Implementing the Maybe monad in Golang

maybe.go

package main

import (
    "fmt"
    "errors"
)

type Maybe interface {
    Return(value interface{}) Maybe
    Bind(func(interface{}) Maybe) Maybe
}

type Just struct {
    Value interface{}
}

type Nothing struct {}

func (j Just) Return(value interface{}) Maybe {
    return Just{ value }
}

func (j Just) Bind(fn func(interface{}) Maybe) Maybe {
    return fn(j.Value)
}

func (n Nothing) Return(value interface{}) Maybe {
    return Nothing{}
}

func (n Nothing) Bind(fn func(interface{}) Maybe) Maybe {
    return Nothing{}
}

// Now instead of writing things like this

func ErroringDouble(a int) (int, error) {
    return 2 * a, errors.New("This is what bothers me about Go the most, I think.")
}

// We can write

func MaybeDouble(a Maybe) Maybe {
    return a.Bind(func (v interface{}) Maybe { 
        return Just { 2 * v.(int) }
    })
}

// Although it certainly isn't as clear as the first definition, and we 
// basically had to throw type safety out the window, we no longer
// have to write code like the following:

/*
// Double a number three times because demonstrations, okay?
a := 2
newA1, err := ErroringDouble(a)
if err != nil {
    fmt.Println("Whoops!")
} else {
    newA2, err2 := ErroringDouble(newA1)
    if err2 != nil {
        fmt.Println("Whoops!!")
    } else {
        newA3, err3 := ErroringDouble(newA3)
        if err3 != nil {
            fmt.Println("Whoops!!!")
        } else {
            // Finally do something with newA3
        }
    }
}
*/

// We can instead write...

/*
a := Just{ 2 }
newA := MaybeDouble(MaybeDouble(MaybeDouble( a )))
newA.bind(func (v interface{}) Maybe) {
    // Do stuff with our new value
})
*/

func main() {
    val := Just{ 3 }
    fmt.Println(val.Bind(func (v interface{}) Maybe {
        value := v.(int)
        return Just{ value * value }
    }).Bind(func (v interface{}) Maybe {
        return Nothing{}
    }).Bind(func (v interface{}) Maybe {
        return Just{ v.(int) + 1 }
    }))
    fmt.Println(MaybeDouble(MaybeDouble(MaybeDouble(val))))
}

Please, do it +

Yes!, now lets go with Try!!, and then Either 😄