diff --git a/core/mr/mapreduce.go b/core/mr/mapreduce.go
index 523b2aac..49da2583 100644
--- a/core/mr/mapreduce.go
+++ b/core/mr/mapreduce.go
@@ -17,28 +17,42 @@ const (
 )
 
 var (
-	ErrCancelWithNil  = errors.New("mapreduce cancelled with nil")
+	// ErrCancelWithNil is an error that mapreduce was cancelled with nil.
+	ErrCancelWithNil = errors.New("mapreduce cancelled with nil")
+	// ErrReduceNoOutput is an error that reduce did not output a value.
 	ErrReduceNoOutput = errors.New("reduce not writing value")
 )
 
 type (
-	GenerateFunc    func(source chan<- interface{})
-	MapFunc         func(item interface{}, writer Writer)
-	VoidMapFunc     func(item interface{})
-	MapperFunc      func(item interface{}, writer Writer, cancel func(error))
-	ReducerFunc     func(pipe <-chan interface{}, writer Writer, cancel func(error))
+	// GenerateFunc is used to let callers send elements into source.
+	GenerateFunc func(source chan<- interface{})
+	// MapFunc is used to do element processing and write the output to writer.
+	MapFunc func(item interface{}, writer Writer)
+	// VoidMapFunc is used to do element processing, but no output.
+	VoidMapFunc func(item interface{})
+	// MapperFunc is used to do element processing and write the output to writer,
+	// use cancel func to cancel the processing.
+	MapperFunc func(item interface{}, writer Writer, cancel func(error))
+	// ReducerFunc is used to reduce all the mapping output and write to writer,
+	// use cancel func to cancel the processing.
+	ReducerFunc func(pipe <-chan interface{}, writer Writer, cancel func(error))
+	// VoidReducerFunc is used to reduce all the mapping output, but no output.
+	// Use cancel func to cancel the processing.
 	VoidReducerFunc func(pipe <-chan interface{}, cancel func(error))
-	Option          func(opts *mapReduceOptions)
+	// Option defines the method to customize the mapreduce.
+	Option func(opts *mapReduceOptions)
 
 	mapReduceOptions struct {
 		workers int
 	}
 
+	// Writer interface wraps Write method.
 	Writer interface {
 		Write(v interface{})
 	}
 )
 
+// Finish runs fns parallelly, cancelled on any error.
 func Finish(fns ...func() error) error {
 	if len(fns) == 0 {
 		return nil
@@ -58,6 +72,7 @@ func Finish(fns ...func() error) error {
 	}, WithWorkers(len(fns)))
 }
 
+// FinishVoid runs fns parallelly.
 func FinishVoid(fns ...func()) {
 	if len(fns) == 0 {
 		return
@@ -73,6 +88,7 @@ func FinishVoid(fns ...func()) {
 	}, WithWorkers(len(fns)))
 }
 
+// Map maps all elements generated from given generate func, and returns an output channel.
 func Map(generate GenerateFunc, mapper MapFunc, opts ...Option) chan interface{} {
 	options := buildOptions(opts...)
 	source := buildSource(generate)
@@ -84,11 +100,14 @@ func Map(generate GenerateFunc, mapper MapFunc, opts ...Option) chan interface{}
 	return collector
 }
 
+// MapReduce maps all elements generated from given generate func,
+// and reduces the output elemenets with given reducer.
 func MapReduce(generate GenerateFunc, mapper MapperFunc, reducer ReducerFunc, opts ...Option) (interface{}, error) {
 	source := buildSource(generate)
 	return MapReduceWithSource(source, mapper, reducer, opts...)
 }
 
+// MapReduceWithSource maps all elements from source, and reduce the output elements with given reducer.
 func MapReduceWithSource(source <-chan interface{}, mapper MapperFunc, reducer ReducerFunc,
 	opts ...Option) (interface{}, error) {
 	options := buildOptions(opts...)
@@ -141,8 +160,10 @@ func MapReduceWithSource(source <-chan interface{}, mapper MapperFunc, reducer R
 	}
 }
 
-func MapReduceVoid(generator GenerateFunc, mapper MapperFunc, reducer VoidReducerFunc, opts ...Option) error {
-	_, err := MapReduce(generator, mapper, func(input <-chan interface{}, writer Writer, cancel func(error)) {
+// MapReduceVoid maps all elements generated from given generate,
+// and reduce the output elements with given reducer.
+func MapReduceVoid(generate GenerateFunc, mapper MapperFunc, reducer VoidReducerFunc, opts ...Option) error {
+	_, err := MapReduce(generate, mapper, func(input <-chan interface{}, writer Writer, cancel func(error)) {
 		reducer(input, cancel)
 		drain(input)
 		// We need to write a placeholder to let MapReduce to continue on reducer done,
@@ -152,12 +173,14 @@ func MapReduceVoid(generator GenerateFunc, mapper MapperFunc, reducer VoidReduce
 	return err
 }
 
+// MapVoid maps all elements from given generate but no output.
 func MapVoid(generate GenerateFunc, mapper VoidMapFunc, opts ...Option) {
 	drain(Map(generate, func(item interface{}, writer Writer) {
 		mapper(item)
 	}, opts...))
 }
 
+// WithWorkers customizes a mapreduce processing with given workers.
 func WithWorkers(workers int) Option {
 	return func(opts *mapReduceOptions) {
 		if workers < minWorkers {