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Published: Nov 22, 2021 License: MIT Imports: 23 Imported by: 3




Golang Struct to HTML Form

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  • Package struc2frm converts a golang struct type into an HTML input form.

  • All your backend forms generated directly from golang structs.

  • HTML input field info is taken from the form struct tag.

  • Decode() and DecodeMultipartForm() transform the HTTP request data back into an instance of the struct type used for the HTML code.

  • Decode() and DecodeMultipartForm() also check the
    auto-generated form token against CSRF attacks.

  • Use Form() to render an HTML form

  • Use Card() to render a read-only HTML card.

Example use

type entryForm struct {
    Department  string   `json:"department,omitempty"    form:"subtype='select',accesskey='p',onchange='true',label='Department/Abteilung',title='loading items'"`
    Separator01 string   `json:"separator01,omitempty"   form:"subtype='separator'"`
    HashKey     string   `json:"hashkey,omitempty"       form:"maxlength='16',size='16',autocapitalize='off',suffix='salt, changes randomness'"` // the , instead of , prevents wrong parsing
    Groups      int      `json:"groups,omitempty"        form:"min=1,max='100',maxlength='3',size='3'"`
    Items       string   `json:"items,omitempty"         form:"subtype='textarea',cols='22',rows='4',maxlength='4000',label='Textarea of<br>line items',title='add times - delimited by newline (enter)'"`
    Items2      []string `json:"items2,omitempty"        form:"subtype='select',size='3',multiple='true',label='Multi<br>select<br>dropdown',autofocus='true'"`
    Group01     string   `json:"group01,omitempty"       form:"subtype='fieldset'"`
    Date        string   `json:"date,omitempty"          form:"subtype='date',nobreak=true,min='1989-10-29',max='2030-10-29'"`
    Time        string   `json:"time,omitempty"          form:"subtype='time',maxlength='12',inputmode='numeric',size='12'"`
    Group02     string   `json:"group02,omitempty"       form:"subtype='fieldset'"`
    DateLayout  string   `json:"date_layout,omitempty"   form:"accesskey='t',maxlength='16',size='16',pattern='[0-9\\.\\-/]{2&comma;10}',placeholder='2006/01/02 15:04',label='Layout of the date'"` // 2006-01-02 15:04
    CheckThis   bool     `json:"check_this,omitempty"    form:"suffix='without consequence'"`

    // Requires distinct way of form parsing
    // Upload     []byte `json:"upload,omitempty"       form:"accesskey='u',accept='.xlsx'"`

// Validate checks whether form entries as a whole are "submittable";
// more than just 'populated'
// Validate generates error messages
func (frm entryForm) Validate() (map[string]string, bool) {
    errs := map[string]string{}
    g1 := frm.Department != ""
    if !g1 {
        errs["department"] = "Missing department"
    g2 := frm.CheckThis
    if !g2 {
        errs["check_this"] = "You need to comply"
    g3 := frm.Items != ""
    if !g3 {
        errs["items"] = "No items"
    return errs, g1 && g2 && g3

// getting a converter
s2f := struc2frm.New()  // or clone existing one
s2f.ShowHeadline = true // set options
s2f.SetOptions("department", []string{"ub", "fm"}, []string{"UB", "FM"})

// init values - non-multiple
frm := entryForm{
    HashKey: time.Now().Format("2006-01-02"),
    Groups:  2,
    Date:    time.Now().Format("2006-01-02"),
    Time:    time.Now().Format("15:04"),

// pulling in values from http request
populated, err := Decode(req, &frm)
if populated && err != nil {
    s2f.AddError("global", fmt.Sprintf("cannot decode form: %v<br>\n <pre>%v</pre>", err, indentedDump(r.Form)))
    log.Printf("cannot decode form: %v<br>\n <pre>%v</pre>", err, indentedDump(r.Form))

// init values - multiple
if !populated {
    if len(frm.Items2) == 0 {
        frm.Items2 = []string{"berta", "dora"}

errs, valid := frm.Validate()

if populated {
    if !valid {
        s2f.AddErrors(errs) // add errors only for a populated form
    } else {
        // further processing with valid form data

if !valid {
    // render to HTML for user input / error correction
    fmt.Fprint(w, s2f.Form(frm))

Global options

  • ShowHeadline - show a headline derived from struct name; default false.

  • FormTag - suppress the surrounding <form ...> ... </form> if you want to compose a form from multiple structs.

  • Name - form name attribute; default frmMain

  • Method - GET or POST; default POST

  • Salt and FormTimeout - parameters to generate CSRF token

  • FocusFirstError - focus on inputs with errors; default true

  • ForceSubmit - show submit button despite onchange=form.submit(); default false

  • Indent, IndentAddenum, VerticalSpacer - change indentation in px; vertical spacing in rem

  • CSS - default CSS classes for reasonable appearance.
    Incorporate similar rules into your application style sheet,
    and set to empty string.

Attributes for field types

  • Use float64 or int to create number inputs - with attributes min=1,max=100,step=2.
    Notice that step=2 defines maximum precision; uneven numbers become invalid.
    This is an HTML5 restriction.

  • string supports attribute placeholder='2006/01/02 15:04' to show a pattern to the user (placeholder).

  • string supports attribute pattern='[0-9\\.\\-/]{10}' to restrict the entry to a regular expression.

  • Use attributes maxlength='16' and size='16'
    determine width and maximum content length respectively for input and textarea.
    Attribute size determines height for select/dropdown elements.

  • Use string field with subtype textarea and attributes cols='32',rows='22'

  • Use string field with subtype date and attributes min='1989-10-29' or max=...

  • Use string field with subtype time

  • Use bool to create a checkbox

Separator and fieldset

These are dummmy fields for formatting only

  • Every string field with subtype separator is rendered into a horizontal line

    • If the struct tag form has as label, then its contents are rendered.
      Serving as static text paragraph.
  • Every string field with subtype fieldset is rendered into grouping box with label

Select / dropdown inputs
  • Use string | int | float64 | bool field with subtype select

  • Use size=1 or size=5 to determine the height

  • Use SetOptions() to fill input[select] elements

  • Use DefaultOptionKey() to read the pre-selected option on clean forms

  • Use onchange='true' for onchange submit


Like select / dropdown,
but rendered as radio inputs.

Select multiple
  • Use subtype select with multiple='true' to enable the selection of multiple items
    in conjunction with struct field type []string | []int | []float64 | []bool

  • Use wildcardselect='true' to show an additional input after the select,
    accepting wildcard expressions with * for selecting options from the select.

    • Wildcard expressions are case sensitive.
    • Multiple wildcard expressions can be chained using ;.
    • Multiple expressions are applied successively additively.
    • Any wildcard expression can be negated by ! prefixing, resulting in unselect.
    • Example Car*;Bike*;!Carsharing.
    • To debug, open the Javascript console of your browser and type wildcardselectDebug = true;
  • Parsing of HTTP request into form struct for multiple fields
    is additive.
    => Init values should not be set before parsing but afterwards.

// ...
populated, err := Decode(req, &frm)
// ...

if len(frm.Items2) == 0 {
    frm.Items2 = []string{"berta", "dora"} // setting defaults if request parsing did not yield any user input

Submit button

If your form only has select inputs with onchange='this.form.submit()'
then no submit button is shown.

This can be overridden by setting struc2frm.New().ShowSubmit to true.

General field attributes

  • Use form:"-" to exclude fields from being rendered
    neither in form view nor in card view

  • Every field can have an attribute label=...,
    appearing before the input element,
    if not specified, json:"[name]..." is labelized and used

    • label-style can specify an individual CSS styles for the label tag
  • Every field can have an attribute suffix=...,
    appearing after the input element

  • Every field can have an attribute title=...
    for mouse-over tooltips

  • Values inside of label='...', suffix='...', title='...', placeholder='...', pattern='...'
    need &comma; instead of ,

  • Every field can have an attribute accesskey='[a-z]'
    Accesskeys are not put into the label, but into the input tag

  • Every field can have an attribute nobreak='true'
    so that the next input remains on the same line

  • Every field can have an attribute autofocus='true'
    setting the keyboard focus to this input.
    Use this only once per form.
    autofocus='true' is overwritten by FocusFirstError==true; see below.

Field attributes for mobile phones
  • inputmode="numeric" opens the numbers keyboard on mobile phones

  • autocapitalize=off switches off first letter upper casing

Validation and errors

The Validator interface is non mandatory helper interface for form structs.

type Validator interface {
    Validate() (map[string]string, bool)

It returns error messages suitable for s2f.AddErrors().

if populated {
    errs, valid := frm.Validate()
    if !valid {
        s2f.AddErrors(errs) // add errors only for a populated form
        // render to HTML for user input / error correction
        fmt.Fprint(w, s2f.Form(frm))

A valid form struct enables further processing.

    } else {
        // further processing
  • Keep FocusFirstError=true to focus the first input having an error message.

  • This overrides autofocus='true'.

File upload

  • input[file] must have golang type []byte

  • input[file] should be named upload
    and requires ParseMultipartForm() instead of ParseForm()

  • DecodeMultipartForm() and ExtractUploadedFile() are helper funcs
    to extract file upload data


type entryForm struct {
    TextField string `json:"text_field,omitempty"   form:"maxlength='16',size='16'"`
    // Requires distinct way of form parsing
    Upload []byte `json:"upload,omitempty"          form:"accesskey='u',accept='.txt',suffix='*.txt files'"`

s2f := struc2frm.New()  // or clone existing one
s2f.ShowHeadline = true // set options
s2f.Indent = 80

// init values
frm := entryForm{
    TextField: "some-init-text",

populated, err := DecodeMultipartForm(req, &frm)
if populated && err != nil {
    s2f.AddError("global", fmt.Sprintf("cannot decode multipart form: %v<br>\n <pre>%v</pre>", err, indentedDump(req.Form)))
    log.Printf("cannot decode multipart form: %v<br>\n <pre>%v</pre>", err, indentedDump(req.Form))

bts, excelFileName, err := ExtractUploadedFile(req)
if err != nil {
    fmt.Fprintf(w, "Cannot extract file from POST form: %v<br>\n", err)

fileMsg := ""
if populated {
    fileMsg = fmt.Sprintf("%v bytes read from excel file -%v- <br>\n", len(bts), excelFileName)
    fileMsg = fmt.Sprintf("%vFile content is --%v-- <br>\n", fileMsg, string(bts))
} else {
    fileMsg = "No upload filename - or empty file<br>\n"



See handler-file-upload_test.go on how to programmatically POST a file and key-values.

CSS Styling

  • Styling is done via CSS selectors
    and can be customized
    by changing or appending struc2frm.New().CSS

  • If you already have good styles in your website,
    set CSS = ""

div.struc2frm {
    padding: 4px;
div.struc2frm  input {
    margin:  4px;

The media query CSS block in default struc2frm.New().CSS
can be used to change the label width depending on screen width.

Label width can also be changed by setting
via struc2frm.New().Indent and struc2frm.New().IndentAddenum to none-zero values.

div.struc2frm-34323168 H3 {
    margin-left: 116px; /* programmatically set via s3f.Indent for each form */
/* change specific inputs */
div.struc2frm label[for="time"] {
    min-width: 20px;
div.struc2frm select[name="department"] {
    background-color: darkkhaki;

Technical stuff

Language files blank comment code
Go 5 123 68 672
Markdown 1 61 0 150
CSS 1 26 8 120
HTML 1 6 1 30
  • Default CSS is init-loaded from an in-package file default.css,
    mostly to have syntax highlighting while editing it.


  • Can we use 0x2C instead of &comma; ?

  • radiogroup needs more finesse

  • Low Prio: Move JavaScript code for multiselect into a JS file



Package struc2frm creates an HTML input form for a given struct type; see for details.



This section is empty.


This section is empty.


func CardH added in v1.2.8

func CardH(w http.ResponseWriter, req *http.Request)

CardH is an example http handler func

func Decode added in v1.0.8

func Decode(r *http.Request, ptr2Struct interface{}) (populated bool, err error)

Decode the http request form into ptr2Struct; validating the CSRF token (; deriving the 'populated' return value from the existence of the CSRF token. We *could* call Validate() on ptr2Struct if implemented; but valid is *more* than just populated.

func DecodeMultipartForm added in v1.0.15

func DecodeMultipartForm(r *http.Request, ptr2Struct interface{}) (populated bool, err error)

DecodeMultipartForm decodes the form into an instance of struct and checks the token against CSRF attacks (

func ExtractUploadedFile

func ExtractUploadedFile(r *http.Request, names ...string) (bts []byte, fname string, err error)

ExtractUploadedFile extracts a file from an HTTP POST request. It needs the request form to be prepared with ParseMultipartForm.

func FileUploadH

func FileUploadH(w http.ResponseWriter, req *http.Request)

FileUploadH is an http handler func for file upload

func FormH

func FormH(w http.ResponseWriter, req *http.Request)

FormH is an example http handler func

func HTML

func HTML(intf interface{}) template.HTML

HTML takes a struct instance and uses the default formatter to turns it into an HTML form.

func New

func New() *s2FT

New converter

func ParseMultipartForm

func ParseMultipartForm(r *http.Request) error

ParseMultipartForm parses an HTTP request form with file attachments

func ValToString added in v1.1.9

func ValToString(val reflect.Value) string

ValToString converts reflect.Value to string.

go-playground/form.Decode nicely converts all kins of request.Form strings into the desired struct types.

But to render the form to HTML, we have to convert those types back to string.

val.String() of a   bool yields "<bool Value>"
val.String() of an   int yields "<int Value>"
val.String() of a  float yields "<float64 Value>"


type CardViewOptions added in v1.2.4

type CardViewOptions struct {
	SkipEmpty bool // Fields with value "" are not rendered
	SuffixPos int  // 0 - no suffix rendered, 1 - suffix after label, 2 - suffix after value

CardViewOptions governs the display of the rendering of Card()

type Validator added in v1.1.0

type Validator interface {
	Validate() (map[string]string, bool)

Validator interface is non mandatory helper interface for form structs; it returns error messages suitable for s2f.AddErrors; a valid form struct enables further processing;


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