README

csvutil GoDoc Build Status Build status Go Report Card codecov

Package csvutil provides fast and idiomatic mapping between CSV and Go (golang) values.

This package does not provide a CSV parser itself, it is based on the Reader and Writer interfaces which are implemented by eg. std Go (golang) csv package. This gives a possibility of choosing any other CSV writer or reader which may be more performant.

Installation

go get github.com/jszwec/csvutil

Requirements

  • Go1.7+

Index

  1. Examples
    1. Unmarshal
    2. Marshal
    3. Unmarshal and metadata
    4. But my CSV file has no header...
    5. Decoder.Map - data normalization
    6. Different separator/delimiter
    7. Decoder and interface values
    8. Custom time.Time format
    9. Custom struct tags
    10. Slice and Map fields
    11. Nested/Embedded structs
    12. Inline tag
  2. Performance
    1. Unmarshal
    2. Marshal

Example

Unmarshal

Nice and easy Unmarshal is using the Go std csv.Reader with its default options. Use Decoder for streaming and more advanced use cases.

	var csvInput = []byte(`
name,age,CreatedAt
jacek,26,2012-04-01T15:00:00Z
john,,0001-01-01T00:00:00Z`,
	)

	type User struct {
		Name      string `csv:"name"`
		Age       int    `csv:"age,omitempty"`
		CreatedAt time.Time
	}

	var users []User
	if err := csvutil.Unmarshal(csvInput, &users); err != nil {
		fmt.Println("error:", err)
	}

	for _, u := range users {
		fmt.Printf("%+v\n", u)
	}

	// Output:
	// {Name:jacek Age:26 CreatedAt:2012-04-01 15:00:00 +0000 UTC}
	// {Name:john Age:0 CreatedAt:0001-01-01 00:00:00 +0000 UTC}
Marshal

Marshal is using the Go std csv.Writer with its default options. Use Encoder for streaming or to use a different Writer.

	type Address struct {
		City    string
		Country string
	}

	type User struct {
		Name string
		Address
		Age       int `csv:"age,omitempty"`
		CreatedAt time.Time
	}

	users := []User{
		{
			Name:      "John",
			Address:   Address{"Boston", "USA"},
			Age:       26,
			CreatedAt: time.Date(2010, 6, 2, 12, 0, 0, 0, time.UTC),
		},
		{
			Name:    "Alice",
			Address: Address{"SF", "USA"},
		},
	}

	b, err := csvutil.Marshal(users)
	if err != nil {
		fmt.Println("error:", err)
	}
	fmt.Println(string(b))

	// Output:
	// Name,City,Country,age,CreatedAt
	// John,Boston,USA,26,2010-06-02T12:00:00Z
	// Alice,SF,USA,,0001-01-01T00:00:00Z
Unmarshal and metadata

It may happen that your CSV input will not always have the same header. In addition to your base fields you may get extra metadata that you would still like to store. Decoder provides Unused method, which after each call to Decode can report which header indexes were not used during decoding. Based on that, it is possible to handle and store all these extra values.

	type User struct {
		Name      string            `csv:"name"`
		City      string            `csv:"city"`
		Age       int               `csv:"age"`
		OtherData map[string]string `csv:"-"`
	}

	csvReader := csv.NewReader(strings.NewReader(`
name,age,city,zip
alice,25,la,90005
bob,30,ny,10005`))

	dec, err := csvutil.NewDecoder(csvReader)
	if err != nil {
		log.Fatal(err)
	}

	header := dec.Header()
	var users []User
	for {
		u := User{OtherData: make(map[string]string)}

		if err := dec.Decode(&u); err == io.EOF {
			break
		} else if err != nil {
			log.Fatal(err)
		}

		for _, i := range dec.Unused() {
			u.OtherData[header[i]] = dec.Record()[i]
		}
		users = append(users, u)
	}

	fmt.Println(users)

	// Output:
	// [{alice la 25 map[zip:90005]} {bob ny 30 map[zip:10005]}]
But my CSV file has no header...

Some CSV files have no header, but if you know how it should look like, it is possible to define a struct and generate it. All that is left to do, is to pass it to a decoder.

	type User struct {
		ID   int
		Name string
		Age  int `csv:",omitempty"`
		City string
	}

	csvReader := csv.NewReader(strings.NewReader(`
1,John,27,la
2,Bob,,ny`))

	// in real application this should be done once in init function.
	userHeader, err := csvutil.Header(User{}, "csv")
	if err != nil {
		log.Fatal(err)
	}

	dec, err := csvutil.NewDecoder(csvReader, userHeader...)
	if err != nil {
		log.Fatal(err)
	}

	var users []User
	for {
		var u User
		if err := dec.Decode(&u); err == io.EOF {
			break
		} else if err != nil {
			log.Fatal(err)
		}
		users = append(users, u)
	}

	fmt.Printf("%+v", users)

	// Output:
	// [{ID:1 Name:John Age:27 City:la} {ID:2 Name:Bob Age:0 City:ny}]
Decoder.Map - data normalization

The Decoder's Map function is a powerful tool that can help clean up or normalize the incoming data before the actual decoding takes place.

Lets say we want to decode some floats and the csv input contains some NaN values, but these values are represented by the 'n/a' string. An attempt to decode 'n/a' into float will end up with error, because strconv.ParseFloat expects 'NaN'. Knowing that, we can implement a Map function that will normalize our 'n/a' string and turn it to 'NaN' only for float types.

	dec, err := NewDecoder(r)
	if err != nil {
		log.Fatal(err)
	}

	dec.Map = func(field, column string, v interface{}) string {
		if _, ok := v.(float64); ok && field == "n/a" {
			return "NaN"
		}
		return field
	}

Now our float64 fields will be decoded properly into NaN. What about float32, float type aliases and other NaN formats? Look at the full example here.

Different separator/delimiter

Some files may use different value separators, for example TSV files would use \t. The following examples show how to set up a Decoder and Encoder for such use case.

Decoder:
	csvReader := csv.NewReader(r)
	csvReader.Comma = '\t'

	dec, err := NewDecoder(csvReader)
	if err != nil {
		log.Fatal(err)
	}

	var users []User
	for {
		var u User
		if err := dec.Decode(&u); err == io.EOF {
			break
		} else if err != nil {
			log.Fatal(err)
		}
		users = append(users, u)
	}

Encoder:
	var buf bytes.Buffer

	w := csv.NewWriter(&buf)
        w.Comma = '\t'
	enc := csvutil.NewEncoder(w)

	for _, u := range users {
		if err := enc.Encode(u); err != nil {
			log.Fatal(err)
		}
        }

	w.Flush()
	if err := w.Error(); err != nil {
		log.Fatal(err)
	}
Decoder and interface values

In the case of interface struct fields data is decoded into strings. However, if Decoder finds out that these fields were initialized with pointer values of a specific type prior to decoding, it will try to decode data into that type.

Why only pointer values? Because these values must be both addressable and settable, otherwise Decoder will have to initialize these types on its own, which could result in losing some unexported information.

If interface stores a non-pointer value it will be replaced with a string.

This example will show how this feature could be useful:

package main

import (
	"bytes"
	"encoding/csv"
	"fmt"
	"io"
	"log"

	"github.com/jszwec/csvutil"
)

// Value defines one record in the csv input. In this example it is important
// that Type field is defined before Value. Decoder reads headers and values
// in the same order as struct fields are defined.
type Value struct {
	Type  string      `csv:"type"`
	Value interface{} `csv:"value"`
}

func main() {
	// lets say our csv input defines variables with their types and values.
	data := []byte(`
type,value
string,string_value
int,10
`)

	dec, err := csvutil.NewDecoder(csv.NewReader(bytes.NewReader(data)))
	if err != nil {
		log.Fatal(err)
	}

	// we would like to read every variable and store their already parsed values
	// in the interface field. We can use Decoder.Map function to initialize
	// interface with proper values depending on the input.
	var value Value
	dec.Map = func(field, column string, v interface{}) string {
		if column == "type" {
			switch field {
			case "int": // csv input tells us that this variable contains an int.
				var n int
				value.Value = &n // lets initialize interface with an initialized int pointer.
			default:
				return field
			}
		}
		return field
	}

	for {
		value = Value{}
		if err := dec.Decode(&value); err == io.EOF {
			break
		} else if err != nil {
			log.Fatal(err)
		}

		if value.Type == "int" {
			// our variable type is int, Map func already initialized our interface
			// as int pointer, so we can safely cast it and use it.
			n, ok := value.Value.(*int)
			if !ok {
				log.Fatal("expected value to be *int")
			}
			fmt.Printf("value_type: %s; value: (%T) %d\n", value.Type, value.Value, *n)
		} else {
			fmt.Printf("value_type: %s; value: (%T) %v\n", value.Type, value.Value, value.Value)
		}
	}

	// Output:
	// value_type: string; value: (string) string_value
	// value_type: int; value: (*int) 10
}
Custom time.Time format

Type time.Time can be used as is in the struct fields by both Decoder and Encoder due to the fact that both have builtin support for encoding.TextUnmarshaler and encoding.TextMarshaler. This means that by default Time has a specific format; look at MarshalText and UnmarshalText. There are two ways to override it, which one you choose depends on your use case:

  1. Via Register func (based on encoding/json)
const format = "2006/01/02 15:04:05"

marshalTime := func(t time.Time) ([]byte, error) {
	return t.AppendFormat(nil, format), nil
}

unmarshalTime := func(data []byte, t *time.Time) error {
	tt, err := time.Parse(format, string(data))
	if err != nil {
		return err
	}
	*t = tt
	return nil
}

enc := csvutil.NewEncoder(w)
enc.Register(marshalTime)

dec, err := csvutil.NewDecoder(r)
if err != nil {
	return err
}
dec.Register(unmarshalTime)
  1. With custom type:
type Time struct {
	time.Time
}

const format = "2006/01/02 15:04:05"

func (t Time) MarshalCSV() ([]byte, error) {
	var b [len(format)]byte
	return t.AppendFormat(b[:0], format), nil
}

func (t *Time) UnmarshalCSV(data []byte) error {
	tt, err := time.Parse(format, string(data))
	if err != nil {
		return err
	}
	*t = Time{Time: tt}
	return nil
}
Custom struct tags

Like in other Go encoding packages struct field tags can be used to set custom names or options. By default encoders and decoders are looking at csv tag. However, this can be overriden by manually setting the Tag field.

	type Foo struct {
		Bar int `custom:"bar"`
	}
	dec, err := csvutil.NewDecoder(r)
	if err != nil {
		log.Fatal(err)
	}
	dec.Tag = "custom"
	enc := csvutil.NewEncoder(w)
	enc.Tag = "custom"
Slice and Map fields

There is no default encoding/decoding support for slice and map fields because there is no CSV spec for such values. In such case, it is recommended to create a custom type alias and implement Marshaler and Unmarshaler interfaces. Please note that slice and map aliases behave differently than aliases of other types - there is no need for type casting.

	type Strings []string

	func (s Strings) MarshalCSV() ([]byte, error) {
		return []byte(strings.Join(s, ",")), nil // strings.Join takes []string but it will also accept Strings
	}

	type StringMap map[string]string

	func (sm StringMap) MarshalCSV() ([]byte, error) {
		return []byte(fmt.Sprint(sm)), nil
	}

	func main() {
		b, err := csvutil.Marshal([]struct {
			Strings Strings   `csv:"strings"`
			Map     StringMap `csv:"map"`
		}{
			{[]string{"a", "b"}, map[string]string{"a": "1"}}, // no type casting is required for slice and map aliases
			{Strings{"c", "d"}, StringMap{"b": "1"}},
		})

		if err != nil {
			log.Fatal(err)
		}

		fmt.Printf("%s\n", b)

		// Output:
		// strings,map
		// "a,b",map[a:1]
		// "c,d",map[b:1]
	}
Nested/Embedded structs

Both Encoder and Decoder support nested or embedded structs.

Playground: https://play.golang.org/p/ZySjdVkovbf

package main

import (
	"fmt"

	"github.com/jszwec/csvutil"
)

type Address struct {
	Street string `csv:"street"`
	City   string `csv:"city"`
}

type User struct {
	Name string `csv:"name"`
	Address
}

func main() {
	users := []User{
		{
			Name: "John",
			Address: Address{
				Street: "Boylston",
				City:   "Boston",
			},
		},
	}

	b, err := csvutil.Marshal(users)
	if err != nil {
		panic(err)
	}

	fmt.Printf("%s\n", b)

	var out []User
	if err := csvutil.Unmarshal(b, &out); err != nil {
		panic(err)
	}

	fmt.Printf("%+v\n", out)

	// Output:
	//
	// name,street,city
	// John,Boylston,Boston
	//
	// [{Name:John Address:{Street:Boylston City:Boston}}]
}
Inline tag

Fields with inline tag behave similarly to embedded struct fields. However, it gives a possibility to specify the prefix for all underlying fields. This can be useful when one structure can define multiple CSV columns because they are different from each other only by a certain prefix. Look at the example below.

Playground: https://play.golang.org/p/jyEzeskSnj7

package main

import (
	"fmt"

	"github.com/jszwec/csvutil"
)

func main() {
	type Address struct {
		Street string `csv:"street"`
		City   string `csv:"city"`
	}

	type User struct {
		Name        string  `csv:"name"`
		Address     Address `csv:",inline"`
		HomeAddress Address `csv:"home_address_,inline"`
		WorkAddress Address `csv:"work_address_,inline"`
		Age         int     `csv:"age,omitempty"`
	}

	users := []User{
		{
			Name:        "John",
			Address:     Address{"Washington", "Boston"},
			HomeAddress: Address{"Boylston", "Boston"},
			WorkAddress: Address{"River St", "Cambridge"},
			Age:         26,
		},
	}

	b, err := csvutil.Marshal(users)
	if err != nil {
		fmt.Println("error:", err)
	}

	fmt.Printf("%s\n", b)

	// Output:
	// name,street,city,home_address_street,home_address_city,work_address_street,work_address_city,age
	// John,Washington,Boston,Boylston,Boston,River St,Cambridge,26
}

Performance

csvutil provides the best encoding and decoding performance with small memory usage.

Unmarshal

benchmark code

csvutil:
BenchmarkUnmarshal/csvutil.Unmarshal/1_record-12         	  280696	      4516 ns/op	    7332 B/op	      26 allocs/op
BenchmarkUnmarshal/csvutil.Unmarshal/10_records-12       	   95750	     11517 ns/op	    8356 B/op	      35 allocs/op
BenchmarkUnmarshal/csvutil.Unmarshal/100_records-12      	   14997	     83146 ns/op	   18532 B/op	     125 allocs/op
BenchmarkUnmarshal/csvutil.Unmarshal/1000_records-12     	    1485	    750143 ns/op	  121094 B/op	    1025 allocs/op
BenchmarkUnmarshal/csvutil.Unmarshal/10000_records-12    	     154	   7587205 ns/op	 1136662 B/op	   10025 allocs/op
BenchmarkUnmarshal/csvutil.Unmarshal/100000_records-12   	      14	  76126616 ns/op	11808744 B/op	  100025 allocs/op
gocsv:
BenchmarkUnmarshal/gocsv.Unmarshal/1_record-12           	  141330	      7499 ns/op	    7795 B/op	      97 allocs/op
BenchmarkUnmarshal/gocsv.Unmarshal/10_records-12         	   54252	     21664 ns/op	   13891 B/op	     307 allocs/op
BenchmarkUnmarshal/gocsv.Unmarshal/100_records-12        	    6920	    159662 ns/op	   72644 B/op	    2380 allocs/op
BenchmarkUnmarshal/gocsv.Unmarshal/1000_records-12       	     752	   1556083 ns/op	  650248 B/op	   23083 allocs/op
BenchmarkUnmarshal/gocsv.Unmarshal/10000_records-12      	      72	  17086623 ns/op	 7017469 B/op	  230092 allocs/op
BenchmarkUnmarshal/gocsv.Unmarshal/100000_records-12     	       7	 163610749 ns/op	75004923 B/op	 2300105 allocs/op
easycsv:
BenchmarkUnmarshal/easycsv.ReadAll/1_record-12           	  101527	     10662 ns/op	    8855 B/op	      81 allocs/op
BenchmarkUnmarshal/easycsv.ReadAll/10_records-12         	   23325	     51437 ns/op	   24072 B/op	     391 allocs/op
BenchmarkUnmarshal/easycsv.ReadAll/100_records-12        	    2402	    447296 ns/op	  170538 B/op	    3454 allocs/op
BenchmarkUnmarshal/easycsv.ReadAll/1000_records-12       	     272	   4370854 ns/op	 1595683 B/op	   34057 allocs/op
BenchmarkUnmarshal/easycsv.ReadAll/10000_records-12      	      24	  47502457 ns/op	18861808 B/op	  340068 allocs/op
BenchmarkUnmarshal/easycsv.ReadAll/100000_records-12     	       3	 468974170 ns/op	189427066 B/op	 3400082 allocs/op
Marshal

benchmark code

csvutil:
BenchmarkMarshal/csvutil.Marshal/1_record-12         	  279558	      4390 ns/op	    9952 B/op	      12 allocs/op
BenchmarkMarshal/csvutil.Marshal/10_records-12       	   82478	     15608 ns/op	   10800 B/op	      21 allocs/op
BenchmarkMarshal/csvutil.Marshal/100_records-12      	   10275	    117288 ns/op	   28208 B/op	     112 allocs/op
BenchmarkMarshal/csvutil.Marshal/1000_records-12     	    1075	   1147473 ns/op	  168508 B/op	    1014 allocs/op
BenchmarkMarshal/csvutil.Marshal/10000_records-12    	     100	  11985382 ns/op	 1525973 B/op	   10017 allocs/op
BenchmarkMarshal/csvutil.Marshal/100000_records-12   	       9	 113640813 ns/op	22455873 B/op	  100021 allocs/op
gocsv:
BenchmarkMarshal/gocsv.Marshal/1_record-12           	  203052	      6077 ns/op	    5914 B/op	      81 allocs/op
BenchmarkMarshal/gocsv.Marshal/10_records-12         	   50132	     24585 ns/op	    9284 B/op	     360 allocs/op
BenchmarkMarshal/gocsv.Marshal/100_records-12        	    5480	    212008 ns/op	   51916 B/op	    3151 allocs/op
BenchmarkMarshal/gocsv.Marshal/1000_records-12       	     514	   2053919 ns/op	  444506 B/op	   31053 allocs/op
BenchmarkMarshal/gocsv.Marshal/10000_records-12      	      52	  21066666 ns/op	 4332377 B/op	  310064 allocs/op
BenchmarkMarshal/gocsv.Marshal/100000_records-12     	       5	 207408929 ns/op	51169419 B/op	 3100077 allocs/op

Documentation

Overview

    Package csvutil provides fast and idiomatic mapping between CSV and Go values.

    This package does not provide a CSV parser itself, it is based on the Reader and Writer interfaces which are implemented by eg. std csv package. This gives a possibility of choosing any other CSV writer or reader which may be more performant.

    Index

    Examples

    Constants

    This section is empty.

    Variables

    View Source
    var ErrFieldCount = errors.New("wrong number of fields in record")

      ErrFieldCount is returned when header's length doesn't match the length of the read record.

      Functions

      func Header(v interface{}, tag string) ([]string, error)

        Header scans the provided struct type and generates a CSV header for it.

        Field names are written in the same order as struct fields are defined. Embedded struct's fields are treated as if they were part of the outer struct. Fields that are embedded types and that are tagged are treated like any other field.

        Unexported fields and fields with tag "-" are ignored.

        Tagged fields have the priority over non tagged fields with the same name.

        Following the Go visibility rules if there are multiple fields with the same name (tagged or not tagged) on the same level and choice between them is ambiguous, then all these fields will be ignored.

        It is a good practice to call Header once for each type. The suitable place for calling it is init function. Look at Decoder.DecodingDataWithNoHeader example.

        If tag is left empty the default "csv" will be used.

        Header will return UnsupportedTypeError if the provided value is nil or is not a struct.

        Example
        Output:
        
        [ID Name Age City zip_code]
        

        func Marshal

        func Marshal(v interface{}) ([]byte, error)

          Marshal returns the CSV encoding of slice or array v. If v is not a slice or elements are not structs then Marshal returns InvalidMarshalError.

          Marshal uses the std encoding/csv.Writer with its default settings for csv encoding.

          Marshal will always encode the CSV header even for the empty slice.

          For the exact encoding rules look at Encoder.Encode method.

          Example
          Output:
          
          Name,City,Country,age,CreatedAt
          John,Boston,USA,26,2010-06-02T12:00:00Z
          Alice,SF,USA,,0001-01-01T00:00:00Z
          
          Example (CustomMarshalCSV)
          Output:
          
          ID,Status
          1,success
          2,failure
          
          Example (SliceMap)
          Output:
          
          strings,map
          "a,b",map[a:1]
          "c,d",map[b:1]
          

          func Unmarshal

          func Unmarshal(data []byte, v interface{}) error

            Unmarshal parses the CSV-encoded data and stores the result in the slice or the array pointed to by v. If v is nil or not a pointer to a struct slice or struct array, Unmarshal returns an InvalidUnmarshalError.

            Unmarshal uses the std encoding/csv.Reader for parsing and csvutil.Decoder for populating the struct elements in the provided slice. For exact decoding rules look at the Decoder's documentation.

            The first line in data is treated as a header. Decoder will use it to map csv columns to struct's fields.

            In case of success the provided slice will be reinitialized and its content fully replaced with decoded data.

            Example
            Output:
            
            {Name:jacek Age:26 CreatedAt:2012-04-01 15:00:00 +0000 UTC}
            {Name:john Age:0 CreatedAt:0001-01-01 00:00:00 +0000 UTC}
            

            Types

            type Decoder

            type Decoder struct {
            	// Tag defines which key in the struct field's tag to scan for names and
            	// options (Default: 'csv').
            	Tag string
            
            	// If not nil, Map is a function that is called for each field in the csv
            	// record before decoding the data. It allows mapping certain string values
            	// for specific columns or types to a known format. Decoder calls Map with
            	// the current column name (taken from header) and a zero non-pointer value
            	// of a type to which it is going to decode data into. Implementations
            	// should use type assertions to recognize the type.
            	//
            	// The good example of use case for Map is if NaN values are represented by
            	// eg 'n/a' string, implementing a specific Map function for all floats
            	// could map 'n/a' back into 'NaN' to allow successful decoding.
            	//
            	// Use Map with caution. If the requirements of column or type are not met
            	// Map should return 'field', since it is the original value that was
            	// read from the csv input, this would indicate no change.
            	//
            	// If struct field is an interface v will be of type string, unless the
            	// struct field contains a settable pointer value - then v will be a zero
            	// value of that type.
            	//
            	// Map must be set before the first call to Decode and not changed after it.
            	Map func(field, col string, v interface{}) string
            	// contains filtered or unexported fields
            }

              A Decoder reads and decodes string records into structs.

              Example (CustomUnmarshalCSV)
              Output:
              
              [{Int:5 Bar:10} {Int:6 Bar:11}]
              
              Example (DecodeEmbedded)
              Output:
              
              [{{0 la ca} 1 alice 25} {{0 ny ny} 2 bob 30}]
              
              Example (DecodingDataWithNoHeader)
              Output:
              
              [{ID:1 Name:John Age:27 State:0 City:la ZIP:90005} {ID:2 Name:Bob Age:0 State:0 City:ny ZIP:10005}]
              
              Example (InterfaceValues)
              Output:
              
              value_type: string; value: (string) string_value
              value_type: int; value: (*int) 10
              

              func NewDecoder

              func NewDecoder(r Reader, header ...string) (dec *Decoder, err error)

                NewDecoder returns a new decoder that reads from r.

                Decoder will match struct fields according to the given header.

                If header is empty NewDecoder will read one line and treat it as a header.

                Records coming from r must be of the same length as the header.

                NewDecoder may return io.EOF if there is no data in r and no header was provided by the caller.

                func (*Decoder) Decode

                func (d *Decoder) Decode(v interface{}) (err error)

                  Decode reads the next string record or records from its input and stores it in the value pointed to by v which must be a pointer to a struct, struct slice or struct array.

                  Decode matches all exported struct fields based on the header. Struct fields can be adjusted by using tags.

                  The "omitempty" option specifies that the field should be omitted from the decoding if record's field is an empty string.

                  Examples of struct field tags and their meanings:

                  // Decode matches this field with "myName" header column.
                  Field int `csv:"myName"`
                  
                  // Decode matches this field with "Field" header column.
                  Field int
                  
                  // Decode matches this field with "myName" header column and decoding is not
                  // called if record's field is an empty string.
                  Field int `csv:"myName,omitempty"`
                  
                  // Decode matches this field with "Field" header column and decoding is not
                  // called if record's field is an empty string.
                  Field int `csv:",omitempty"`
                  
                  // Decode ignores this field.
                  Field int `csv:"-"`
                  
                  // Decode treats this field exactly as if it was an embedded field and
                  // matches header columns that start with "my_prefix_" to all fields of this
                  // type.
                  Field Struct `csv:"my_prefix_,inline"`
                  
                  // Decode treats this field exactly as if it was an embedded field.
                  Field Struct `csv:",inline"`
                  

                  By default decode looks for "csv" tag, but this can be changed by setting Decoder.Tag field.

                  To Decode into a custom type v must implement csvutil.Unmarshaler or encoding.TextUnmarshaler.

                  Anonymous struct fields with tags are treated like normal fields and they must implement csvutil.Unmarshaler or encoding.TextUnmarshaler unless inline tag is specified.

                  Anonymous struct fields without tags are populated just as if they were part of the main struct. However, fields in the main struct have bigger priority and they are populated first. If main struct and anonymous struct field have the same fields, the main struct's fields will be populated.

                  Fields of type []byte expect the data to be base64 encoded strings.

                  Float fields are decoded to NaN if a string value is 'NaN'. This check is case insensitive.

                  Interface fields are decoded to strings unless they contain settable pointer value.

                  Pointer fields are decoded to nil if a string value is empty.

                  If v is a slice, Decode resets it and reads the input until EOF, storing all decoded values in the given slice. Decode returns nil on EOF.

                  If v is an array, Decode reads the input until EOF or until it decodes all corresponding array elements. If the input contains less elements than the array, the additional Go array elements are set to zero values. Decode returns nil on EOF unless there were no records decoded.

                  Fields with inline tags that have a non-empty prefix must not be cyclic structures. Passing such values to Decode will result in an infinite loop.

                  Example
                  Output:
                  
                  [{<nil> alice la 25} {<nil> bob ny 30}]
                  
                  Example (Array)
                  Output:
                  
                  [{<nil> alice la 25} {<nil> bob ny 30}]
                  
                  Example (Inline)
                  Output:
                  
                  [{John {Washington Boston} {Boylston Boston} {River St Cambridge} 26}]
                  
                  Example (Slice)
                  Output:
                  
                  [{<nil> alice la 25} {<nil> bob ny 30}]
                  

                  func (*Decoder) Header

                  func (d *Decoder) Header() []string

                    Header returns the first line that came from the reader, or returns the defined header by the caller.

                    func (*Decoder) Record

                    func (d *Decoder) Record() []string

                      Record returns the most recently read record. The slice is valid until the next call to Decode.

                      func (*Decoder) Register

                      func (d *Decoder) Register(f interface{})

                        Register registers a custom decoding function for a concrete type or interface. The argument f must be of type:

                        func([]byte, T) error
                        

                        T must be a concrete type such as *time.Time, or interface that has at least one method.

                        During decoding, fields are matched by the concrete type first. If match is not found then Decoder looks if field implements any of the registered interfaces in order they were registered.

                        Register panics if:

                        - f does not match the right signature
                        - f is an empty interface
                        - f was already registered
                        

                        Register is based on the encoding/json proposal: https://github.com/golang/go/issues/5901.

                        Example
                        Output:
                        
                        12:00PM,15,10,{Value:34}
                        

                        func (*Decoder) Unused

                        func (d *Decoder) Unused() []int

                          Unused returns a list of column indexes that were not used during decoding due to lack of matching struct field.

                          Example
                          Output:
                          
                          [{alice la 25 map[zip:90005]} {bob ny 30 map[zip:10005]}]
                          

                          type Encoder

                          type Encoder struct {
                          	// Tag defines which key in the struct field's tag to scan for names and
                          	// options (Default: 'csv').
                          	Tag string
                          
                          	// If AutoHeader is true, a struct header is encoded during the first call
                          	// to Encode automatically (Default: true).
                          	AutoHeader bool
                          	// contains filtered or unexported fields
                          }

                            Encoder writes structs CSV representations to the output stream.

                            func NewEncoder

                            func NewEncoder(w Writer) *Encoder

                              NewEncoder returns a new encoder that writes to w.

                              func (*Encoder) Encode

                              func (e *Encoder) Encode(v interface{}) error

                                Encode writes the CSV encoding of v to the output stream. The provided argument v must be a struct, struct slice or struct array.

                                Only the exported fields will be encoded.

                                First call to Encode will write a header unless EncodeHeader was called first or AutoHeader is false. Header names can be customized by using tags ('csv' by default), otherwise original Field names are used.

                                Header and fields are written in the same order as struct fields are defined. Embedded struct's fields are treated as if they were part of the outer struct. Fields that are embedded types and that are tagged are treated like any other field, but they have to implement Marshaler or encoding.TextMarshaler interfaces.

                                Marshaler interface has the priority over encoding.TextMarshaler.

                                Tagged fields have the priority over non tagged fields with the same name.

                                Following the Go visibility rules if there are multiple fields with the same name (tagged or not tagged) on the same level and choice between them is ambiguous, then all these fields will be ignored.

                                Nil values will be encoded as empty strings. Same will happen if 'omitempty' tag is set, and the value is a default value like 0, false or nil interface.

                                Bool types are encoded as 'true' or 'false'.

                                Float types are encoded using strconv.FormatFloat with precision -1 and 'G' format. NaN values are encoded as 'NaN' string.

                                Fields of type []byte are being encoded as base64-encoded strings.

                                Fields can be excluded from encoding by using '-' tag option.

                                Examples of struct tags:

                                // Field appears as 'myName' header in CSV encoding.
                                Field int `csv:"myName"`
                                
                                // Field appears as 'Field' header in CSV encoding.
                                Field int
                                
                                // Field appears as 'myName' header in CSV encoding and is an empty string
                                // if Field is 0.
                                Field int `csv:"myName,omitempty"`
                                
                                // Field appears as 'Field' header in CSV encoding and is an empty string
                                // if Field is 0.
                                Field int `csv:",omitempty"`
                                
                                // Encode ignores this field.
                                Field int `csv:"-"`
                                
                                // Encode treats this field exactly as if it was an embedded field and adds
                                // "my_prefix_" to each field's name.
                                Field Struct `csv:"my_prefix_,inline"`
                                
                                // Encode treats this field exactly as if it was an embedded field.
                                Field Struct `csv:",inline"`
                                

                                Fields with inline tags that have a non-empty prefix must not be cyclic structures. Passing such values to Encode will result in an infinite loop.

                                Encode doesn't flush data. The caller is responsible for calling Flush() if the used Writer supports it.

                                Example (All)
                                Output:
                                
                                Name,City,Country,age
                                John,Boston,USA,26
                                Bob,LA,USA,27
                                Alice,SF,USA,
                                
                                Example (Inline)
                                Output:
                                
                                name,street,city,home_address_street,home_address_city,work_address_street,work_address_city,age
                                John,Washington,Boston,Boylston,Boston,River St,Cambridge,26
                                
                                Example (Streaming)
                                Output:
                                
                                Name,City,Country,age
                                John,Boston,USA,26
                                Bob,LA,USA,27
                                Alice,SF,USA,
                                

                                func (*Encoder) EncodeHeader

                                func (e *Encoder) EncodeHeader(v interface{}) error

                                  EncodeHeader writes the CSV header of the provided struct value to the output stream. The provided argument v must be a struct value.

                                  The first Encode method call will not write header if EncodeHeader was called before it. This method can be called in cases when a data set could be empty, but header is desired.

                                  EncodeHeader is like Header function, but it works with the Encoder and writes directly to the output stream. Look at Header documentation for the exact header encoding rules.

                                  Example
                                  Output:
                                  
                                  Name,age
                                  

                                  func (*Encoder) Register

                                  func (e *Encoder) Register(f interface{})

                                    Register registers a custom encoding function for a concrete type or interface. The argument f must be of type:

                                    func(T) ([]byte, error)
                                    

                                    T must be a concrete type such as Foo or *Foo, or interface that has at least one method.

                                    During encoding, fields are matched by the concrete type first. If match is not found then Encoder looks if field implements any of the registered interfaces in order they were registered.

                                    Register panics if:

                                    - f does not match the right signature
                                    - f is an empty interface
                                    - f was already registered
                                    

                                    Register is based on the encoding/json proposal: https://github.com/golang/go/issues/5901.

                                    Example
                                    Output:
                                    
                                    time,hex,ptr_hex,buffer
                                    12:00PM,f,NULL,hello
                                    

                                    type InvalidDecodeError

                                    type InvalidDecodeError struct {
                                    	Type reflect.Type
                                    }

                                      An InvalidDecodeError describes an invalid argument passed to Decode. (The argument to Decode must be a non-nil struct pointer)

                                      func (*InvalidDecodeError) Error

                                      func (e *InvalidDecodeError) Error() string

                                      type InvalidEncodeError

                                      type InvalidEncodeError struct {
                                      	Type reflect.Type
                                      }

                                        InvalidEncodeError is returned by Encode when the provided value was invalid.

                                        func (*InvalidEncodeError) Error

                                        func (e *InvalidEncodeError) Error() string

                                        type InvalidMarshalError

                                        type InvalidMarshalError struct {
                                        	Type reflect.Type
                                        }

                                          InvalidMarshalError is returned by Marshal when the provided value was invalid.

                                          func (*InvalidMarshalError) Error

                                          func (e *InvalidMarshalError) Error() string

                                          type InvalidUnmarshalError

                                          type InvalidUnmarshalError struct {
                                          	Type reflect.Type
                                          }

                                            An InvalidUnmarshalError describes an invalid argument passed to Unmarshal. (The argument to Unmarshal must be a non-nil slice of structs pointer)

                                            func (*InvalidUnmarshalError) Error

                                            func (e *InvalidUnmarshalError) Error() string

                                            type Marshaler

                                            type Marshaler interface {
                                            	MarshalCSV() ([]byte, error)
                                            }

                                              Marshaler is the interface implemented by types that can marshal themselves into valid string.

                                              type MarshalerError

                                              type MarshalerError struct {
                                              	Type          reflect.Type
                                              	MarshalerType string
                                              	Err           error
                                              }

                                                MarshalerError is returned by Encoder when MarshalCSV or MarshalText returned an error.

                                                func (*MarshalerError) Error

                                                func (e *MarshalerError) Error() string

                                                type Reader

                                                type Reader interface {
                                                	Read() ([]string, error)
                                                }

                                                  Reader provides the interface for reading a single CSV record.

                                                  If there is no data left to be read, Read returns (nil, io.EOF).

                                                  It is implemented by csv.Reader.

                                                  type UnmarshalTypeError

                                                  type UnmarshalTypeError struct {
                                                  	Value string       // string value
                                                  	Type  reflect.Type // type of Go value it could not be assigned to
                                                  }

                                                    An UnmarshalTypeError describes a string value that was not appropriate for a value of a specific Go type.

                                                    func (*UnmarshalTypeError) Error

                                                    func (e *UnmarshalTypeError) Error() string

                                                    type Unmarshaler

                                                    type Unmarshaler interface {
                                                    	UnmarshalCSV([]byte) error
                                                    }

                                                      Unmarshaler is the interface implemented by types that can unmarshal a single record's field description of themselves.

                                                      type UnsupportedTypeError

                                                      type UnsupportedTypeError struct {
                                                      	Type reflect.Type
                                                      }

                                                        An UnsupportedTypeError is returned when attempting to encode or decode a value of an unsupported type.

                                                        func (*UnsupportedTypeError) Error

                                                        func (e *UnsupportedTypeError) Error() string

                                                        type Writer

                                                        type Writer interface {
                                                        	Write([]string) error
                                                        }

                                                          Writer provides the interface for writing a single CSV record.

                                                          It is implemented by csv.Writer.