1 files changed, 27 insertions, 2 deletions
diff --git a/skate/map.go b/skate/map.go
index 8d8094b..b06bf96 100644
--- a/skate/map.go
+++ b/skate/map.go
@@ -2,6 +2,7 @@ package skate
 
 import (
 	"bytes"
+	"errors"
 	"reflect"
 	"runtime"
 	"strconv"
@@ -13,8 +14,12 @@ import (
 var (
 	bTab     = []byte("\t")
 	bNewline = []byte("\n")
+
+	ErrZeroFields       = errors.New("zero fields")
+	ErrMissingFieldName = errors.New("missing field name")
 )
 
+// Title is a document with a title.
 type TitleDoc struct {
 	Title string `json:"title"`
 }
@@ -35,9 +40,9 @@ type PartialDoc struct {
 // doc). We want fields, but we do not want to bake in TSV into each function.
 type Mapper func([]byte) ([][]byte, error)
 
-// TSV serialized the result of a field mapper as TSV. This is a slim adapter,
+// AsTSV serialized the result of a field mapper as AsTSV. This is a slim adapter,
 // e.g. to parallel.Processor, which expects this function signature.
-func (f Mapper) TSV(p []byte) ([]byte, error) {
+func (f Mapper) AsTSV(p []byte) ([]byte, error) {
 	fields, err := f(p)
 	if err != nil {
 		return nil, err
@@ -45,6 +50,21 @@ func (f Mapper) TSV(p []byte) ([]byte, error) {
 	return append(bytes.Join(fields, bTab), bNewline...), nil
 }
 
+// WithPrefix adds a given prefix to the first element.
+func WithPrefix(f Mapper, prefix string) Mapper {
+	return func(p []byte) ([][]byte, error) {
+		fields, err := f(p)
+		if err != nil {
+			return fields, err
+		}
+		if len(fields) == 0 {
+			return nil, ErrZeroFields
+		}
+		fields[0] = append([]byte(prefix+":"), fields[0]...)
+		return fields, err
+	}
+}
+
 // NameOf returns name of value, e.g. the name of a function.
 func NameOf(f interface{}) string {
 	v := reflect.ValueOf(f)
@@ -64,6 +84,11 @@ func Identity(p []byte) ([][]byte, error) {
 // CreateFixedMapper extract the value from a given fixed top level json key.
 // Returns a function that maps doc to (v, doc).
 func CreateFixedMapper(field string) Mapper {
+	if field == "" {
+		return func(p []byte) ([][]byte, error) {
+			return nil, ErrMissingFieldName
+		}
+	}
 	f := func(p []byte) ([][]byte, error) {
 		var (
 			doc map[string]interface{}