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A library that provides Go Generics friendly "optional" features.

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# CDN으로 사용하기

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// repository documentation

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A library that provides [Go Generics](https://go.dev/blog/generics-proposal) friendly "optional" features.

## Synopsis

```go
some := optional.Some[int](123)
fmt.Printf("%v\n", some.IsSome()) // => true
fmt.Printf("%v\n", some.IsNone()) // => false

v, err := some.Take()
fmt.Printf("err is nil: %v\n", err == nil) // => err is nil: true
fmt.Printf("%d\n", v) // => 123

mapped := optional.Map(some, func (v int) int {
    return v * 2
})
fmt.Printf("%v\n", mapped.IsSome()) // => true

mappedValue, _ := some.Take()
fmt.Printf("%d\n", mappedValue) // => 246
```

```go
none := optional.None[int]()
fmt.Printf("%v\n", none.IsSome()) // => false
fmt.Printf("%v\n", none.IsNone()) // => true

_, err := none.Take()
fmt.Printf("err is nil: %v\n", err == nil) // => err is nil: false
// the error must be `ErrNoneValueTaken`

mapped := optional.Map(none, func (v int) int {
    return v * 2
})
fmt.Printf("%v\n", mapped.IsNone()) // => true
```

and more detailed examples are here: [./examples_test.go](./examples_test.go).

## Docs

[![GoDoc](https://godoc.org/github.com/moznion/go-optional?status.svg)](https://godoc.org/github.com/moznion/go-optional)

### Supported Operations

#### Value Factory Methods

- [Some[T]\() Option[T]](https://pkg.go.dev/github.com/moznion/go-optional#Some)
- [None[T]\() Option[T]](https://pkg.go.dev/github.com/moznion/go-optional#None)
- [FromNillable[T]\() Option[T]](https://pkg.go.dev/github.com/moznion/go-optional#FromNillable)
- [PtrFromNillable[T]\() Option[T]](https://pkg.go.dev/github.com/moznion/go-optional#PtrFromNillable)

#### Option value handler methods

- [Option[T]#IsNone() bool](https://pkg.go.dev/github.com/moznion/go-optional#Option.IsNone)
- [Option[T]#IsSome() bool](https://pkg.go.dev/github.com/moznion/go-optional#Option.IsSome)
- [Option[T]#Unwrap() T](https://pkg.go.dev/github.com/moznion/go-optional#Option.Unwrap)
- [Option[T]#UnwrapAsPtr() \*T](https://pkg.go.dev/github.com/moznion/go-optional#Option.UnwrapAsPtr)
- [Option[T]#Take() (T, error)](https://pkg.go.dev/github.com/moznion/go-optional#Option.Take)
- [Option[T]#TakeOr(fallbackValue T) T](https://pkg.go.dev/github.com/moznion/go-optional#Option.TakeOr)
- [Option[T]#TakeOrElse(fallbackFunc func() T) T](https://pkg.go.dev/github.com/moznion/go-optional#Option.TakeOrElse)
- [Option[T]#Or(fallbackOptionValue Option[T]) Option[T]](https://pkg.go.dev/github.com/moznion/go-optional#Option.Or)
- [Option[T]#OrElse(fallbackOptionFunc func() Option[T]) Option[T]](https://pkg.go.dev/github.com/moznion/go-optional#Option.OrElse)
- [Option[T]#Filter(predicate func(v T) bool) Option[T]](https://pkg.go.dev/github.com/moznion/go-optional#Option.Filter)
- [Option[T]#IfSome(f func(v T))](https://pkg.go.dev/github.com/moznion/go-optional#Option.IfSome)
- [Option[T]#IfSomeWithError(f func(v T) error) error](https://pkg.go.dev/github.com/moznion/go-optional#Option.IfSomeWithError)
- [Option[T]#IfNone(f func())](https://pkg.go.dev/github.com/moznion/go-optional#Option.IfNone)
- [Option[T]#IfNoneWithError(f func() error) error](https://pkg.go.dev/github.com/moznion/go-optional#Option.IfNoneWithError)
- [Option.Map[T, U any](option Option[T], mapper func(v T) U) Option[U]](https://pkg.go.dev/github.com/moznion/go-optional#Map)
- [Option.MapOr[T, U any](option Option[T], fallbackValue U, mapper func(v T) U) U](https://pkg.go.dev/github.com/moznion/go-optional#MapOr)
- [Option.MapWithError[T, U any](option Option[T], mapper func(v T) (U, error)) (Option[U], error)](https://pkg.go.dev/github.com/moznion/go-optional#MapWithError)
- [Option.MapOrWithError[T, U any](option Option[T], fallbackValue U, mapper func(v T) (U, error)) (U, error)](https://pkg.go.dev/github.com/moznion/go-optional#MapOrWithError)
- [Option.FlatMap[T, U any](option Option[T], mapper func(v T) Option[U]) Option[U]](https://pkg.go.dev/github.com/moznion/go-optional#FlatMap)
- [Option.FlatMapOr[T, U any](option Option[T], fallbackValue U, mapper func(v T) Option[U]) U](https://pkg.go.dev/github.com/moznion/go-optional#FlatMapOr)
- [Option.FlatMapWithError[T, U any](option Option[T], mapper func(v T) (Option[U], error)) (Option[U], error)](https://pkg.go.dev/github.com/moznion/go-optional#FlatMapWithError)
- [Option.FlatMapOrWithError[T, U any](option Option[T], fallbackValue U, mapper func(v T) (Option[U], error)) (U, error)](https://pkg.go.dev/github.com/moznion/go-optional#FlatMapOrWithError)
- [Option.Zip[T, U any](opt1 Option[T], opt2 Option[U]) Option[Pair[T, U]]](https://pkg.go.dev/github.com/moznion/go-optional#Zip)
- [Option.ZipWith[T, U, V any](opt1 Option[T], opt2 Option[U], zipper func(opt1 T, opt2 U) V) Option[V]](https://pkg.go.dev/github.com/moznion/go-optional#ZipWith)
- [Option.Unzip[T, U any](zipped Option[Pair[T, U]]) (Option[T], Option[U])](https://pkg.go.dev/github.com/moznion/go-optional#Unzip)
- [Option.UnzipWith[T, U, V any](zipped Option[V], unzipper func(zipped V) (T, U)) (Option[T], Option[U])](https://pkg.go.dev/github.com/moznion/go-optional#UnzipWith)

### nil == None[T]

This library deals with `nil` as same as `None[T]`. So it works with like the following example:

```go
var nilValue Option[int] = nil
fmt.Printf("%v\n", nilValue.IsNone()) // => true
fmt.Printf("%v\n", nilValue.IsSome()) // => false
```

### JSON marshal/unmarshal support

This `Option[T]` type supports JSON marshal and unmarshal.

If the value wanted to marshal is `Some[T]` then it marshals that value into the JSON bytes simply, and in unmarshaling, if the given JSON string/bytes has the actual value on corresponded property, it unmarshals that value into `Some[T]` value.

example:

```go
type JSONStruct struct {
	Val Option[int] `json:"val"`
}

some := Some[int](123)
jsonStruct := &JSONStruct{Val: some}

marshal, err := json.Marshal(jsonStruct)
if err != nil {
	return err
}
fmt.Printf("%s\n", marshal) // => {"val":123}

var unmarshalJSONStruct JSONStruct
err = json.Unmarshal(marshal, &unmarshalJSONStruct)
if err != nil {
	return err
}
// unmarshalJSONStruct.Val == Some[int](123)
```

Elsewise, when the value is `None[T]`, the marshaller serializes that value as `null`. And if the unmarshaller gets the JSON `null` value on a property corresponding to the `Optional[T]` value, or the value of a property is missing, that deserializes that value as `None[T]`.

example:

```go
type JSONStruct struct {
	Val Option[int] `json:"val"`
}

none := None[int]()
jsonStruct := &JSONStruct{Val: none}

marshal, err := json.Marshal(jsonStruct)
if err != nil {
	return err
}
fmt.Printf("%s\n", marshal) // => {"val":null}

var unmarshalJSONStruct JSONStruct
err = json.Unmarshal(marshal, &unmarshalJSONStruct)
if err != nil {
	return err
}
// unmarshalJSONStruct.Val == None[int]()
```

And this also supports `omitempty` option for JSON unmarshaling. If the value of the property is `None[T]` and that property has `omitempty` option, it omits that property.

ref:

> The "omitempty" option specifies that the field should be omitted from the encoding if the field has an empty value, defined as false, 0, a nil pointer, a nil interface value, and any empty array, slice, map, or string.
> https://pkg.go.dev/encoding/json#Marshal

example:

```go
type JSONStruct struct {
	OmitemptyVal Option[string] `json:"omitemptyVal,omitempty"` // this should be omitted
}

jsonStruct := &JSONStruct{OmitemptyVal: None[string]()}
marshal, err := json.Marshal(jsonStruct)
if err != nil {
	return err
}
fmt.Printf("%s\n", marshal) // => {}
```

### SQL Driver Support

`Option[T]` satisfies [sql/driver.Valuer](https://pkg.go.dev/database/sql/driver#Valuer) and [sql.Scanner](https://pkg.go.dev/database/sql#Scanner), so this type can be used by SQL interface on Golang.

example of the primitive usage:

```go
sqlStmt := "CREATE TABLE tbl (id INTEGER NOT NULL PRIMARY KEY, name VARCHAR(32));"
db.Exec(sqlStmt)

tx, _ := db.Begin()
func() {
    stmt, _ := tx.Prepare("INSERT INTO tbl(id, name) values(?, ?)")
    defer stmt.Close()
    stmt.Exec(1, "foo")
}()
func() {
    stmt, _ := tx.Prepare("INSERT INTO tbl(id) values(?)")
    defer stmt.Close()
    stmt.Exec(2) // name is NULL
}()
tx.Commit()

var maybeName Option[string]

row := db.QueryRow("SELECT name FROM tbl WHERE id = 1")
row.Scan(&maybeName)
fmt.Println(maybeName) // Some[foo]

row := db.QueryRow("SELECT name FROM tbl WHERE id = 2")
row.Scan(&maybeName)
fmt.Println(maybeName) // None[]
```

## Known Issues

The runtime raises a compile error like "methods cannot have type parameters", so `Map()`, `MapOr()`, `MapWithError()`, `MapOrWithError()`, `Zip()`, `ZipWith()`, `Unzip()` and `UnzipWith()` have been providing as functions. Basically, it would be better to provide them as the methods, but currently, it compromises with the limitation.

## Author

moznion (<moznion@mail.moznion.net>)

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