";
+                printAmbiguous(*i->value, symbols, str, seen, depth - 1);
+                str << "; ";
+            }
+            str << "}";
+        }
+        break;
+    }
+    case nList:
+        if (seen && v.listSize() && !seen->insert(v.listElems()).second)
+            str << "«repeated»";
+        else {
+            str << "[ ";
+            for (auto v2 : v.listItems()) {
+                if (v2)
+                    printAmbiguous(*v2, symbols, str, seen, depth - 1);
+                else
+                    str << "(nullptr)";
+                str << " ";
+            }
+            str << "]";
+        }
+        break;
+    case nThunk:
+        if (!v.isBlackhole()) {
+            str << "<CODE>";
+        } else {
+            // Although we know for sure that it's going to be an infinite recursion
+            // when this value is accessed _in the current context_, it's likely
+            // that the user will misinterpret a simpler «infinite recursion» output
+            // as a definitive statement about the value, while in fact it may be
+            // a valid value after `builtins.trace` and perhaps some other steps
+            // have completed.
+            str << "«potential infinite recursion»";
+        }
+        break;
+    case nFunction:
+        if (v.isLambda()) {
+            str << "<LAMBDA>";
+        } else if (v.isPrimOp()) {
+            str << "<PRIMOP>";
+        } else if (v.isPrimOpApp()) {
+            str << "<PRIMOP-APP>";
+        }
+        break;
+    case nExternal:
+        str << *v.external;
+        break;
+    case nFloat:
+        str << v.fpoint;
+        break;
+    default:
+        printError("Nix evaluator internal error: printAmbiguous: invalid value type");
+        abort();
+    }
+}
+
+}

src/libexpr/print-ambiguous.hh

@@ -0,0 +1,24 @@
+#pragma once
+
+#include "value.hh"
+
+namespace nix {
+
+/**
+ * Print a value in the deprecated format used by `nix-instantiate --eval` and
+ * `nix-env` (for manifests).
+ *
+ * This output can't be changed because it's part of the `nix-instantiate` API,
+ * but it produces ambiguous output; unevaluated thunks and lambdas (and a few
+ * other types) are printed as Nix path syntax like `<CODE>`.
+ *
+ * See: https://github.com/NixOS/nix/issues/9730
+ */
+void printAmbiguous(
+    Value &v,
+    const SymbolTable &symbols,
+    std::ostream &str,
+    std::set<const void *> *seen,
+    int depth);
+
+}

src/libexpr/print-options.hh

@@ -0,0 +1,52 @@
+#pragma once
+/**
+ * @file
+ * @brief Options for printing Nix values.
+ */
+
+#include <limits>
+
+namespace nix {
+
+/**
+ * Options for printing Nix values.
+ */
+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 an attribute set to print.
+     */
+    size_t maxAttrs = std::numeric_limits<size_t>::max();
+    /**
+     * Maximum number of list items to print.
+     */
+    size_t maxListItems = std::numeric_limits<size_t>::max();
+    /**
+     * Maximum string length to print.
+     */
+    size_t maxStringLength = std::numeric_limits<size_t>::max();
+};
+
+}

src/libexpr/print.cc

@@ -1,24 +1,64 @@
-#include "print.hh"
+#include <limits>
 #include <unordered_set>
 
+#include "print.hh"
+#include "ansicolor.hh"
+#include "store-api.hh"
+#include "english.hh"
+
 namespace nix {
 
-std::ostream &
+void printElided(
-printLiteralString(std::ostream & str, const std::string_view string)
+    std::ostream & output,
+    unsigned int value,
+    const std::string_view single,
+    const std::string_view plural,
+    bool ansiColors)
 {
+    if (ansiColors)
+        output << ANSI_FAINT;
+    output << " «";
+    pluralize(output, value, single, plural);
+    output << " elided»";
+    if (ansiColors)
+        output << ANSI_NORMAL;
+}
+
+
+std::ostream &
+printLiteralString(std::ostream & str, const std::string_view string, size_t maxLength, bool ansiColors)
+{
+    size_t charsPrinted = 0;
+    if (ansiColors)
+        str << ANSI_MAGENTA;
     str << "\"";
     for (auto i = string.begin(); i != string.end(); ++i) {
+        if (charsPrinted >= maxLength) {
+            str << "\"";
+            printElided(str, string.length() - charsPrinted, "byte", "bytes", ansiColors);
+            return str;
+        }
+
         if (*i == '\"' || *i == '\\') str << "\\" << *i;
         else if (*i == '\n') str << "\\n";
         else if (*i == '\r') str << "\\r";
         else if (*i == '\t') str << "\\t";
         else if (*i == '$' && *(i+1) == '{') str << "\\" << *i;
         else str << *i;
+        charsPrinted++;
     }
     str << "\"";
+    if (ansiColors)
+        str << ANSI_NORMAL;
     return str;
 }
 
+std::ostream &
+printLiteralString(std::ostream & str, const std::string_view string)
+{
+    return printLiteralString(str, string, std::numeric_limits<size_t>::max(), false);
+}
+
 std::ostream &
 printLiteralBool(std::ostream & str, bool boolean)
 {
@@ -90,5 +130,373 @@ printAttributeName(std::ostream & str, std::string_view name) {
     return str;
 }
 
+bool isImportantAttrName(const std::string& attrName)
+{
+    return attrName == "type" || attrName == "_type";
+}
+
+typedef std::pair<std::string, Value *> AttrPair;
+
+struct ImportantFirstAttrNameCmp
+{
+
+    bool operator()(const AttrPair& lhs, const AttrPair& rhs) const
+    {
+        auto lhsIsImportant = isImportantAttrName(lhs.first);
+        auto rhsIsImportant = isImportantAttrName(rhs.first);
+        return std::forward_as_tuple(!lhsIsImportant, lhs.first)
+            < std::forward_as_tuple(!rhsIsImportant, rhs.first);
+    }
+};
+
+typedef std::set<Value *> ValuesSeen;
+
+class Printer
+{
+private:
+    std::ostream & output;
+    EvalState & state;
+    PrintOptions options;
+    std::optional<ValuesSeen> seen;
+
+    void printRepeated()
+    {
+        if (options.ansiColors)
+            output << ANSI_MAGENTA;
+        output << "«repeated»";
+        if (options.ansiColors)
+            output << ANSI_NORMAL;
+    }
+
+    void printNullptr()
+    {
+        if (options.ansiColors)
+            output << ANSI_MAGENTA;
+        output << "«nullptr»";
+        if (options.ansiColors)
+            output << ANSI_NORMAL;
+    }
+
+    void printElided(unsigned int value, const std::string_view single, const std::string_view plural)
+    {
+        ::nix::printElided(output, value, single, plural, options.ansiColors);
+    }
+
+    void printInt(Value & v)
+    {
+        if (options.ansiColors)
+            output << ANSI_CYAN;
+        output << v.integer;
+        if (options.ansiColors)
+            output << ANSI_NORMAL;
+    }
+
+    void printFloat(Value & v)
+    {
+        if (options.ansiColors)
+            output << ANSI_CYAN;
+        output << v.fpoint;
+        if (options.ansiColors)
+            output << ANSI_NORMAL;
+    }
+
+    void printBool(Value & v)
+    {
+        if (options.ansiColors)
+            output << ANSI_CYAN;
+        printLiteralBool(output, v.boolean);
+        if (options.ansiColors)
+            output << ANSI_NORMAL;
+    }
+
+    void printString(Value & v)
+    {
+        printLiteralString(output, v.string.s, options.maxStringLength, options.ansiColors);
+    }
+
+    void printPath(Value & v)
+    {
+        if (options.ansiColors)
+            output << ANSI_GREEN;
+        output << v.path().to_string(); // !!! escaping?
+        if (options.ansiColors)
+            output << ANSI_NORMAL;
+    }
+
+    void printNull()
+    {
+        if (options.ansiColors)
+            output << ANSI_CYAN;
+        output << "null";
+        if (options.ansiColors)
+            output << ANSI_NORMAL;
+    }
+
+    void printDerivation(Value & v)
+    {
+        try {
+            Bindings::iterator i = v.attrs->find(state.sDrvPath);
+            NixStringContext context;
+            std::string storePath;
+            if (i != v.attrs->end())
+                storePath = state.store->printStorePath(state.coerceToStorePath(i->pos, *i->value, context, "while evaluating the drvPath of a derivation"));
+
+            if (options.ansiColors)
+                output << ANSI_GREEN;
+            output << "«derivation";
+            if (!storePath.empty()) {
+                output << " " << storePath;
+            }
+            output << "»";
+            if (options.ansiColors)
+                output << ANSI_NORMAL;
+        } catch (BaseError & e) {
+            printError_(e);
+        }
+    }
+
+    void printAttrs(Value & v, size_t depth)
+    {
+        if (seen && !seen->insert(&v).second) {
+            printRepeated();
+            return;
+        }
+
+        if (options.force && options.derivationPaths && state.isDerivation(v)) {
+            printDerivation(v);
+        } else if (depth < options.maxDepth) {
+            output << "{ ";
+
+            std::vector<std::pair<std::string, Value *>> sorted;
+            for (auto & i : *v.attrs)
+                sorted.emplace_back(std::pair(state.symbols[i.name], i.value));
+
+            if (options.maxAttrs == std::numeric_limits<size_t>::max())
+                std::sort(sorted.begin(), sorted.end());
+            else
+                std::sort(sorted.begin(), sorted.end(), ImportantFirstAttrNameCmp());
+
+            size_t attrsPrinted = 0;
+            for (auto & i : sorted) {
+                if (attrsPrinted >= options.maxAttrs) {
+                    printElided(sorted.size() - attrsPrinted, "attribute", "attributes");
+                    break;
+                }
+
+                printAttributeName(output, i.first);
+                output << " = ";
+                print(*i.second, depth + 1);
+                output << "; ";
+                attrsPrinted++;
+            }
+
+            output << "}";
+        } else
+            output << "{ ... }";
+    }
+
+    void printList(Value & v, size_t depth)
+    {
+        if (seen && v.listSize() && !seen->insert(&v).second) {
+            printRepeated();
+            return;
+        }
+
+        output << "[ ";
+        if (depth < options.maxDepth) {
+            size_t listItemsPrinted = 0;
+            for (auto elem : v.listItems()) {
+                if (listItemsPrinted >= options.maxListItems) {
+                    printElided(v.listSize() - listItemsPrinted, "item", "items");
+                    break;
+                }
+
+                if (elem) {
+                    print(*elem, depth + 1);
+                } else {
+                    printNullptr();
+                }
+                output << " ";
+                listItemsPrinted++;
+            }
+        }
+        else
+            output << "... ";
+        output << "]";
+    }
+
+    void printFunction(Value & v)
+    {
+        if (options.ansiColors)
+            output << ANSI_BLUE;
+        output << "«";
+
+        if (v.isLambda()) {
+            output << "lambda";
+            if (v.lambda.fun) {
+                if (v.lambda.fun->name) {
+                    output << " " << state.symbols[v.lambda.fun->name];
+                }
+
+                std::ostringstream s;
+                s << state.positions[v.lambda.fun->pos];
+                output << " @ " << filterANSIEscapes(s.str());
+            }
+        } else if (v.isPrimOp()) {
+            if (v.primOp)
+                output << *v.primOp;
+            else
+                output << "primop";
+        } else if (v.isPrimOpApp()) {
+            output << "partially applied ";
+            auto primOp = v.primOpAppPrimOp();
+            if (primOp)
+                output << *primOp;
+            else
+                output << "primop";
+        } else {
+            abort();
+        }
+
+        output << "»";
+        if (options.ansiColors)
+            output << ANSI_NORMAL;
+    }
+
+    void printThunk(Value & v)
+    {
+        if (v.isBlackhole()) {
+            // Although we know for sure that it's going to be an infinite recursion
+            // when this value is accessed _in the current context_, it's likely
+            // that the user will misinterpret a simpler «infinite recursion» output
+            // as a definitive statement about the value, while in fact it may be
+            // a valid value after `builtins.trace` and perhaps some other steps
+            // have completed.
+            if (options.ansiColors)
+                output << ANSI_RED;
+            output << "«potential infinite recursion»";
+            if (options.ansiColors)
+                output << ANSI_NORMAL;
+        } else if (v.isThunk() || v.isApp()) {
+            if (options.ansiColors)
+                    output << ANSI_MAGENTA;
+            output << "«thunk»";
+            if (options.ansiColors)
+                    output << ANSI_NORMAL;
+        } else {
+            abort();
+        }
+    }
+
+    void printExternal(Value & v)
+    {
+        v.external->print(output);
+    }
+
+    void printUnknown()
+    {
+        if (options.ansiColors)
+            output << ANSI_RED;
+        output << "«unknown»";
+        if (options.ansiColors)
+            output << ANSI_NORMAL;
+    }
+
+    void printError_(BaseError & e)
+    {
+        if (options.ansiColors)
+            output << ANSI_RED;
+        output << "«" << e.msg() << "»";
+        if (options.ansiColors)
+            output << ANSI_NORMAL;
+    }
+
+    void print(Value & v, size_t depth)
+    {
+        output.flush();
+        checkInterrupt();
+
+        if (options.force) {
+            try {
+                state.forceValue(v, v.determinePos(noPos));
+            } catch (BaseError & e) {
+                printError_(e);
+                return;
+            }
+        }
+
+        switch (v.type()) {
+
+        case nInt:
+            printInt(v);
+            break;
+
+        case nFloat:
+            printFloat(v);
+            break;
+
+        case nBool:
+            printBool(v);
+            break;
+
+        case nString:
+            printString(v);
+            break;
+
+        case nPath:
+            printPath(v);
+            break;
+
+        case nNull:
+            printNull();
+            break;
+
+        case nAttrs:
+            printAttrs(v, depth);
+            break;
+
+        case nList:
+            printList(v, depth);
+            break;
+
+        case nFunction:
+            printFunction(v);
+            break;
+
+        case nThunk:
+            printThunk(v);
+            break;
+
+        case nExternal:
+            printExternal(v);
+            break;
+
+        default:
+            printUnknown();
+            break;
+        }
+    }
+
+public:
+    Printer(std::ostream & output, EvalState & state, PrintOptions options)
+        : output(output), state(state), options(options) { }
+
+    void print(Value & v)
+    {
+        if (options.trackRepeated) {
+            seen.emplace();
+        } else {
+            seen.reset();
+        }
+
+        ValuesSeen seen;
+        print(v, 0);
+    }
+};
+
+void printValue(EvalState & state, std::ostream & output, Value & v, PrintOptions options)
+{
+    Printer(output, state, options).print(v);
+}
 
 }

src/libexpr/print.hh

@@ -9,6 +9,9 @@
 
 #include <iostream>
 
+#include "eval.hh"
+#include "print-options.hh"
+
 namespace nix {
 
 /**
@@ -16,6 +19,7 @@ namespace nix {
  *
  * Quotes and fairly minimal escaping are added.
  *
+ * @param o The output stream to print to
  * @param s The logical string
  */
 std::ostream & printLiteralString(std::ostream & o, std::string_view s);
@@ -53,4 +57,6 @@ bool isReservedKeyword(const std::string_view str);
  */
 std::ostream & printIdentifier(std::ostream & o, std::string_view s);
 
+void printValue(EvalState & state, std::ostream & str, Value & v, PrintOptions options = PrintOptions {});
+
 }

src/libexpr/value.hh

@@ -8,6 +8,7 @@
 #include "value/context.hh"
 #include "input-accessor.hh"
 #include "source-path.hh"
+#include "print-options.hh"
 
 #if HAVE_BOEHMGC
 #include <gc/gc_allocator.h>
@@ -70,7 +71,7 @@ class StorePath;
 class Store;
 class EvalState;
 class XMLWriter;
-
+class Printer;
 
 typedef int64_t NixInt;
 typedef double NixFloat;
@@ -82,6 +83,7 @@ typedef double NixFloat;
 class ExternalValueBase
 {
     friend std::ostream & operator << (std::ostream & str, const ExternalValueBase & v);
+    friend class Printer;
     protected:
     /**
      * Print out the value
@@ -139,11 +141,9 @@ private:
 
     friend std::string showType(const Value & v);
 
-    void print(const SymbolTable &symbols, std::ostream &str, std::set<const void *> *seen, int depth) const;
-
 public:
 
-    void print(const SymbolTable &symbols, std::ostream &str, bool showRepeated = false, int depth = INT_MAX) const;
+    void print(EvalState &state, std::ostream &str, PrintOptions options = PrintOptions {});
 
     // Functions needed to distinguish the type
     // These should be removed eventually, by putting the functionality that's
@@ -351,10 +351,15 @@ public:
     inline void mkPrimOpApp(Value * l, Value * r)
     {
         internalType = tPrimOpApp;
-        app.left = l;
+        primOpApp.left = l;
-        app.right = r;
+        primOpApp.right = r;
     }
 
+    /**
+     * For a `tPrimOpApp` value, get the original `PrimOp` value.
+     */
+    PrimOp * primOpAppPrimOp() const;
+
     inline void mkExternal(ExternalValueBase * e)
     {
         clearValue();

src/libutil/english.cc

@@ -0,0 +1,18 @@
+#include "english.hh"
+
+namespace nix {
+
+std::ostream & pluralize(
+    std::ostream & output,
+    unsigned int count,
+    const std::string_view single,
+    const std::string_view plural)
+{
+    if (count == 1)
+        output << "1 " << single;
+    else
+        output << count << " " << plural;
+    return output;
+}
+
+}

src/libutil/english.hh

@@ -0,0 +1,18 @@
+#pragma once
+
+#include <iostream>
+
+namespace nix {
+
+/**
+ * Pluralize a given value.
+ *
+ * If `count == 1`, prints `1 {single}` to `output`, otherwise prints `{count} {plural}`.
+ */
+std::ostream & pluralize(
+    std::ostream & output,
+    unsigned int count,
+    const std::string_view single,
+    const std::string_view plural);
+
+}

src/nix-env/user-env.cc

@@ -9,6 +9,8 @@
 #include "eval.hh"
 #include "eval-inline.hh"
 #include "profiles.hh"
+#include "print-ambiguous.hh"
+#include <limits>
 
 
 namespace nix {
@@ -106,7 +108,7 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
        the store; we need it for future modifications of the
        environment. */
     std::ostringstream str;
-    manifest.print(state.symbols, str, true);
+    printAmbiguous(manifest, state.symbols, str, nullptr, std::numeric_limits<int>::max());
     auto manifestFile = state.store->addTextToStore("env-manifest.nix",
         str.str(), references);
 

src/nix-instantiate/nix-instantiate.cc

@@ -1,4 +1,5 @@
 #include "globals.hh"
+#include "print-ambiguous.hh"
 #include "shared.hh"
 #include "eval.hh"
 #include "eval-inline.hh"
@@ -25,7 +26,6 @@ static int rootNr = 0;
 
 enum OutputKind { okPlain, okXML, okJSON };
 
-
 void processExpr(EvalState & state, const Strings & attrPaths,
     bool parseOnly, bool strict, Bindings & autoArgs,
     bool evalOnly, OutputKind output, bool location, Expr * e)
@@ -57,7 +57,8 @@ void processExpr(EvalState & state, const Strings & attrPaths,
                 std::cout << std::endl;
             } else {
                 if (strict) state.forceValueDeep(vRes);
-                vRes.print(state.symbols, std::cout);
+                std::set<const void *> seen;
+                printAmbiguous(vRes, state.symbols, std::cout, &seen, std::numeric_limits<int>::max());
                 std::cout << std::endl;
             }
         } else {

tests/functional/lang/eval-okay-print.err.exp

@@ -1 +1 @@
-trace: [ <CODE> ]
+trace: [ «thunk» ]

tests/functional/lang/eval-okay-repeated-empty-attrs.exp

@@ -0,0 +1 @@
+[ { } { } ]

tests/functional/lang/eval-okay-repeated-empty-attrs.nix

@@ -0,0 +1,2 @@
+# Tests that empty attribute sets are not printed as `«repeated»`.
+[ {} {} ]

tests/functional/lang/eval-okay-repeated-empty-list.exp

@@ -0,0 +1 @@
+[ [ ] [ ] ]

tests/functional/lang/eval-okay-repeated-empty-list.nix

@@ -0,0 +1 @@
+[ [] [] ]

tests/unit/libexpr/value/print.cc

@@ -1,6 +1,7 @@
 #include "tests/libexpr.hh"
 
 #include "value.hh"
+#include "print.hh"
 
 namespace nix {
 
@@ -12,7 +13,7 @@ struct ValuePrintingTests : LibExprTest
     void test(Value v, std::string_view expected, A... args)
     {
         std::stringstream out;
-        v.print(state.symbols, out, args...);
+        v.print(state, out, args...);
         ASSERT_EQ(out.str(), expected);
     }
 };
@@ -84,7 +85,7 @@ TEST_F(ValuePrintingTests, tList)
     vList.bigList.elems[1] = &vTwo;
     vList.bigList.size = 3;
 
-    test(vList, "[ 1 2 (nullptr) ]");
+    test(vList, "[ 1 2 «nullptr» ]");
 }
 
 TEST_F(ValuePrintingTests, vThunk)
@@ -92,7 +93,7 @@ TEST_F(ValuePrintingTests, vThunk)
     Value vThunk;
     vThunk.mkThunk(nullptr, nullptr);
 
-    test(vThunk, "<CODE>");
+    test(vThunk, "«thunk»");
 }
 
 TEST_F(ValuePrintingTests, vApp)
@@ -100,32 +101,55 @@ TEST_F(ValuePrintingTests, vApp)
     Value vApp;
     vApp.mkApp(nullptr, nullptr);
 
-    test(vApp, "<CODE>");
+    test(vApp, "«thunk»");
 }
 
 TEST_F(ValuePrintingTests, vLambda)
 {
-    Value vLambda;
+    Env env {
-    vLambda.mkLambda(nullptr, nullptr);
+        .up = nullptr,
+        .values = { }
+    };
+    PosTable::Origin origin((std::monostate()));
+    auto posIdx = state.positions.add(origin, 1, 1);
+    auto body = ExprInt(0);
+    auto formals = Formals {};
 
-    test(vLambda, "<LAMBDA>");
+    ExprLambda eLambda(posIdx, createSymbol("a"), &formals, &body);
+
+    Value vLambda;
+    vLambda.mkLambda(&env, &eLambda);
+
+    test(vLambda, "«lambda @ «none»:1:1»");
+
+    eLambda.setName(createSymbol("puppy"));
+
+    test(vLambda, "«lambda puppy @ «none»:1:1»");
 }
 
 TEST_F(ValuePrintingTests, vPrimOp)
 {
     Value vPrimOp;
-    PrimOp primOp{};
+    PrimOp primOp{
+        .name = "puppy"
+    };
     vPrimOp.mkPrimOp(&primOp);
 
-    test(vPrimOp, "<PRIMOP>");
+    test(vPrimOp, "«primop puppy»");
 }
 
 TEST_F(ValuePrintingTests, vPrimOpApp)
 {
-    Value vPrimOpApp;
+    PrimOp primOp{
-    vPrimOpApp.mkPrimOpApp(nullptr, nullptr);
+        .name = "puppy"
+    };
+    Value vPrimOp;
+    vPrimOp.mkPrimOp(&primOp);
 
-    test(vPrimOpApp, "<PRIMOP-APP>");
+    Value vPrimOpApp;
+    vPrimOpApp.mkPrimOpApp(&vPrimOp, nullptr);
+
+    test(vPrimOpApp, "«partially applied primop puppy»");
 }
 
 TEST_F(ValuePrintingTests, vExternal)
@@ -176,9 +200,14 @@ TEST_F(ValuePrintingTests, depthAttrs)
     Value vTwo;
     vTwo.mkInt(2);
 
+    BindingsBuilder builderEmpty(state, state.allocBindings(0));
+    Value vAttrsEmpty;
+    vAttrsEmpty.mkAttrs(builderEmpty.finish());
+
     BindingsBuilder builder(state, state.allocBindings(10));
     builder.insert(state.symbols.create("one"), &vOne);
     builder.insert(state.symbols.create("two"), &vTwo);
+    builder.insert(state.symbols.create("nested"), &vAttrsEmpty);
 
     Value vAttrs;
     vAttrs.mkAttrs(builder.finish());
@@ -191,10 +220,10 @@ TEST_F(ValuePrintingTests, depthAttrs)
     Value vNested;
     vNested.mkAttrs(builder2.finish());
 
-    test(vNested, "{ nested = «too deep»; one = «too deep»; two = «too deep»; }", false, 1);
+    test(vNested, "{ nested = { ... }; one = 1; two = 2; }", PrintOptions { .maxDepth = 1 });
-    test(vNested, "{ nested = { one = «too deep»; two = «too deep»; }; one = 1; two = 2; }", false, 2);
+    test(vNested, "{ nested = { nested = { ... }; one = 1; two = 2; }; one = 1; two = 2; }", PrintOptions { .maxDepth = 2 });
-    test(vNested, "{ nested = { one = 1; two = 2; }; one = 1; two = 2; }", false, 3);
+    test(vNested, "{ nested = { nested = { }; one = 1; two = 2; }; one = 1; two = 2; }", PrintOptions { .maxDepth = 3 });
-    test(vNested, "{ nested = { one = 1; two = 2; }; one = 1; two = 2; }", false, 4);
+    test(vNested, "{ nested = { nested = { }; one = 1; two = 2; }; one = 1; two = 2; }", PrintOptions { .maxDepth = 4 });
 }
 
 TEST_F(ValuePrintingTests, depthList)
@@ -227,11 +256,561 @@ TEST_F(ValuePrintingTests, depthList)
     vList.bigList.elems[2] = &vNested;
     vList.bigList.size = 3;
 
-    test(vList, "[ «too deep» «too deep» «too deep» ]", false, 1);
+    test(vList, "[ 1 2 { ... } ]", PrintOptions { .maxDepth = 1 });
-    test(vList, "[ 1 2 { nested = «too deep»; one = «too deep»; two = «too deep»; } ]", false, 2);
+    test(vList, "[ 1 2 { nested = { ... }; one = 1; two = 2; } ]", PrintOptions { .maxDepth = 2 });
-    test(vList, "[ 1 2 { nested = { one = «too deep»; two = «too deep»; }; one = 1; two = 2; } ]", false, 3);
+    test(vList, "[ 1 2 { nested = { one = 1; two = 2; }; one = 1; two = 2; } ]", PrintOptions { .maxDepth = 3 });
-    test(vList, "[ 1 2 { nested = { one = 1; two = 2; }; one = 1; two = 2; } ]", false, 4);
+    test(vList, "[ 1 2 { nested = { one = 1; two = 2; }; one = 1; two = 2; } ]", PrintOptions { .maxDepth = 4 });
-    test(vList, "[ 1 2 { nested = { one = 1; two = 2; }; one = 1; two = 2; } ]", false, 5);
+    test(vList, "[ 1 2 { nested = { one = 1; two = 2; }; one = 1; two = 2; } ]", PrintOptions { .maxDepth = 5 });
+}
+
+struct StringPrintingTests : LibExprTest
+{
+    template<class... A>
+    void test(std::string_view literal, std::string_view expected, unsigned int maxLength, A... args)
+    {
+        Value v;
+        v.mkString(literal);
+
+        std::stringstream out;
+        printValue(state, out, v, PrintOptions {
+            .maxStringLength = maxLength
+        });
+        ASSERT_EQ(out.str(), expected);
+    }
+};
+
+TEST_F(StringPrintingTests, maxLengthTruncation)
+{
+    test("abcdefghi", "\"abcdefghi\"", 10);
+    test("abcdefghij", "\"abcdefghij\"", 10);
+    test("abcdefghijk", "\"abcdefghij\" «1 byte elided»", 10);
+    test("abcdefghijkl", "\"abcdefghij\" «2 bytes elided»", 10);
+    test("abcdefghijklm", "\"abcdefghij\" «3 bytes elided»", 10);
+}
+
+// Check that printing an attrset shows 'important' attributes like `type`
+// first, but only reorder the attrs when we have a maxAttrs budget.
+TEST_F(ValuePrintingTests, attrsTypeFirst)
+{
+    Value vType;
+    vType.mkString("puppy");
+
+    Value vApple;
+    vApple.mkString("apple");
+
+    BindingsBuilder builder(state, state.allocBindings(10));
+    builder.insert(state.symbols.create("type"), &vType);
+    builder.insert(state.symbols.create("apple"), &vApple);
+
+    Value vAttrs;
+    vAttrs.mkAttrs(builder.finish());
+
+    test(vAttrs,
+         "{ type = \"puppy\"; apple = \"apple\"; }",
+         PrintOptions {
+            .maxAttrs = 100
+         });
+
+    test(vAttrs,
+         "{ apple = \"apple\"; type = \"puppy\"; }",
+         PrintOptions { });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsInt)
+{
+    Value v;
+    v.mkInt(10);
+
+    test(v,
+         ANSI_CYAN "10" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsFloat)
+{
+    Value v;
+    v.mkFloat(1.6);
+
+    test(v,
+         ANSI_CYAN "1.6" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsBool)
+{
+    Value v;
+    v.mkBool(true);
+
+    test(v,
+         ANSI_CYAN "true" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsString)
+{
+    Value v;
+    v.mkString("puppy");
+
+    test(v,
+         ANSI_MAGENTA "\"puppy\"" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true
+        });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsStringElided)
+{
+    Value v;
+    v.mkString("puppy");
+
+    test(v,
+         ANSI_MAGENTA "\"pup\"" ANSI_FAINT " «2 bytes elided»" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true,
+             .maxStringLength = 3
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsPath)
+{
+    Value v;
+    v.mkPath(state.rootPath(CanonPath("puppy")));
+
+    test(v,
+         ANSI_GREEN "/puppy" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsNull)
+{
+    Value v;
+    v.mkNull();
+
+    test(v,
+         ANSI_CYAN "null" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsAttrs)
+{
+    Value vOne;
+    vOne.mkInt(1);
+
+    Value vTwo;
+    vTwo.mkInt(2);
+
+    BindingsBuilder builder(state, state.allocBindings(10));
+    builder.insert(state.symbols.create("one"), &vOne);
+    builder.insert(state.symbols.create("two"), &vTwo);
+
+    Value vAttrs;
+    vAttrs.mkAttrs(builder.finish());
+
+    test(vAttrs,
+         "{ one = " ANSI_CYAN "1" ANSI_NORMAL "; two = " ANSI_CYAN "2" ANSI_NORMAL "; }",
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsDerivation)
+{
+    Value vDerivation;
+    vDerivation.mkString("derivation");
+
+    BindingsBuilder builder(state, state.allocBindings(10));
+    builder.insert(state.sType, &vDerivation);
+
+    Value vAttrs;
+    vAttrs.mkAttrs(builder.finish());
+
+    test(vAttrs,
+         ANSI_GREEN "«derivation»" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true,
+             .force = true,
+             .derivationPaths = true
+         });
+
+    test(vAttrs,
+         "{ type = " ANSI_MAGENTA "\"derivation\"" ANSI_NORMAL "; }",
+         PrintOptions {
+             .ansiColors = true,
+             .force = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsError)
+{
+    Value throw_ = state.getBuiltin("throw");
+    Value message;
+    message.mkString("uh oh!");
+    Value vError;
+    vError.mkApp(&throw_, &message);
+
+    test(vError,
+         ANSI_RED
+         "«"
+         ANSI_RED
+         "error:"
+         ANSI_NORMAL
+         "\n       … while calling the '"
+         ANSI_MAGENTA
+         "throw"
+         ANSI_NORMAL
+         "' builtin\n\n       "
+         ANSI_RED
+         "error:"
+         ANSI_NORMAL
+         " uh oh!»"
+         ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true,
+             .force = true,
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsDerivationError)
+{
+    Value throw_ = state.getBuiltin("throw");
+    Value message;
+    message.mkString("uh oh!");
+    Value vError;
+    vError.mkApp(&throw_, &message);
+
+    Value vDerivation;
+    vDerivation.mkString("derivation");
+
+    BindingsBuilder builder(state, state.allocBindings(10));
+    builder.insert(state.sType, &vDerivation);
+    builder.insert(state.sDrvPath, &vError);
+
+    Value vAttrs;
+    vAttrs.mkAttrs(builder.finish());
+
+    test(vAttrs,
+         "{ drvPath = "
+         ANSI_RED
+         "«"
+         ANSI_RED
+         "error:"
+         ANSI_NORMAL
+         "\n       … while calling the '"
+         ANSI_MAGENTA
+         "throw"
+         ANSI_NORMAL
+         "' builtin\n\n       "
+         ANSI_RED
+         "error:"
+         ANSI_NORMAL
+         " uh oh!»"
+         ANSI_NORMAL
+         "; type = "
+         ANSI_MAGENTA
+         "\"derivation\""
+         ANSI_NORMAL
+         "; }",
+         PrintOptions {
+             .ansiColors = true,
+             .force = true
+         });
+
+    test(vAttrs,
+         ANSI_RED
+         "«"
+         ANSI_RED
+         "error:"
+         ANSI_NORMAL
+         "\n       … while calling the '"
+         ANSI_MAGENTA
+         "throw"
+         ANSI_NORMAL
+         "' builtin\n\n       "
+         ANSI_RED
+         "error:"
+         ANSI_NORMAL
+         " uh oh!»"
+         ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true,
+             .force = true,
+             .derivationPaths = true,
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsAssert)
+{
+    ExprVar eFalse(state.symbols.create("false"));
+    eFalse.bindVars(state, state.staticBaseEnv);
+    ExprInt eInt(1);
+
+    ExprAssert expr(noPos, &eFalse, &eInt);
+
+    Value v;
+    state.mkThunk_(v, &expr);
+
+    test(v,
+         ANSI_RED "«" ANSI_RED "error:" ANSI_NORMAL " assertion '" ANSI_MAGENTA "false" ANSI_NORMAL "' failed»" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true,
+             .force = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsList)
+{
+    Value vOne;
+    vOne.mkInt(1);
+
+    Value vTwo;
+    vTwo.mkInt(2);
+
+    Value vList;
+    state.mkList(vList, 5);
+    vList.bigList.elems[0] = &vOne;
+    vList.bigList.elems[1] = &vTwo;
+    vList.bigList.size = 3;
+
+    test(vList,
+         "[ " ANSI_CYAN "1" ANSI_NORMAL " " ANSI_CYAN "2" ANSI_NORMAL " " ANSI_MAGENTA "«nullptr»" ANSI_NORMAL " ]",
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsLambda)
+{
+    Env env {
+        .up = nullptr,
+        .values = { }
+    };
+    PosTable::Origin origin((std::monostate()));
+    auto posIdx = state.positions.add(origin, 1, 1);
+    auto body = ExprInt(0);
+    auto formals = Formals {};
+
+    ExprLambda eLambda(posIdx, createSymbol("a"), &formals, &body);
+
+    Value vLambda;
+    vLambda.mkLambda(&env, &eLambda);
+
+    test(vLambda,
+         ANSI_BLUE "«lambda @ «none»:1:1»" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true,
+             .force = true
+         });
+
+    eLambda.setName(createSymbol("puppy"));
+
+    test(vLambda,
+         ANSI_BLUE "«lambda puppy @ «none»:1:1»" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true,
+             .force = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsPrimOp)
+{
+    PrimOp primOp{
+        .name = "puppy"
+    };
+    Value v;
+    v.mkPrimOp(&primOp);
+
+    test(v,
+         ANSI_BLUE "«primop puppy»" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsPrimOpApp)
+{
+    PrimOp primOp{
+        .name = "puppy"
+    };
+    Value vPrimOp;
+    vPrimOp.mkPrimOp(&primOp);
+
+    Value v;
+    v.mkPrimOpApp(&vPrimOp, nullptr);
+
+    test(v,
+         ANSI_BLUE "«partially applied primop puppy»" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsThunk)
+{
+    Value v;
+    v.mkThunk(nullptr, nullptr);
+
+    test(v,
+         ANSI_MAGENTA "«thunk»" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsBlackhole)
+{
+    Value v;
+    v.mkBlackhole();
+
+    test(v,
+         ANSI_RED "«potential infinite recursion»" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsAttrsRepeated)
+{
+    BindingsBuilder emptyBuilder(state, state.allocBindings(1));
+
+    Value vEmpty;
+    vEmpty.mkAttrs(emptyBuilder.finish());
+
+    BindingsBuilder builder(state, state.allocBindings(10));
+    builder.insert(state.symbols.create("a"), &vEmpty);
+    builder.insert(state.symbols.create("b"), &vEmpty);
+
+    Value vAttrs;
+    vAttrs.mkAttrs(builder.finish());
+
+    test(vAttrs,
+         "{ a = { }; b = " ANSI_MAGENTA "«repeated»" ANSI_NORMAL "; }",
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsListRepeated)
+{
+    BindingsBuilder emptyBuilder(state, state.allocBindings(1));
+
+    Value vEmpty;
+    vEmpty.mkAttrs(emptyBuilder.finish());
+
+    Value vList;
+    state.mkList(vList, 3);
+    vList.bigList.elems[0] = &vEmpty;
+    vList.bigList.elems[1] = &vEmpty;
+    vList.bigList.size = 2;
+
+    test(vList,
+         "[ { } " ANSI_MAGENTA "«repeated»" ANSI_NORMAL " ]",
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, listRepeated)
+{
+    BindingsBuilder emptyBuilder(state, state.allocBindings(1));
+
+    Value vEmpty;
+    vEmpty.mkAttrs(emptyBuilder.finish());
+
+    Value vList;
+    state.mkList(vList, 3);
+    vList.bigList.elems[0] = &vEmpty;
+    vList.bigList.elems[1] = &vEmpty;
+    vList.bigList.size = 2;
+
+    test(vList, "[ { } «repeated» ]", PrintOptions { });
+    test(vList,
+         "[ { } { } ]",
+         PrintOptions {
+             .trackRepeated = false
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsAttrsElided)
+{
+    Value vOne;
+    vOne.mkInt(1);
+
+    Value vTwo;
+    vTwo.mkInt(2);
+
+    BindingsBuilder builder(state, state.allocBindings(10));
+    builder.insert(state.symbols.create("one"), &vOne);
+    builder.insert(state.symbols.create("two"), &vTwo);
+
+    Value vAttrs;
+    vAttrs.mkAttrs(builder.finish());
+
+    test(vAttrs,
+         "{ one = " ANSI_CYAN "1" ANSI_NORMAL "; " ANSI_FAINT " «1 attribute elided»" ANSI_NORMAL "}",
+         PrintOptions {
+             .ansiColors = true,
+             .maxAttrs = 1
+         });
+
+    Value vThree;
+    vThree.mkInt(3);
+
+    builder.insert(state.symbols.create("three"), &vThree);
+    vAttrs.mkAttrs(builder.finish());
+
+    test(vAttrs,
+         "{ one = " ANSI_CYAN "1" ANSI_NORMAL "; " ANSI_FAINT " «2 attributes elided»" ANSI_NORMAL "}",
+         PrintOptions {
+             .ansiColors = true,
+             .maxAttrs = 1
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsListElided)
+{
+    BindingsBuilder emptyBuilder(state, state.allocBindings(1));
+
+    Value vOne;
+    vOne.mkInt(1);
+
+    Value vTwo;
+    vTwo.mkInt(2);
+
+    Value vList;
+    state.mkList(vList, 4);
+    vList.bigList.elems[0] = &vOne;
+    vList.bigList.elems[1] = &vTwo;
+    vList.bigList.size = 2;
+
+    test(vList,
+         "[ " ANSI_CYAN "1" ANSI_NORMAL " " ANSI_FAINT " «1 item elided»" ANSI_NORMAL "]",
+         PrintOptions {
+             .ansiColors = true,
+             .maxListItems = 1
+         });
+
+    Value vThree;
+    vThree.mkInt(3);
+
+    vList.bigList.elems[2] = &vThree;
+    vList.bigList.size = 3;
+
+    test(vList,
+         "[ " ANSI_CYAN "1" ANSI_NORMAL " " ANSI_FAINT " «2 items elided»" ANSI_NORMAL "]",
+         PrintOptions {
+             .ansiColors = true,
+             .maxListItems = 1
+         });
 }
 
 } // namespace nix