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
NixOS#9554 (comment))

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`.

src/libcmd/repl.cc

@@ -93,9 +93,16 @@ struct NixRepl
     void evalString(std::string s, Value & v);
     void loadDebugTraceEnv(DebugTrace & dt);
 
-    typedef std::set<Value *> ValuesSeen;
-    std::ostream & printValue(std::ostream & str, Value & v, unsigned int maxDepth);
-    std::ostream & printValue(std::ostream & str, Value & v, unsigned int maxDepth, ValuesSeen & seen);
+    void printValue(std::ostream & str,
+                              Value & v,
+                              unsigned int maxDepth = std::numeric_limits<unsigned int>::max())
+    {
+        ::nix::printValue(*state, str, v, PrintOptions {
+            .ansiColors = true,
+            .force = true,
+            .maxDepth = maxDepth
+        });
+    }
 };
 
 std::string removeWhitespace(std::string s)
@@ -708,7 +715,8 @@ 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") {
@@ -770,7 +778,8 @@ 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;
         }
     }
 
@@ -892,144 +901,6 @@ 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, [&]() { return 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_view());
-        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

@@ -103,101 +103,9 @@ 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_view());
-        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:
-        str << "<CODE>";
-        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;
-    case tBlackhole:
-        // 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;
-    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);
+void Value::print(EvalState &state, std::ostream &str,
+                  PrintOptions options) {
+    printValue(state, str, *this, options);
 }
 
 // Pretty print types for assertion errors
@@ -206,10 +114,10 @@ std::ostream & operator << (std::ostream & os, const ValueType 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();
 }
 

src/libexpr/eval.hh

@@ -132,7 +132,7 @@ 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-options.hh

@@ -0,0 +1,45 @@
+#pragma once
+/**
+ * @file
+ * @brief Options for printing Nix values.
+ */
+
+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, 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.
+     */
+    unsigned int maxDepth = std::numeric_limits<unsigned int>::max();
+    /**
+     * Maximum number of attributes in an attribute set to print.
+     */
+    unsigned int maxAttrs = std::numeric_limits<unsigned int>::max();
+    /**
+     * Maximum number of list items to print.
+     */
+    unsigned int maxListItems = std::numeric_limits<unsigned int>::max();
+    /**
+     * Maximum string length to print.
+     */
+    unsigned int maxStringLength = std::numeric_limits<unsigned int>::max();
+};
+
+}