Print value on type error

Adds the failing value to `value is <TYPE> while a <TYPE> is expected`
error messages.

src/libexpr/eval-inline.hh

@@ -1,6 +1,7 @@
 #pragma once
 ///@file
 
+#include "print.hh"
 #include "eval.hh"
 
 namespace nix {
@@ -114,7 +115,10 @@ inline void EvalState::forceAttrs(Value & v, Callable getPos, std::string_view e
     PosIdx pos = getPos();
     forceValue(v, pos);
     if (v.type() != nAttrs) {
-        error("value is %1% while a set was expected", showType(v)).withTrace(pos, errorCtx).debugThrow<TypeError>();
+        error("expected a set but found %1%: %2%",
+              showType(v),
+              ValuePrinter(*this, v, errorPrintOptions))
+            .withTrace(pos, errorCtx).debugThrow<TypeError>();
     }
 }
 
@@ -124,7 +128,10 @@ inline void EvalState::forceList(Value & v, const PosIdx pos, std::string_view e
 {
     forceValue(v, pos);
     if (!v.isList()) {
-        error("value is %1% while a list was expected", showType(v)).withTrace(pos, errorCtx).debugThrow<TypeError>();
+        error("expected a list but found %1%: %2%",
+              showType(v),
+              ValuePrinter(*this, v, errorPrintOptions))
+            .withTrace(pos, errorCtx).debugThrow<TypeError>();
     }
 }
 

src/libexpr/eval.cc

@@ -2,6 +2,7 @@
 #include "eval-settings.hh"
 #include "hash.hh"
 #include "primops.hh"
+#include "print-options.hh"
 #include "types.hh"
 #include "util.hh"
 #include "store-api.hh"
@@ -29,9 +30,9 @@
 #include <unistd.h>
 #include <sys/time.h>
 #include <sys/resource.h>
-#include <iostream>
 #include <fstream>
 #include <functional>
+#include <iostream>
 
 #include <sys/resource.h>
 #include <nlohmann/json.hpp>
@@ -1153,7 +1154,10 @@ inline bool EvalState::evalBool(Env & env, Expr * e, const PosIdx pos, std::stri
         Value v;
         e->eval(*this, env, v);
         if (v.type() != nBool)
-            error("value is %1% while a Boolean was expected", showType(v)).withFrame(env, *e).debugThrow<TypeError>();
+            error("expected a Boolean but found %1%: %2%",
+                  showType(v),
+                  ValuePrinter(*this, v, errorPrintOptions))
+                .withFrame(env, *e).debugThrow<TypeError>();
         return v.boolean;
     } catch (Error & e) {
         e.addTrace(positions[pos], errorCtx);
@@ -1167,7 +1171,10 @@ inline void EvalState::evalAttrs(Env & env, Expr * e, Value & v, const PosIdx po
     try {
         e->eval(*this, env, v);
         if (v.type() != nAttrs)
-            error("value is %1% while a set was expected", showType(v)).withFrame(env, *e).debugThrow<TypeError>();
+            error("expected a set but found %1%: %2%",
+                  showType(v),
+                  ValuePrinter(*this, v, errorPrintOptions))
+                .withFrame(env, *e).debugThrow<TypeError>();
     } catch (Error & e) {
         e.addTrace(positions[pos], errorCtx);
         throw;
@@ -2076,7 +2083,10 @@ NixInt EvalState::forceInt(Value & v, const PosIdx pos, std::string_view errorCt
     try {
         forceValue(v, pos);
         if (v.type() != nInt)
-            error("value is %1% while an integer was expected", showType(v)).debugThrow<TypeError>();
+            error("expected an integer but found %1%: %2%",
+                  showType(v),
+                  ValuePrinter(*this, v, errorPrintOptions))
+                .debugThrow<TypeError>();
         return v.integer;
     } catch (Error & e) {
         e.addTrace(positions[pos], errorCtx);
@@ -2092,7 +2102,10 @@ NixFloat EvalState::forceFloat(Value & v, const PosIdx pos, std::string_view err
         if (v.type() == nInt)
             return v.integer;
         else if (v.type() != nFloat)
-            error("value is %1% while a float was expected", showType(v)).debugThrow<TypeError>();
+            error("expected a float but found %1%: %2%",
+                  showType(v),
+                  ValuePrinter(*this, v, errorPrintOptions))
+                .debugThrow<TypeError>();
         return v.fpoint;
     } catch (Error & e) {
         e.addTrace(positions[pos], errorCtx);
@@ -2106,7 +2119,10 @@ bool EvalState::forceBool(Value & v, const PosIdx pos, std::string_view errorCtx
     try {
         forceValue(v, pos);
         if (v.type() != nBool)
-            error("value is %1% while a Boolean was expected", showType(v)).debugThrow<TypeError>();
+            error("expected a Boolean but found %1%: %2%",
+                  showType(v),
+                  ValuePrinter(*this, v, errorPrintOptions))
+                .debugThrow<TypeError>();
         return v.boolean;
     } catch (Error & e) {
         e.addTrace(positions[pos], errorCtx);
@@ -2126,7 +2142,10 @@ void EvalState::forceFunction(Value & v, const PosIdx pos, std::string_view erro
     try {
         forceValue(v, pos);
         if (v.type() != nFunction && !isFunctor(v))
-            error("value is %1% while a function was expected", showType(v)).debugThrow<TypeError>();
+            error("expected a function but found %1%: %2%",
+                  showType(v),
+                  ValuePrinter(*this, v, errorPrintOptions))
+                .debugThrow<TypeError>();
     } catch (Error & e) {
         e.addTrace(positions[pos], errorCtx);
         throw;
@@ -2139,7 +2158,10 @@ std::string_view EvalState::forceString(Value & v, const PosIdx pos, std::string
     try {
         forceValue(v, pos);
         if (v.type() != nString)
-            error("value is %1% while a string was expected", showType(v)).debugThrow<TypeError>();
+            error("expected a string but found %1%: %2%",
+                  showType(v),
+                  ValuePrinter(*this, v, errorPrintOptions))
+                .debugThrow<TypeError>();
         return v.string_view();
     } catch (Error & e) {
         e.addTrace(positions[pos], errorCtx);

src/libexpr/primops.cc

@@ -997,7 +997,7 @@ static void prim_trace(EvalState & state, const PosIdx pos, Value * * args, Valu
     if (args[0]->type() == nString)
         printError("trace: %1%", args[0]->string_view());
     else
-        printError("trace: %1%", printValue(state, *args[0]));
+        printError("trace: %1%", ValuePrinter(state, *args[0]));
     state.forceValue(*args[1], pos);
     v = *args[1];
 }

src/libexpr/print-ambiguous.cc

@@ -1,6 +1,7 @@
 #include "print-ambiguous.hh"
 #include "print.hh"
 #include "signals.hh"
+#include "eval.hh"
 
 namespace nix {
 

src/libexpr/print-options.hh

@@ -49,4 +49,16 @@ struct PrintOptions
     size_t maxStringLength = std::numeric_limits<size_t>::max();
 };
 
+/**
+ * `PrintOptions` for unknown and therefore potentially large values in error messages,
+ * to avoid printing "too much" output.
+ */
+static PrintOptions errorPrintOptions = PrintOptions {
+    .ansiColors = true,
+    .maxDepth = 10,
+    .maxAttrs = 10,
+    .maxListItems = 10,
+    .maxStringLength = 1024
+};
+
 }

src/libexpr/print.cc

@@ -7,6 +7,7 @@
 #include "store-api.hh"
 #include "terminal.hh"
 #include "english.hh"
+#include "eval.hh"
 
 namespace nix {
 
@@ -501,4 +502,10 @@ void printValue(EvalState & state, std::ostream & output, Value & v, PrintOption
     Printer(output, state, options).print(v);
 }
 
+std::ostream & operator<<(std::ostream & output, const ValuePrinter & printer)
+{
+    printValue(printer.state, output, printer.value, printer.options);
+    return output;
+}
+
 }

src/libexpr/print.hh

@@ -9,11 +9,13 @@
 
 #include <iostream>
 
-#include "eval.hh"
 #include "print-options.hh"
 
 namespace nix {
 
+class EvalState;
+struct Value;
+
 /**
  * Print a string as a Nix string literal.
  *
@@ -59,4 +61,21 @@ std::ostream & printIdentifier(std::ostream & o, std::string_view s);
 
 void printValue(EvalState & state, std::ostream & str, Value & v, PrintOptions options = PrintOptions {});
 
+/**
+ * A partially-applied form of `printValue` which can be formatted using `<<`
+ * without allocating an intermediate string.
+ */
+class ValuePrinter {
+    friend std::ostream & operator << (std::ostream & output, const ValuePrinter & printer);
+private:
+    EvalState & state;
+    Value & value;
+    PrintOptions options;
+
+public:
+    ValuePrinter(EvalState & state, Value & value, PrintOptions options = PrintOptions {})
+        : state(state), value(value), options(options) { }
+};
+
+std::ostream & operator<<(std::ostream & output, const ValuePrinter & printer);
 }