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Package form Decodes url.Values into Go value(s) and Encodes Go value(s) into url.Values.

It has the following features:

Supports map of almost all types.
Supports both Numbered and Normal arrays eg. "Array[0]" and just "Array" with multiple values passed.
Slice honours the specified index. eg. if "Slice[2]" is the only Slice value passed down, it will be put at index 2; if slice isn't big enough it will be expanded.
Array honours the specified index. eg. if "Array[2]" is the only Array value passed down, it will be put at index 2; if array isn't big enough a warning will be printed and value ignored.
Only creates objects as necessary eg. if no array or map values are passed down, the array and map are left as their default values in the struct.
Allows for Custom Type registration.
Handles time.Time using RFC3339 time format by default, but can easily be changed by registering a Custom Type, see below.
Handles Encoding & Decoding of almost all Go types eg. can Decode into struct, array, map, int... and Encode a struct, array, map, int...

Common Questions

Does it support encoding.TextUnmarshaler? No because TextUnmarshaler only accepts []byte but posted values can have multiple values, so is not suitable.
Mixing array/slice with array[idx]/slice[idx], in which order are they parsed? array/slice then array[idx]/slice[idx]

Supported Types ( out of the box )

string
bool
int, int8, int16, int32, int64
uint, uint8, uint16, uint32, uint64
float32, float64
struct and anonymous struct
interface{}
time.Time - by default using RFC3339
a pointer to one of the above types
slice, array
map
custom types can override any of the above types
many other types may be supported inherently

NOTE: map, struct and slice nesting are ad infinitum.

Installation

Use go get.

go get github.com/go-playground/form

Then import the form package into your own code.

import "github.com/go-playground/form/v4"

Usage

Use symbol . for separating fields/structs. (eg. structfield.field)
Use [index or key] for access to index of a slice/array or key for map. (eg. arrayfield[0], mapfield[keyvalue])

<form method="POST">
  <input type="text" name="Name" value="joeybloggs"/>
  <input type="text" name="Age" value="3"/>
  <input type="text" name="Gender" value="Male"/>
  <input type="text" name="Address[0].Name" value="26 Here Blvd."/>
  <input type="text" name="Address[0].Phone" value="9(999)999-9999"/>
  <input type="text" name="Address[1].Name" value="26 There Blvd."/>
  <input type="text" name="Address[1].Phone" value="1(111)111-1111"/>
  <input type="text" name="active" value="true"/>
  <input type="text" name="MapExample[key]" value="value"/>
  <input type="text" name="NestedMap[key][key]" value="value"/>
  <input type="text" name="NestedArray[0][0]" value="value"/>
  <input type="submit"/>
</form>

Examples

Decoding

package main

import (
	"fmt"
	"log"
	"net/url"

	"github.com/go-playground/form/v4"
)

// Address contains address information
type Address struct {
	Name  string
	Phone string
}

// User contains user information
type User struct {
	Name        string
	Age         uint8
	Gender      string
	Address     []Address
	Active      bool `form:"active"`
	MapExample  map[string]string
	NestedMap   map[string]map[string]string
	NestedArray [][]string
}

// use a single instance of Decoder, it caches struct info
var decoder *form.Decoder

func main() {
	decoder = form.NewDecoder()

	// this simulates the results of http.Request's ParseForm() function
	values := parseForm()

	var user User

	// must pass a pointer
	err := decoder.Decode(&user, values)
	if err != nil {
		log.Panic(err)
	}

	fmt.Printf("%#v\n", user)
}

// this simulates the results of http.Request's ParseForm() function
func parseForm() url.Values {
	return url.Values{
		"Name":                []string{"joeybloggs"},
		"Age":                 []string{"3"},
		"Gender":              []string{"Male"},
		"Address[0].Name":     []string{"26 Here Blvd."},
		"Address[0].Phone":    []string{"9(999)999-9999"},
		"Address[1].Name":     []string{"26 There Blvd."},
		"Address[1].Phone":    []string{"1(111)111-1111"},
		"active":              []string{"true"},
		"MapExample[key]":     []string{"value"},
		"NestedMap[key][key]": []string{"value"},
		"NestedArray[0][0]":   []string{"value"},
	}
}

Encoding

package main

import (
	"fmt"
	"log"

	"github.com/go-playground/form/v4"
)

// Address contains address information
type Address struct {
	Name  string
	Phone string
}

// User contains user information
type User struct {
	Name        string
	Age         uint8
	Gender      string
	Address     []Address
	Active      bool `form:"active"`
	MapExample  map[string]string
	NestedMap   map[string]map[string]string
	NestedArray [][]string
}

// use a single instance of Encoder, it caches struct info
var encoder *form.Encoder

func main() {
	encoder = form.NewEncoder()

	user := User{
		Name:   "joeybloggs",
		Age:    3,
		Gender: "Male",
		Address: []Address{
			{Name: "26 Here Blvd.", Phone: "9(999)999-9999"},
			{Name: "26 There Blvd.", Phone: "1(111)111-1111"},
		},
		Active:      true,
		MapExample:  map[string]string{"key": "value"},
		NestedMap:   map[string]map[string]string{"key": {"key": "value"}},
		NestedArray: [][]string{{"value"}},
	}

	// must pass a pointer
	values, err := encoder.Encode(&user)
	if err != nil {
		log.Panic(err)
	}

	fmt.Printf("%#v\n", values)
}

Registering Custom Types

Decoder

decoder.RegisterCustomTypeFunc(func(vals []string) (interface{}, error) {
	return time.Parse("2006-01-02", vals[0])
}, time.Time{})

ADDITIONAL: if a struct type is registered, the function will only be called if a url.Value exists for the struct and not just the struct fields eg. url.Values{"User":"Name%3Djoeybloggs"} will call the custom type function with 'User' as the type, however url.Values{"User.Name":"joeybloggs"} will not.

Encoder

encoder.RegisterCustomTypeFunc(func(x interface{}) ([]string, error) {
	return []string{x.(time.Time).Format("2006-01-02")}, nil
}, time.Time{})

Ignoring Fields

you can tell form to ignore fields using - in the tag

type MyStruct struct {
	Field string `form:"-"`
}

Omitempty

you can tell form to omit empty fields using ,omitempty or FieldName,omitempty in the tag

type MyStruct struct {
	Field  string `form:",omitempty"`
	Field2 string `form:"CustomFieldName,omitempty"`
}

Notes

To maximize compatibility with other systems the Encoder attempts to avoid using array indexes in url.Values if at all possible.

eg.

// A struct field of
Field []string{"1", "2", "3"}

// will be output a url.Value as
"Field": []string{"1", "2", "3"}

and not
"Field[0]": []string{"1"}
"Field[1]": []string{"2"}
"Field[2]": []string{"3"}

// however there are times where it is unavoidable, like with pointers
i := int(1)
Field []*string{nil, nil, &i}

// to avoid index 1 and 2 must use index
"Field[2]": []string{"1"}

Benchmarks

Run on M1 MacBook Pro using go version go1.20.6 darwin/amd64

NOTE: the 1 allocation and B/op in the first 4 decodes is actually the struct allocating when passing it in, so primitives are actually zero allocation.

go test -run=NONE -bench=. -benchmem ./...
goos: darwin
goarch: arm64
pkg: github.com/go-playground/form/v4
cpu: Apple M3 Max
BenchmarkNestedArrayDecode100-16     	      75	  15782643 ns/op	18754349 B/op	  360810 allocs/op
BenchmarkNestedArrayDecode1000-16    	       1	2227892458 ns/op	1877558216 B/op	36011385 allocs/op
PASS
ok  	github.com/go-playground/form/v4	4.251s
goos: darwin
goarch: arm64
pkg: github.com/go-playground/form/v4/benchmarks
cpu: Apple M3 Max
BenchmarkSimpleUserDecodeStruct-16                              	12669696	        94.60 ns/op	      64 B/op	       1 allocs/op
BenchmarkSimpleUserDecodeStructParallel-16                      	46715631	        27.79 ns/op	      64 B/op	       1 allocs/op
BenchmarkSimpleUserEncodeStruct-16                              	 4624094	       256.7 ns/op	     485 B/op	      10 allocs/op
BenchmarkSimpleUserEncodeStructParallel-16                      	 7386290	       166.2 ns/op	     485 B/op	      10 allocs/op
BenchmarkPrimitivesDecodeStructAllPrimitivesTypes-16            	 3533421	       332.3 ns/op	      96 B/op	       1 allocs/op
BenchmarkPrimitivesDecodeStructAllPrimitivesTypesParallel-16    	20706642	        59.43 ns/op	      96 B/op	       1 allocs/op
BenchmarkPrimitivesEncodeStructAllPrimitivesTypes-16            	 1228750	       966.4 ns/op	    1465 B/op	      34 allocs/op
BenchmarkPrimitivesEncodeStructAllPrimitivesTypesParallel-16    	 1962678	       607.2 ns/op	    1465 B/op	      34 allocs/op
BenchmarkComplexArrayDecodeStructAllTypes-16                    	  21

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