trycopy - uses a try monad to copy a file

trycopy.go

package try // https://github.com/dnmfarrell/try

import (
	"fmt"
	"io"
	"os"
)

// based on:
// https://go.googlesource.com/proposal/+/master/design/go2draft-error-handling-overview.md

func Open(src string) Try[*os.File] {
	r, err := os.Open(src)
	if err != nil {
		return Fail[*os.File](err)
	}
	return Succeed[*os.File](r)
}

func Copy(r, w *os.File) Try[*os.File] {
	_, err := io.Copy(w, r)
	r.Close()
	if err != nil {
		w.Close()
		return Fail[*os.File](err)
	}
	return Succeed[*os.File](w)
}

func Close(fd *os.File) Try[int] { // should be option
	err := fd.Close()
	if err != nil {
		return Fail[int](err)
	}
	return Succeed[int](0)
}

func CopyFile(src, dst string) error {
	createNCopy := func(r *os.File) Try[*os.File] {
		w, err := os.Create(dst)
		if err != nil {
			r.Close()
			return Fail[*os.File](err)
		}
		return Copy(r, w)
	}
	err := Bind(Bind(Open(src), createNCopy), Close).Err
	if err != nil {
		os.Remove(dst)
		return fmt.Errorf("copy %s %s: %v", src, dst, err)
	}
	return nil
}