The qs package can marshal and unmarshal structs into/from url query strings. The interface of qs is very similar to that of some standard marshaler packages like encoding/json, encoding/xml.

Note that html forms are often POST-ed in the HTTP request body in the same format as query strings (with an encoding called application/x-www-form-urlencoded) so this package can be used for that as well.

The go standard library can convert only between the (query) string and the standard url.Values data type (which is a map[string][]string). This qs package adds struct marshaling and unmarshaling to your arsenal:

                  +--------------+
+---------------->| query string +------------------+
|                 +---------+----+                  |
|                      ^    |                       |
|    url.Values.Encode |    | url.ParseQuery        |
|                      |    v                       |
|                 +----+---------+                  |
|                 |  url.Values  |                  |
|                 +---------+----+                  |
|                      ^    |                       |
|     qs.MarshalValues |    | qs.UnmarshalValues    |
|                      |    v                       |
|                 +----+---------+                  |
+-----------------+    struct    |<-----------------+
    qs.Marshal    +--------------+    qs.Unmarshal

Example:

package main

import "fmt"
import "github.com/pasztorpisti/qs"

type Query struct {
	Search     string
	Page       int
	PageSize   int
	Categories []string `qs:"category"`
}

func main() {
	queryStr, err := qs.Marshal(&Query{
		Search:     "my search",
		Page:       2,
		PageSize:   50,
		Categories: []string{"c1", "c2"},
	})
	fmt.Println("Marshal-Result:", queryStr, err)

	var q Query
	err = qs.Unmarshal(&q, queryStr)
	fmt.Println("Unmarshal-Result:", q, err)

	// Output:
	// Marshal-Result: category=c1&category=c2&page=2&page_size=50&search=my+search <nil>
	// Unmarshal-Result: {my search 2 50 [c1 c2]} <nil>
}

• Support for primitive types (bool, int, etc...), pointers, slices, arrays, maps, structs, time.Time and url.URL.
• A custom type can implement the MarshalQS and/or UnmarshalQS interfaces to handle its own marshaling/unmarshaling.
• The marshaler and unmarshaler are modular and can be extended to support new types. This makes it possible to do several tricks. One of them is being able to override existing type marshalers (e.g.: the []byte array marshaler).
• It can tell whether a type is marshallable before actually marshaling an object of that type. Most marshalers (including the standard encoding/json) can't do this! E.g.: If you have an empty slice that has a non-marshallable item type (e.g.: function) then encoding/json and many other marshalers happily marshal it as an empty slice - they return with an error only if the given slice has at least one item and they realise that it can't be marshaled.
• You can create custom marshaler objects that define different defaults for the marshaling process:
  • A struct-to-query_string name transformer func that is used when the struct field tag doesn't set a custom name for the field. The default function converts CamelCase go struct field names to snake_case which is the standard in case of query strings.
  • When a struct field tag specifies none of the keepempty and omitempty options the marshaler uses keepempty by default. By creating a custom marshaler you can change the default to omitempty.
  • When a struct field tag doesn't specify any of the opt, nil, req options the unmarshaler uses opt by default. By creating a custom unmarshaler you can change this default.
• A struct field tag can be used to:
  • Exclude a field from marshaling/unmarshaling by specifying - as the field name (qs:"-").
  • Set custom name for the field in the marshaled query string.
  • Set one of the keepempty, omitempty options for marshaling.
  • Set one of the opt, nil, req options for unmarshaling.

The godoc of the qs package contains more detailed documentation with working examples.