binding

Reflectionless data binding for Go's net/http

Features

• HTTP request data binding
• Data validation (custom and built-in)
• Error handling

Benefits

• Moves data binding, validation, and error handling out of your application's handler
• Reads Content-Type to deserialize form, multipart form, and JSON data from requests
• No middleware: just a function call
• Usable in any setting where net/http is present (Negroni, gocraft/web, std lib, etc.)
• No reflection

Usage example

package main

import (
	"fmt"
	"net/http"

	"github.com/mholt/binding"
)

// First define a type to hold the data
// (If the data comes from JSON, see: http://mholt.github.io/json-to-go)
type ContactForm struct {
	User struct {
		ID int
	}
	Email   string
	Message string
}

// Then provide a field mapping (pointer receiver is vital)
func (cf *ContactForm) FieldMap(req *http.Request) binding.FieldMap {
	return binding.FieldMap{
		&cf.User.ID: "user_id",
		&cf.Email:   "email",
		&cf.Message: binding.Field{
			Form:     "message",
			Required: true,
		},
	}
}

// Now your handlers can stay clean and simple
func handler(resp http.ResponseWriter, req *http.Request) {
	contactForm := new(ContactForm)
	errs := binding.Bind(req, contactForm)
	if errs.Handle(resp) {
		return
	}
	fmt.Fprintf(resp, "From:    %d\n", contactForm.User.ID)
	fmt.Fprintf(resp, "Message: %s\n", contactForm.Message)
}

func main() {
	http.HandleFunc("/contact", handler)
	http.ListenAndServe(":3000", nil)
}

Multipart/form-data usage example

package main

import (
	"bytes"
	"fmt"
	"github.com/mholt/binding"
	"io"
	"log"
	"mime/multipart"
	"net/http"
)

// We expect a multipart/form-data upload with
// a file field named 'data'
type MultipartForm struct {
	Data *multipart.FileHeader `json:"data"`
}

func (f *MultipartForm) FieldMap(req *http.Request) binding.FieldMap {
	return binding.FieldMap{
		&f.Data: "data",
	}
}

// Handlers are still clean and simple
func handler(resp http.ResponseWriter, req *http.Request) {
	multipartForm := new(MultipartForm)
	errs := binding.Bind(req, multipartForm)
	if errs.Handle(resp) {
		return
	}

	// To access the file data you need to Open the file
	// handler and read the bytes out.
	var fh io.ReadCloser
	var err error
	if fh, err = multipartForm.Data.Open(); err != nil {
		http.Error(resp,
			fmt.Sprint("Error opening Mime::Data %+v", err),
			http.StatusInternalServerError)
		return
	}
	defer fh.Close()
	dataBytes := bytes.Buffer{}
	var size int64
	if size, err = dataBytes.ReadFrom(fh); err != nil {
		http.Error(resp,
			fmt.Sprint("Error reading Mime::Data %+v", err),
			http.StatusInternalServerError)
		return
	}
	// Now you have the attachment in databytes.
	// Maximum size is default is 10MB.
	log.Printf("Read %v bytes with filename %s",
		size, multipartForm.Data.Filename)
}

func main() {
	http.HandleFunc("/upload", handler)
	http.ListenAndServe(":3000", nil)
}

You can test from CLI using the excellent httpie client

http -f POST localhost:3000/upload data@myupload

Custom data validation

You may optionally have your type implement the binding.Validator interface to perform your own data validation. The .Validate() method is called after the struct is populated.

func (cf ContactForm) Validate(req *http.Request, errs binding.Errors) binding.Errors {
	if cf.Message == "Go needs generics" {
		errs = append(errs, binding.Error{
			FieldNames:     []string{"message"},
			Classification: "ComplaintError",
			Message:        "Go has generics. They're called interfaces.",
		})
	}
	return errs
}

Error Handling

binding.Bind() and each deserializer returns errors. You don't have to use them, but the binding.Errors type comes with a kind of built-in "handler" to write the errors to the response as JSON for you. For example, you might do this in your HTTP handler:

if binding.Bind(req, contactForm).Handle(resp) {
	return
}

As you can see, if .Handle(resp) wrote errors to the response, your handler may gracefully exit.

Binding custom types

For types you've defined, you can bind form data to it by implementing the Binder interface. Here's a contrived example:

type MyType map[string]string

func (t *MyType) Bind(fieldName string, strVals []string, errs binding.Errors) binding.Errors {
	t["formData"] = strVals[0]
	return errs
}

If you can't add a method to the type, you can still specify a Binder func in the field spec. Here's a contrived example that binds an integer (not necessary, but you get the idea):

func (t *MyType) FieldMap() binding.FieldMap {
	return binding.FieldMap{
		"number": binding.Field{
			Binder: func(fieldName string, formVals []string, errs binding.Errors) binding.Errors {
				val, err := strconv.Atoi(formVals[0])
				if err != nil {
					errs.Add([]string{fieldName}, binding.DeserializationError, err.Error())
				}
				t.SomeNumber = val
				return errs
			},
		},
	}
}

Notice that the binding.Errors type has a convenience method .Add() which you can use to append to the slice if you prefer.

Supported types (forms)

The following types are supported in form deserialization by default. (JSON requests are delegated to encoding/json.)

• uint, *uint, []uint, uint8, *uint8, []uint8, uint16, *uint16, []uint16, uint32, *uint32, []uint32, uint64, *uint64, []uint64
• int, *int, []int, int8, *int8, []int8, int16, *int16, []int16, int32, *int32, []int32, int64, *int64, []int64
• float32, *float32, []float32, float64, *float64, []float64
• bool, *bool, []bool
• string, *string, []string
• time.Time, *time.Time, []time.Time
• *multipart.FileHeader, []*multipart.FileHeader