Callin' a generic function with different type arguments based on runtime values

type_dispatch.rs

macro_rules! type_dispatch {
    ($function:ident,
     ($($arg:expr),*),
     $base_ty:ident,
     <$($param_ty:ty),*>,
     [ $value:expr => primitive_type ] $($rest:tt)*) => {
        match $value {
            "bool" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* bool>, $($rest)*),
            "uint8" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* u8>, $($rest)*),
            "uint16" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* u16>, $($rest)*),
            "uint32" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* u32>, $($rest)*),
            "uint64" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* u64>, $($rest)*),
            "uint128" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* U128>, $($rest)*),
            "uint256" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* U256>, $($rest)*),
            _ => { println!("unsupported: {}", $value); Ok(()) },
        }
    };
    ($function:ident,
     ($($arg:expr),*),
     $base_ty:ident,
     <$($param_ty:ty),*>,
     [ $value:expr => typenum ] $($rest:tt)*) => {
        match $value {
            // DO YOU LIKE NUMBERS?
            "0" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* U0>, $($rest)*),
            "1" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* U1>, $($rest)*),
            "2" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* U2>, $($rest)*),
            "3" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* U3>, $($rest)*),
            "4" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* U4>, $($rest)*),
            "5" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* U5>, $($rest)*),
            "6" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* U6>, $($rest)*),
            "7" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* U7>, $($rest)*),
            "8" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* U8>, $($rest)*),
            "16" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* U16>, $($rest)*),
            "32" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* U32>, $($rest)*),
            "64" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* U64>, $($rest)*),
            "128" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* U128>, $($rest)*),
            "256" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* U256>, $($rest)*),
            "512" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* U512>, $($rest)*),
            "1024" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* U1024>, $($rest)*),
            "2048" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* U2048>, $($rest)*),
            "4096" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* U4096>, $($rest)*),
            "8192" => type_dispatch!($function, ($($arg),*), $base_ty, <$($param_ty,)* U8192>, $($rest)*),
            _ => { println!("unsupported: {}", $value); Ok(()) },
        }
    };
    ($function:ident, ($($arg:expr),*), $base_type_name:ident, <$($param_ty:ty),*>,) => {
        $function::<$base_type_name<$($param_ty),*>>($($arg),*)
    }
}

// Example usage
fn main() {
  type_dispatch!(
    ssz_generic_test,
    (&self.path),
    FixedVector,
    <>,
    [elem_ty => primitive_type]
    [length => typenum]
  )?;
}