Transforming return type when using map

[Luviz]

Hi I have been looking to use this package in my solution however every time I have tried to so I feel like something is missing.

In our solution we want to use Options and Results every time we are uncertain of the return value.

I do use monads in python and node (typescript) and it's great there is a a lot that we can do with elegantly, and I do like MarshalJSON function that you have implemented.

My issue with mo is that when we need to map a type to another type, see example below.

this code is more like a pseudocode

type UserRequestForm struct {
	Email     string
	FirstName string
	LastName  string
	Address   string // can be empty
	Title     string // can be empty
}

type User struct {
	Id        string // uuid/guid
	Email     string
	Name      string
	FirstName string
	LastName  string
	Address   string // can be empty
	Title     string // can be empty
}

func (data UserRequestForm) TDO() User {
	return User{
		Id:        uuidx.NewV4().String(),
		Email:     data.Email,
		Name:      fmt.Sprintf("%v %v", data.FirstName, data.LastName),
		FirstName: data.FirstName,
		LastName:  data.LastName,
		Address:   data.Address,
		Title:     data.Title,
	}
}

func UserRequestFormToDTO(value UserRequestForm) mo.Option[User] {
	return mo.Some(value.TDO())
}

Here I want to get a option of UserRequestForm and if it has a value I wan't to convert it to a TDO and push it to a DB.

So something looking like this:

func CreateUser(user mo.Option[UserRequestForm]) error {
	return user.FlatMap(UserRequestFormToDTO).Match(
		func(value User) {
			db.CreateUser(value) // Value should be of type User
		},
		func() (error) {
			return fmt.Errorf("Form data could to be converted to User database object ")
		})
}

I'm kind new to GO thereby I might be a bit ignorerat, so there might be that writing something like the code above is might not be a good idea.
However I know that there is good support generics and type inference in GO. So I don't see any issues with creating functions looking like this:

func Map[T any, R any](value T, callback func(T) R) R {
	return callback(value)
}

func main() {
       u := Map(data, UserRequestFormToDTO) // typeof u is User
}

Here is my question, is there a reson why the Map and FlatMap function is hard code as generic. Or is just that you haven't implemented support for it yet, and in that case can I try to make PR?

[axaluss]

this is needed badly

[Luviz]

I found som commented out code ./options/transforms.go.

I looks like someone tried it but gave up.

[axaluss]

I happened to come up with the same code as workaround. Why would this not be provided? Beste Grüße Alexander Thom

…

On Tue, Mar 11, 2025 at 5:54 AM Bardia Jedi ***@***.***> wrote: I found som commented out code ./options/transforms.go <https://github.com/samber/mo/blob/master/options/transforms.go>. I looks like someone tried it but gave up. — Reply to this email directly, view it on GitHub <#66 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AACTUZSWUMRC5VWTGQNYJCD2T2XDRAVCNFSM6AAAAABYMEMOKSVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDOMJTGQZDQMJYHA> . You are receiving this because you commented.Message ID: ***@***.***> [image: Luviz]*Luviz* left a comment (samber/mo#66) <#66 (comment)> I found som commented out code ./options/transforms.go <https://github.com/samber/mo/blob/master/options/transforms.go>. I looks like someone tried it but gave up. — Reply to this email directly, view it on GitHub <#66 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AACTUZSWUMRC5VWTGQNYJCD2T2XDRAVCNFSM6AAAAABYMEMOKSVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDOMJTGQZDQMJYHA> . You are receiving this because you commented.Message ID: ***@***.***>

[Luviz]

We could always fork it and make the change. @samber haven't responded and if you were able to find a workaround, I would love to check it out and see we could use it as well.
Also I could help as well if it's needed.

[axaluss]

I'm using Result a lot. The code can be copied and adapted for any of the mo types.
I'm considering moving to https://github.com/IBM/fp-go because it's way more feature complete ...but looks more complicated than need be what i'm going for....

func FlatMap[T any, V any](
	result mo.Result[T],
	f func(T) mo.Result[V],
) mo.Result[V] {
	v, err := result.Get()
	if err != nil {
		return mo.Err[V](err)
	}
	return f(v)
}