Some notational convenience for formatting strings

We can now write

  throw Error("file '%s' not found", path);

instead of

  throw Error(format("file '%s' not found") % path);

and similarly

  printError("file '%s' not found", path);

instead of

  printMsg(lvlError, format("file '%s' not found") % path);

src/libstore/build.cc

@@ -3768,7 +3768,7 @@ void LocalStore::buildPaths(const PathSet & drvPaths, BuildMode buildMode)
         }
 
     if (!failed.empty())
-        throw Error(format("build of %1% failed") % showPaths(failed), worker.exitStatus());
+        throw Error(worker.exitStatus(), "build of %s failed",showPaths(failed));
 }
 
 
@@ -3804,7 +3804,7 @@ void LocalStore::ensurePath(const Path & path)
     worker.run(goals);
 
     if (goal->getExitCode() != Goal::ecSuccess)
-        throw Error(format("path ‘%1%’ does not exist and cannot be created") % path, worker.exitStatus());
+        throw Error(worker.exitStatus(), "path ‘%s’ does not exist and cannot be created", path);
 }
 
 
@@ -3825,7 +3825,7 @@ void LocalStore::repairPath(const Path & path)
             goals.insert(worker.makeDerivationGoal(deriver, StringSet(), bmRepair));
             worker.run(goals);
         } else
-            throw Error(format("cannot repair path ‘%1%’") % path, worker.exitStatus());
+            throw Error(worker.exitStatus(), "cannot repair path ‘%s’", path);
     }
 }
 

src/libstore/remote-store.cc

@@ -564,7 +564,7 @@ void RemoteStore::Connection::processStderr(Sink * sink, Source * source)
     if (msg == STDERR_ERROR) {
         string error = readString(from);
         unsigned int status = readInt(from);
-        throw Error(format("%1%") % error, status);
+        throw Error(status, error);
     }
     else if (msg != STDERR_LAST)
         throw Error("protocol error processing standard error");

src/libutil/args.hh

@@ -8,7 +8,7 @@
 
 namespace nix {
 
-MakeError(UsageError, nix::Error);
+MakeError(UsageError, Error);
 
 enum HashType : char;
 

src/libutil/logging.hh

@@ -66,14 +66,19 @@ Logger * makeDefaultLogger();
 
 extern Verbosity verbosity; /* suppress msgs > this */
 
-#define printMsg(level, f) \
+/* Print a message if the current log level is at least the specified
+   level. Note that this has to be implemented as a macro to ensure
+   that the arguments are evaluated lazily. */
+#define printMsg(level, args...) \
     do { \
         if (level <= nix::verbosity) { \
-            logger->log(level, (f)); \
+            logger->log(level, fmt(args)); \
         } \
     } while (0)
 
-#define debug(f) printMsg(lvlDebug, f)
+#define printError(args...) printMsg(lvlError, args)
+#define printInfo(args...) printMsg(lvlInfo, args)
+#define debug(args...) printMsg(lvlDebug, args)
 
 void warnOnce(bool & haveWarned, const FormatOrString & fs);
 

src/libutil/types.hh

@@ -41,6 +41,45 @@ struct FormatOrString
 };
 
 
+/* A helper for formatting strings. ‘fmt(format, a_0, ..., a_n)’ is
+   equivalent to ‘boost::format(format) % a_0 % ... %
+   ... a_n’. However, ‘fmt(s)’ is equivalent to ‘s’ (so no %-expansion
+   takes place). */
+
+inline void formatHelper(boost::format & f)
+{
+}
+
+template<typename T, typename... Args>
+inline void formatHelper(boost::format & f, T x, Args... args)
+{
+    formatHelper(f % x, args...);
+}
+
+inline std::string fmt(const std::string & s)
+{
+    return s;
+}
+
+inline std::string fmt(const char * s)
+{
+    return s;
+}
+
+inline std::string fmt(const FormatOrString & fs)
+{
+    return fs.s;
+}
+
+template<typename... Args>
+inline std::string fmt(const std::string & fs, Args... args)
+{
+    boost::format f(fs);
+    formatHelper(f, args...);
+    return f.str();
+}
+
+
 /* BaseError should generally not be caught, as it has Interrupted as
    a subclass. Catch Error instead. */
 class BaseError : public std::exception
@@ -49,14 +88,28 @@ protected:
     string prefix_; // used for location traces etc.
     string err;
 public:
-    unsigned int status; // exit status
+    unsigned int status = 1; // exit status
-    BaseError(const FormatOrString & fs, unsigned int status = 1);
+
+    template<typename... Args>
+    BaseError(unsigned int status, Args... args)
+        : err(fmt(args...))
+        , status(status)
+    {
+    }
+
+    template<typename... Args>
+    BaseError(Args... args)
+        : err(fmt(args...))
+    {
+    }
+
 #ifdef EXCEPTION_NEEDS_THROW_SPEC
     ~BaseError() throw () { };
     const char * what() const throw () { return err.c_str(); }
 #else
     const char * what() const noexcept { return err.c_str(); }
 #endif
+
     const string & msg() const { return err; }
     const string & prefix() const { return prefix_; }
     BaseError & addPrefix(const FormatOrString & fs);
@@ -66,7 +119,7 @@ public:
     class newClass : public superClass                  \
     {                                                   \
     public:                                             \
-        newClass(const FormatOrString & fs, unsigned int status = 1) : superClass(fs, status) { }; \
+        using superClass::superClass;                   \
     };
 
 MakeError(Error, BaseError)
@@ -75,7 +128,15 @@ class SysError : public Error
 {
 public:
     int errNo;
-    SysError(const FormatOrString & fs);
+
+    template<typename... Args>
+    SysError(Args... args)
+        : Error(addErrno(fmt(args...)))
+    { }
+
+private:
+
+    std::string addErrno(const std::string & s);
 };
 
 

src/libutil/util.cc

@@ -31,13 +31,6 @@ extern char * * environ;
 namespace nix {
 
 
-BaseError::BaseError(const FormatOrString & fs, unsigned int status)
-    : status(status)
-{
-    err = fs.s;
-}
-
-
 BaseError & BaseError::addPrefix(const FormatOrString & fs)
 {
     prefix_ = fs.s + prefix_;
@@ -45,10 +38,10 @@ BaseError & BaseError::addPrefix(const FormatOrString & fs)
 }
 
 
-SysError::SysError(const FormatOrString & fs)
+std::string SysError::addErrno(const std::string & s)
-    : Error(format("%1%: %2%") % fs.s % strerror(errno))
-    , errNo(errno)
 {
+    errNo = errno;
+    return s + ": " + strerror(errNo);
 }