Alternatives and detailed information of set

Package set is a small wrapper around the official reflect package that facilitates loose type conversion, assignment into native Go types, and utilities to populate deeply nested Go structs.

Read the godoc for more detailed explanations and examples but here are some enticing snippets.

Scalars and Type-Coercion

{
    // type coercion
    b, i := true, 42
    set.V(&b).To("False")    // Sets b to false
    set.V(&i).To("3.14")     // Sets i to 3
}

{
    // type coercion
    a := int(0)
    b := uint(42)
    set.V(&a).To(b)             // This coerces b into a if possible.
    set.V(&a).To("-57")         // Also works.
    set.V(&a).To("Hello")       // Returns an error.    
}

Pointer Allocation Plus Type-Coercion

{
    // pointer allocation and type coercion
    var bppp ***bool
    set.V(&bppp).To("True")
    fmt.Println(***bppp) // Prints true
}

Scalars-to-Slices and Slices-to-Scalars

{
    // assign scalars to slices
    var b []bool
    set.V(&b).To("True") // b is []bool{ true }

    // or slices to scalars (last element wins)
    var b bool
    set.V(&b).To([]bool{ false, false, true } ) // b is true, coercion not needed.
    set.V(&b).To([]interface{}{ float32(1), uint(0) }) // b is false, coercion needed.
}

Slices to Slices Including Type-Coercion

{
    // slices to slices with or without type coercion; new slice is always created!
    var t []bool
    var s []interface{}
    s = []interface{}{ "true", 0, float64(1) }
    set.V(&t).To(s) // t is []bool{ true, false, true }
}

{
    var t []bool
    var s []bool
    s = []bool{ true, false, true }
    set.V(&t).To(s) // t is []bool{ true, false, true } and t != s
}

Filling Structs by Field Name

m := map[string]interface{}{
    "Name": "Bob",
    "Age":  42,
    "Address": map[interface{}]string{
        "Street1": "97531 Some Street",
        "Street2": "",
        "City":    "Big City",
        "State":   "ST",
        "Zip":     "12345",
    },
}
myGetter := set.MapGetter(m)

type Address struct {
    Street1 string 
    Street2 string 
    City    string 
    State   string 
    Zip     string 
}
type Person struct {
    Name    string  
    Age     uint    
    Address Address 
}
var t Person
set.V(&t).Fill(myGetter)

Filling Structs by Struct Tag

m := map[string]interface{}{
    "name": "Bob",
    "age":  42,
    "address": map[interface{}]string{
        "street1": "97531 Some Street",
        "street2": "",
        "city":    "Big City",
        "state":   "ST",
        "zip":     "12345",
    },
}
myGetter := set.MapGetter(m)

type Address struct {
    Street1 string `key:"street1"`
    Street2 string `key:"street2"`
    City    string `key:"city"`
    State   string `key:"state"`
    Zip     string `key:"zip"`
}
type Person struct {
    Name    string  `key:"name"`
    Age     uint    `key:"age"`
    Address Address `key:"address"`
}
var t Person
set.V(&t).FillByTag("key", myGetter)

Allocating Struct Pointers and Pointer Fields

type Address struct {
    Street1 string `key:"street1"`
    Street2 string `key:"street2"`
    City    string `key:"city"`
    State   string `key:"state"`
    Zip     string `key:"zip"`
}
type Person struct {
    Name    string   `key:"name"`
    Age     uint     `key:"age"`
    Address *Address `key:"address"`
}
m := map[string]interface{}{
    "name": "Bob",
    "age":  42,
    "address": map[interface{}]string{
        "street1": "97531 Some Street",
        "street2": "",
        "city":    "Big City",
        "state":   "ST",
        "zip":     "12345",
    },
}
getter := set.MapGetter(m)

var t *Person
set.V(&t).FillByTag("key", getter)
fmt.Println(t.Name)                 // Bob
fmt.Println(t.Address.Street1)      // 97531 Some Street

Field Pointers Always Allocated

type Address struct {
    Street1 string `key:"street1"`
    Street2 string `key:"street2"`
    City    string `key:"city"`
    State   string `key:"state"`
    Zip     string `key:"zip"`
}
type Person struct {
    Name    string   `key:"name"`
    Age     uint     `key:"age"`
    Address *Address `key:"address"`
}
m := map[string]interface{}{
    "name": "Bob",
    "age":  42,
    // address is missing!
}
getter := set.MapGetter(m)
var t *Person
set.V(&t).FillByTag("key", getter)
fmt.Printf("%p\n", t.Address) // Prints a memory address; the field was allocated.

Pointer-to-Slices-of-Struct-Pointers -- OH MY!

Also noteworthy in this example is the same fuzzy logic for assigning scalar-to-slice or slice-to-scalar also works for struct-to-[]struct and []struct-to-struct.

func TestValue_fillNestedStructPointerToSlicesAsPointers(t *testing.T) {
    chk := assert.New(t)
    //
    var err error
    type Address struct {
        Street1 string `key:"street1"`
        Street2 string `key:"street2"`
        City    string `key:"city"`
        State   string `key:"state"`
        Zip     string `key:"zip"`
    }
    type Person struct {
        Name    string   `key:"name"`
        Age     uint     `key:"age"`
        Address *Address `key:"address"`
    }
    type Company struct {
        Name         string     `key:"name"`
        // Within the Getter "employees" is a []map so it is intuitive that Company.Employees becomes
        // a slice with as many entries as Getter( "employees" ).
        Employees    *[]*Person `key:"employees"`
        // The "employees" key is reused here but now it is going into a single struct pointer; the
        // last set of data in Getter( "employees" ) wins, aka Sally.
        LastEmployee *Person    `key:"employees"`
        // Note the "slice" key in the getter is not a []map itself; but a single *Person is
        // created and inserted into Company.Slice.
        Slice        *[]*Person `key:"slice"`
    }
    //
    // Also noteworthy is that Company.Employees and Company.Slice are pointers to slices and
    // they are instantiated automatically by this package.  The syntax to use them becomes unwieldly
    // so I don't know why you'd want to do this but hey -- it works and that's neat.
    //
    m := map[string]interface{}{
        "name": "Some Company",
        "slice": map[string]interface{}{
            "name": "Slice",
            "age":  2,
            "address": map[interface{}]string{
                "street1": "Slice Street",
                "street2": "",
                "city":    "Slice City",
                "state":   "SL",
                "zip":     "99999",
            },
        },
        "employees": []map[string]interface{}{
            {
                "name": "Bob",
                "age":  42,
                "address": map[interface{}]string{
                    "street1": "97531 Some Street",
                    "street2": "",
                    "city":    "Big City",
                    "state":   "ST",
                    "zip":     "12345",
                },
            },
            {
                "name": "Sally",
                "age":  48,
                "address": map[interface{}]string{
                    "street1": "555 Small Lane",
                    "street2": "",
                    "city":    "Other City",
                    "state":   "OO",
                    "zip":     "54321",
                },
            },
        },
    }
    getter := set.MapGetter(m)

    var t *Company
    err = set.V(&t).FillByTag("key", getter)
    chk.NoError(err)
    chk.Equal("Some Company", t.Name)
    //
    chk.Equal(2, len(*t.Employees))
    //
    chk.Equal("Bob", (*t.Employees)[0].Name)
    chk.Equal(uint(42), (*t.Employees)[0].Age)
    chk.Equal("97531 Some Street", (*t.Employees)[0].Address.Street1)
    chk.Equal("", (*t.Employees)[0].Address.Street2)
    chk.Equal("Big City", (*t.Employees)[0].Address.City)
    chk.Equal("ST", (*t.Employees)[0].Address.State)
    chk.Equal("12345", (*t.Employees)[0].Address.Zip)
    //
    chk.Equal("Sally", (*t.Employees)[1].Name)
    chk.Equal(uint(48), (*t.Employees)[1].Age)
    chk.Equal("555 Small Lane", (*t.Employees)[1].Address.Street1)
    chk.Equal("", (*t.Employees)[1].Address.Street2)
    chk.Equal("Other City", (*t.Employees)[1].Address.City)
    chk.Equal("OO", (*t.Employees)[1].Address.State)
    chk.Equal("54321", (*t.Employees)[1].Address.Zip)
    //
    chk.Equal("Sally", t.LastEmployee.Name)
    chk.Equal(uint(48), t.LastEmployee.Age)
    chk.Equal("555 Small Lane", t.LastEmployee.Address.Street1)
    chk.Equal("", t.LastEmployee.Address.Street2)
    chk.Equal("Other City", t.LastEmployee.Address.City)
    chk.Equal("OO", t.LastEmployee.Address.State)
    chk.Equal("54321", t.LastEmployee.Address.Zip)
    //
    chk.Equal(1, len(*t.Slice))
    chk.Equal("Slice", (*t.Slice)[0].Name)
    chk.Equal("Slice Street", (*t.Slice)[0].Address.Street1)
    chk.Equal("", (*t.Slice)[0].Address.Street2)
    chk.Equal("Slice City", (*t.Slice)[0].Address.City)
    chk.Equal("SL", (*t.Slice)[0].Address.State)
    chk.Equal("99999", (*t.Slice)[0].Address.Zip)
}

Value.FieldByIndex() and Value.FieldByIndexAsValue()

Similarly to the standard reflect package Value also supports accessing struct types by field indeces. Unlike the standard reflect package this package will instantiate and create nil members as it traverses the struct.

type A struct {
	B struct {
		C *struct {
			CString string
		}
		BString string
	}
}
var a A
var v, f reflect.Value
v = reflect.Indirect(reflect.ValueOf(&a))
f = v.FieldByIndex([]int{0, 1})     // Accesses a.B.BString -- Ok because B is not nil.
f = v.FieldByIndex([]int{0, 0, 0})  // Accesses a.B.C.CString -- panic because C is nil.

// However with set.Value.FieldByIndex()
f := set.V(&a).FieldByIndex([]int{0, 0, 0}) // Creates C and returns reflect.Value for CString.

// FieldByIndex() returns the field as a reflect.Value but you can also return it as a *set.Value:
asValue := set.V(&a).FieldByIndexAsValue([]int{0, 0, 0})

Mapper and Mapping for Nested Struct Access

Mapper traverses a nested struct hierarchy to generate a Mapping. From a Mapping you can use string keys to access the struct members by struct field index.

type CommonDb struct {
    Pk          int    `t:"pk"`
    CreatedTime string `t:"created_time"`
    UpdatedTime string `t:"updated_time"`
}
type Person struct {
    CommonDb `t:"common"`
    Name     string `t:"name"`
    Age      int    `t:"age"`
}
var data Person

Depending on the public Mapper members (aka options) the following mappings can be created:

// Type CommonDb doesn't affect names; join nestings with "_"
mapper := &set.Mapper{
    Elevated: set.NewTypeList(CommonDb{}),
    Join:     "_",
}
generates = `
[0 0] Pk
[0 1] CreatedTime
[0 2] UpdatedTime
[1] Name
[2] Age
`

// lowercase with dot separators
// This time CommonDb is not elevated and becomes part of the generated names.
mapper := &set.Mapper{
    Join:      ".",
    Transform: strings.ToLower,
}
generates = `
[0 0] commondb.pk
[0 1] commondb.createdtime
[0 2] commondb.updatedtime
[1] name
[2] age
`

// specify tags
// CommonDb not elevated again, tags are used to generate names.
mapper := &set.Mapper{
    Join: "_",
    Tags: []string{"t"},
}
generates = `
[0 0] common_pk
[0 1] common_created_time
[0 2] common_updated_time
[1] name
[2] age
`

BoundMappings combine Mapper.Map() and Value.FieldByIndex()

Mapper.Bind() will return a BoundMapping that you can use to set nested struct members on a variable via strings. You can Rebind() a BoundMapping to switch the underlying value. Consider the following struct hierarchy:

type Common struct {
    Id int
}
type Timestamps struct {
    CreatedTime  string
    ModifiedTime string
}
type Person struct {
    Common
    Timestamps // Not used but present anyways
    First      string
    Last       string
}
type Vendor struct {
    Common
    Timestamps  // Not used but present anyways
    Name        string
    Description string
    Contact     Person
}
type T struct {
    Common
    Timestamps
    //
    Price    int
    Quantity int
    Total    int
    //
    Customer Person
    Vendor   Vendor
}

Create a set.Mapper with your desired options and create a binding:

mapper := &set.Mapper{
    Elevated: set.NewTypeList(Common{}, Timestamps{}),
    Join:     "_",
}
//
dest := new(T)
// bound will become our BoundMapping and the accessor into instances of T
bound := mapper.Bind(&dest)

Now you can iterate a dataset and populate instances of T:

for k := 0; k < b.N; k++ {
    // `rows` is a big pile of randomized data.  We are assigning data from `row` into our new T
    // but using strings to access into the nested structure of T.
    row := rows[k%size]
    // Create a new T
    dest = new(T)
    // Change our binding to the new T
    bound.Rebind(&dest)
    //
    bound.Set("Id", row.Id)
    bound.Set("CreatedTime", row.CreatedTime)
    bound.Set("ModifiedTime", row.ModifiedTime)
    bound.Set("Price", row.Price)
    bound.Set("Quantity", row.Quantity)
    bound.Set("Total", row.Total)
    //
    bound.Set("Customer_Id", row.CustomerId)
    bound.Set("Customer_First", row.CustomerFirst)
    bound.Set("Customer_Last", row.CustomerLast)
    //
    bound.Set("Vendor_Id", row.VendorId)
    bound.Set("Vendor_Name", row.VendorName)
    bound.Set("Vendor_Description", row.VendorDescription)
    bound.Set("Vendor_Contact_Id", row.VendorContactId)
    bound.Set("Vendor_Contact_First", row.VendorContactFirst)
    bound.Set("Vendor_Contact_Last", row.VendorContactLast)
    //
    if err = bound.Err(); err != nil {
        b.Fatalf("Unable to set: %v", err.Error())
    }
}

API Consistency and Breaking Changes

I am making a very concerted effort to break the API as little as possible while adding features or fixing bugs. However this software is currently in a pre-1.0.0 version and breaking changes are allowed under standard semver. As the API approaches a stable 1.0.0 release I will list any such breaking changes here and they will always be signaled by a bump in minor version.

0.3.0 ⭢ 0.4.0
set.Mapper has new field TaggedFieldsOnly. TaggedFieldsOnly=false means no change in behavior. TaggedFieldsOnly=true means set.Mapper only maps exported fields with struct tags.
0.2.3 ⭢ 0.3.0
set.BoundMapping.Assignables has a second argument allowing you to pre-allocate the slice that is returned; you can also set it to nil to keep current behavior.

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