:steam_locomotive: decodes url.Values into struct, creating objects as necessary. Dual Array and Full map support.

go get


Package form

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Package form parses url.Values and fills a struct with values, creating objects as necessary.

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.
  • 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 it will be expanded.
  • 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.

Common Questions

  • Does it support encoding.TextUnmarshaler? No because TextUnmarshaler only accepts []byte but posted values can have multiple values, so is not suitable.

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 (eg. bson.ObjectId is type ObjectId string, which will get populated by the string type

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


Use go get.

go get

Then import the form package into your own code.

import ""


  • 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"/>


package main

import (


// 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 {

    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"},

Registering Custom Types

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

Ignoring Fields

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

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


Run on MacBook Pro (Retina, 15-inch, Late 2013) 2.6 GHz Intel Core i7 16 GB 1600 MHz DDR3 using Go version go1.6.2 darwin/amd64

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

go test -bench=. -benchmem=true

BenchmarkSimpleUserStruct-8                                      5000000           299 ns/op          64 B/op          1 allocs/op
BenchmarkSimpleUserStructParallel-8                             20000000           110 ns/op          64 B/op          1 allocs/op
BenchmarkPrimitivesStructAllPrimitivesTypes-8                    2000000           956 ns/op          96 B/op          1 allocs/op
BenchmarkPrimitivesStructAllPrimitivesTypesParallel-8            5000000           285 ns/op          96 B/op          1 allocs/op
BenchmarkComplexArrayStructAllTypes-8                             100000         20706 ns/op        6776 B/op        159 allocs/op
BenchmarkComplexArrayStructAllTypesParallel-8                     200000          6158 ns/op        6776 B/op        159 allocs/op
BenchmarkComplexMapStructAllTypes-8                                50000         35548 ns/op       20966 B/op        245 allocs/op
BenchmarkComplexMapStructAllTypesParallel-8                       200000         11984 ns/op       20966 B/op        245 allocs/op
BenchmarkArrayMapNestedStruct-8                                   200000          5617 ns/op        2064 B/op         37 allocs/op
BenchmarkArrayMapNestedStructParallel-8                          1000000          2032 ns/op        2064 B/op         37 allocs/op

Competitor benchmarks can be found here

Complimentary Software

Here is a list of software that compliments using this library post decoding.

  • Validator - Go Struct and Field validation, including Cross Field, Cross Struct, Map, Slice and Array diving.
  • Conform - Trims, sanitizes & scrubs data based on struct tags.

Package Versioning

I'm jumping on the vendoring bandwagon, you should vendor this package as I will not be creating different version with like allot of my other libraries.

Why? because my time is spread pretty thin maintaining all of the libraries I have + LIFE, it is so freeing not to worry about it and will help me keep pouring out bigger and better things for you the community.


Distributed under MIT License, please see license file in code for more details.