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nest - Accessing deeply nested values made easy.
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nest is a Go library to work on deeply nested datastructures in a more managed way.

nest is a library that provides an easy way to retreive fields from deeply nested data structures. Structure here can be of any Go type that can store data (slice, maps, structs, primitives).

Fields are accessed by using a specific path string. The path string contains accessors for the nested structures seperated by /.

Accessors can be of two types:

  1. Direct field/index accessor.
  2. Special types.

Each path string must begin with a /. This / is the root structure. Simply getting the path / returns the original structure. e.g.:

var dst = []int{}
var src = []int{1, 2, 3}
Get("/", src, &dst) // dst == []int{1, 2, 3} == src -- deep copy
  1. Direct field/index accessors: After the root path, direct accessor can be used. It can be an index, in case of a slice, or a field name/map key. e.g.:
var dst int
Get("/1", src, &dst) // dst == 2 == src[1]

type Course struct {
  Name      string
  Teacher   string
var src = struct{
  Name    string
  Age     int
  Marks   []int
  Courses []Course
  Name:  "Abh",
  Age:   10,
  Marks: []int{20, 19, 15}
  Courses: []Course{ {"Physics", "Prof. P"} {"Chemistry", "Prof. C"} }

var dst string
Get("/Name", src, &dst) // dst == "Abh" == src.Name

var dst int
Get("/Marks/2", src, &dst) // dst == 15 == src.Marks[2]

var dst string
Get("/Courses/0/Teacher", src, &dst) // dst == "Prof. P"
  1. Special Type Accessors: Use of only direct accessor result in single values, as illustrated above. To get multiple values, special accessors are required. There are two types of these accessors:

The Dot (.) Accessor: The Dot accessor fetches each element in an iterable type (slice, map). For example, consider the following value:

type InnerSimple struct {
  I int
  S string
v := []InnerSimple{{1, "one"}, {2, "two"}, {3, "three"}}

Now, to get all the values in v, we can write:

var result []InnerSimple
Get("/.", v, &result) // result == v

But this is not a very useful result. But we can see that '.' fetched each value in v and put it in a new slice. Now consider the case when you want all the string values from the slice:

var result []int
Get("/./S", v, &result) // result == []string{"one", "two", "three"}

Now this is a more useful result. . can be used multiple times in a path as long as it follows an iterable type in the path. In above path, / is a slice itself and is iterable. The . accesses each element in that slice (/ here) and fetches the elements 'S' field. Note that each element of the slice must either be a struct with a field named 'S' or a map with a key 'S'.

As another example, consider:

type Simple struct {
  SimpleI int
  SimpleS string
  SimpleSlc []InnerSimple
v := []Simple{
  {1, "one", InnerSimple{10, "ten"}, []InnerSimple{InnerSimple{1, "a"}, InnerSimple{2, "b"}}},
  {2, "two", InnerSimple{20, "twenty"}, []InnerSimple{InnerSimple{3, "c"}, InnerSimple{4, "d"}}},
  {3, "three", InnerSimple{30, "thirty"}, []InnerSimple{InnerSimple{5, "e"}, InnerSimple{6, "f"}}},
var result [][]string
Get("/./SimpleSlc/./S", v, &result) // result ==  [][]string{{"a", "b"}, {"c", "d"}, {"e", "f"}}

In above example, the path starts at root, which is a slice ([]Simple). It then accesses each element of this slice. For each element, it accesses the SimpleSlc field. This field is also a slice and so another . can be used to access each element of SimpleSlc. After the second ., 'S' field of each element of SimpleSlc is accessed.

Note the type of the result. It is of [][]string type. That's because each . fills up exactly one slice. Inner . works on a single SimpleSlc and returns a slice containing all the 'S' values. The outer . collect these values in another slice. Hence [][]string. But what if we wanted to merge the inner slices together. That's where our second accessor type comes into play.

The Star (*) Accessor: There are two important rules associated with this accessor.
a) Star accessor can only be used to replace a . in the path.
b) Star accessor cannot be the last accessor (or only) in a path. I.e., path /A/B/*/C is illegal.
What * does is that it breaks the structure or returned result. If in above path /./SimpleSlc/./S, we replace the first . with a star, we get the following result:

var result [][]string
Get("/*/SimpleSlc/./S", v, &result) // result == []string{"a", "b", "c", "d", "e", "f"}

As evident, * merges together the outer slice. This is why * must always be used before a ., because it must have some result to merge together. Many more examples can be found in the test files, which also include map accesses.

Add data updates through path.
Add custom function to process accessed data.
Add interface similar to Marshalling/Unmarshalling similar to encoding.

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