Cannot return a struct implementing a trait in a function

Question

I have this code :

trait State:

    fn to_string(self) -> String:
        ...

struct StateTtt(State):

    fn __init__(inout self):
        pass

    fn to_string(self) -> String:
        return "ABC"

fn get_state() -> State:
    return StateTtt()

fn main():
    var s = get_state()
    print(s.to_string())

But it gives me the error :

cannot implicitly convert 'StateTtt' value to 'State' in return value

But the StateTtt struct implements the State trait, so why it cannot be converted ?

Of course, this is a simplified version of my code. In the real code, the getState function will create a struct instance based on a parameter.

Dimitri · Accepted Answer · 2024-05-16 04:39:54Z

First, welcome to Mojo🔥!

A trait (State) is not a returnable type, but rather a description of how a type can conform to an expected implementation of expected features. You can read more about traits here: https://docs.modular.com/mojo/manual/traits

And here is an example of your code with some modifications:

trait State:
    fn to_string(self) -> String:
        ...

@value
struct StateTtt(State, Stringable):
    var some_state_data: Int

    fn __init__(inout self, some_state_data: Int = 0):
        self.some_state_data = some_state_data

    fn __str__(self) -> String:
        return "StateTtt(" + str(self.some_state_data) + ")"
    
    fn to_string(self) -> String:
        return str(self)

fn get_state() -> StateTtt:
    return StateTtt()

fn main():
    var s = get_state()
    print(s)

Result:

StateTtt(0)

Please note that get_state() is no different than StateTtt(). Since I'm not clear on your objective, I'm just answering the question as I see it and not making architectural tweaks.

Also note that __str__ is the Mojo standard for the already-existing Stringable trait, which allows a struct to be str()'d into a String.

Since your code sample does not lay out full concept behind State and StateTtt, I am unsure if this answer is fully answering the "question behind the question," so to say. But I hope I'm getting you moving in the right direction!

Of note: this answer is current as of Mojo nightly, version 2024.5.1515 (077049dc). The language is still evolving; subtle changes could occur between this post and any future reader.

Collectives™ on Stack Overflow

Cannot return a struct implementing a trait in a function

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related