How to deduce sub generic types from main generic type?

Question

Is it possible to change this code to specify just the main type: impl Writer<MyTable> and let Rust deduce the H and R parameters automatically?

trait Table<H, R> {
    fn rows(&self) -> &Vec<R>;
    fn header(&self) -> &H;
}

// MyTable implements Table

trait Writer<H, R, T: Table<H, R>> {}

impl Writer<MyTableHeader, MyTableRow, MyTable> for WriterImpl {}

On an aside note, consider returning `&[R]` instead of `&Vec`. The caller will be no worse off because they couldn't mutate the vector through a shared reference to begin with, and using a slice avoids hard-coding how the data is stored in the table (other than it being stored contiguously). Also, a slice has one less level of indirection when accessing the data because it contains the pointer to data, whereas a reference to the vector first has to access the vector, and only then the data. — user4815162342, Aug 19 '20 at 12:31
^ That comment in more detail: [Why is it discouraged to accept a reference to a String (&String), Vec (&Vec), or Box (&Box) as a function argument?](https://stackoverflow.com/q/40006219/155423) — Shepmaster, Aug 19 '20 at 13:38

score 4 · Accepted Answer · answered Aug 19 '20 at 11:12

One way to solve your problem would be to move the H and R template parameters into the Table trait as associated types:

trait Table {
    type Header;
    type Row;

    fn rows(&self) -> &Vec<Self::Row>;
    fn header(&self) -> &Self::Header;
}

When implementing this trait, you specify which type to use as Header and Row.

You can then modify your Writer to accept only one template parameter:

trait Writer<T: Table> {}

How to deduce sub generic types from main generic type?

1 Answers1