I’m fairly new to Rust, and as a project to make my hands more used to it, I decided to implement a library about category theory and functional programming in general. Long story short, there is a trait called Monad:
pub trait Monad {
// Associated types and functions here
}
And in rust, Options are Monads, so I went like this:
impl<X> Monad for Option<X> {
// Implementation here
}
Everything’s working properly so far. I decided to add a special Monad called Identity Monad, which is a famous and handy one:
type Identity<A> = A;
impl<X> Monad for Identity<X> {
// Implementation here
}
And right there, I get this error: conflicting implementations for Option<_>. Now I have read this SO post, and it makes sense for that use case, but about mine, as far as I can think through, since Identity is a type (not a trait) it may never be the "Option". Can anyone let me know about why’s that happening and what I can do for it?
>Solution :
They conflict because Identity<Option<X>> is the same type as Option<X>.
type does not define a new type, it defines an alias for an existing type; Identity<X> is just another name for X.
You might be interested in the newtype pattern where you create a single-struct field to define a new semantic type wrapping another:
struct Identity<X>(X);
Now Identity<X> is a different type from X, and the implementations would no longer conflict.
If you decorate Identity with #[repr(transparent)] then it’s even possible to safely transmute &X to &Identity<X>, which might be useful depending on the members of Monad.