gistfile1.diff

diff --git a/log/value.go b/log/value.go
index f2c55b7..4351586 100644
--- a/log/value.go
+++ b/log/value.go
@@ -18,6 +18,12 @@ type Valuer interface {
 	Value() Value
 }
 
+// ValuerFunc is an adapter func type that implements the Valuer interface.
+type ValuerFunc func() Value
+
+// Value implements the Value method of the Valuer interface.
+func (f ValuerFunc) Value() Value { return f() }
+
 // BindValues returns a slice with all value elements (odd indexes) that
 // implement Valuer replaced with the result of calling their Value method. If
 // no value elements implement Valuer, the original slice is returned.
@@ -46,31 +52,27 @@ func containsValuer(keyvals []interface{}) bool {
 	return false
 }
 
-// Timestamp is a Valuer that invokes the underlying function when bound,
-// returning a time.Time. Users will probably want to use DefaultTimestamp or
-// DefaultTimestampUTC.
-type Timestamp func() time.Time
-
-// Value implements Valuer.
-func (t Timestamp) Value() Value { return t() }
-
-// Caller is a Valuer that returns a file and line from a specified depth in
-// the callstack. Users will probably want to use DefaultCaller.
-type Caller int
-
-// Value implements Valuer.
-func (c Caller) Value() Value { return stack.Caller(int(c)) }
-
 var (
+	// Timestamp returns a Valuer that invokes the underlying function when bound,
+	// returning a time.Time. Users will probably want to use DefaultTimestamp or
+	// DefaultTimestampUTC.
+	Timestamp = func(t func() time.Time) Valuer {
+		return ValuerFunc(func() Value { return t() })
+	}
 	// DefaultTimestamp is a Timestamp Valuer that returns the current wallclock
 	// time, respecting time zones, when bound.
-	DefaultTimestamp Timestamp = time.Now
-
+	DefaultTimestamp = Timestamp(time.Now)
 	// DefaultTimestampUTC wraps DefaultTimestamp but ensures the returned
 	// time is always in UTC. Note that it invokes DefaultTimestamp, and so
 	// reflects any changes to the DefaultTimestamp package global.
-	DefaultTimestampUTC Timestamp = func() time.Time { return DefaultTimestamp().UTC() }
-
+	DefaultTimestampUTC = Timestamp(func() time.Time {
+		return DefaultTimestamp.Value().(time.Time).UTC()
+	})
+	// Caller is a Valuer that returns a file and line from a specified depth in
+	// the callstack. Users will probably want to use DefaultCaller.
+	Caller = func(depth int) Valuer {
+		return ValuerFunc(func() Value { return stack.Caller(depth) })
+	}
 	// DefaultCaller is a Valuer that returns the file and line where the Log
 	// method was invoked.
 	DefaultCaller = Caller(4)