test.go

package main

import "errors"
import "fmt"

type just struct {
    fn func () (interface {}, error)
    lastRetval interface {}
}

type _error struct {
    err error
}

type monad interface {
    Do(func () (interface {}, error)) monad
    DoIt() (interface{}, error)
    End(func (err error)) interface {}
}

func (m *just) DoIt() (interface{}, error) {
    if m.fn != nil {
        return m.fn()
    } else {
        return nil, nil
    }
}

func (m *just) Do(what func () (interface {}, error)) monad {
    retval, err := m.DoIt()
    if err != nil {
        return &_error { err }
    }
    return &just { fn: what, lastRetval: retval }
}

func (m *just) End(errorfn func (err error)) interface {} {
    retval, err := m.DoIt()
    if err != nil {
        errorfn(err)
        return nil
    }
    return retval
}

func (m *_error) DoIt() (interface{}, error) {
    return nil, m.err
}

func (m *_error) Do(what func () (interface {}, error)) monad {
    return &_error { m.err }
}

func (m *_error) End(errorfn func (_ error)) interface {} {
    errorfn(m.err)
    return nil
}

func main() {
    monad := &just {}
    monad.Do(func () (interface {}, error) {
        println("test1")
        return nil, nil
    }).Do(func () (interface {}, error) {
        println("test2")
        // return nil, nil
        return nil, errors.New("1")
    }).Do(func () (interface {}, error) {
        println("test3")
        return nil, nil
    }).Do(func () (interface {}, error) {
        println("test4")
        return nil, nil
    }).End(func (err error) {
        fmt.Printf("I got an error: %s\n", err.Error())
    })
}