Providing a unified write api for encoding several rust types

#1

Hi,

So as the title says, I have a small example here:

Code is also listed below:


use std::collections::BTreeMap;

enum MyType {
    Bool(bool),
    Int(i32),
    Str(String)
}

// A unified api which allows to encode any rust type
fn write<T: Into<MyType>>(val: T) {
    // Encoding goes here
}

impl Into<MyType> for bool {
    fn into(self) -> MyType {
        MyType::Bool(self)
    }
}

// error[E0210]: type parameter `T` must be used as the type parameter for some
// local type (e.g. `MyStruct<T>`); only traits defined in the current crate can be 
// implemented for a type parameter
impl<T: Into<MyType>> Into<MyType> for HashMap<String, T> {
    fn into(self) -> MyType {
        unimplemented!();
    }
}

impl Into<MyType> for String {
    fn into(self) -> MyType {
        MyType::Str(self)
    }
}

fn dummy_map() -> BTreeMap<String, String> {
    let mut m = BTreeMap::new();
    m.insert("".to_string(), "".to_string());
    m
}

fn main() {
    write(true);
    write(true);
    // Now i want to write a map
    let mut my_map = dummy_map();

    // my_map.write(my_map);    // This does not work
}

Compiler errors with:
error[E0210]: type parameter T must be used as the type parameter for some local type (e.g. MyStruct<T>); only traits defined in the current crate can be implemented for a type parameter

Sure i can create a wrapper type like: struct WrapperMap<T>(BTreeMap<String, T>); and ask users of my library to wrap their types in this wrapper, but that’s extra thing that library user has to do.

So i wanted to know if there are other ways where my write method can accept the BTreeMap with T as a value ?

#2

You shouldn’t be implementing Into. Implement From<BTreeMap…> for MyType instead.