Code shared from the Rust Playground

playground.rs

/// Demonstration of compile-time key-value type safety.
///
/// This program demonstrates how you use Rust generics to encode type
/// information into the keys for a key-value store. This allows for
/// type-safe access to data in the store, preventing things like writing
/// strings to keys that should only contain integers. Attempting to
/// do so results in a compile-time error.
///
/// The example code is at the top of this file, the implementation details
/// are below the example.
///
use std::marker::PhantomData;

pub const AUTH: TypedKey<&[u8]> = TypedKey("auth", PhantomData);
pub const HOST: TypedKey<&str> = TypedKey("host", PhantomData);
pub const PORT: TypedKey<u16> = TypedKey("port", PhantomData);

fn main() {
    let mut store = KeyValueStore;

    // Setter example.
    store.set_key_value(HOST, "example.com");
    store.set_key_value(PORT, 1234);
    store.set_key_value(AUTH, b"password");

    // Getter example.
    let value: &str = store.get_key_value(HOST).unwrap();
    println!("host -> {:?}", value);
    let value: u16 = store.get_key_value(PORT).unwrap();
    println!("port -> {:?}", value);
    let value: &[u8] = store.get_key_value(AUTH).unwrap();
    println!("auth -> {:?}", value);

    // The lines below this point do not compile,
    // because the type does not match the key.
    //store.set_key_value(HOST, b"example.com");
    //store.set_key_value(PORT, 91234);
    //store.set_key_value(AUTH, "password");
    //let value: u16 = store.get_key_value(HOST).unwrap();
    //let value: () = store.get_key_value(PORT).unwrap();
    //let value: &str = store.get_key_value(AUTH).unwrap();
}

///////////////////////////////////////////////////////////////////////////////
// IMPLEMENTATION DETAILS /////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////////////

use std::convert::{From, Into, TryFrom, TryInto};

/// Key class for our key-value store that also encodes value type information.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct TypedKey<T: 'static + ?Sized>(&'static str, PhantomData<&'static T>);

/// The underlying value format of our key-value store. This could also be a
/// JSON datastore, or an opaque binary blob of some sort.
#[derive(Debug)]
pub enum CheckedValue<'a> {
    Empty,
    String(&'a str),
    Integer(u32),
    Bytes(&'a [u8]),
}

// Every type that is used with a `TypeKey` that will be "set" needs to have an
// implementation of `std::convert::From` to convert the native value into the
// underlying value format:

impl<'a> From<u16> for CheckedValue<'a> {
    fn from(value: u16) -> Self {
        CheckedValue::Integer(value as u32)
    }
}

impl<'a> From<&'a str> for CheckedValue<'a> {
    fn from(value: &'a str) -> Self {
        CheckedValue::String(value)
    }
}

impl<'a> From<&'a [u8]> for CheckedValue<'a> {
    fn from(value: &'a [u8]) -> Self {
        CheckedValue::Bytes(value)
    }
}

impl<'a> From<()> for CheckedValue<'a> {
    fn from(_: ()) -> Self {
        CheckedValue::Empty
    }
}

// Every type that is used with a `TypeKey` that will be retrieved needs to
// have an implementation of `std::convert::TryFrom` to convert from the
// underlying format to the native value:

impl<'a> TryFrom<CheckedValue<'a>> for u16 {
    type Error = ();

    fn try_from(value: CheckedValue<'a>) -> Result<Self, Self::Error> {
        match value {
            CheckedValue::Integer(x) => Ok(x as u16),
            _ => Err(()),
        }
    }
}

impl<'a> TryFrom<CheckedValue<'a>> for &'a str {
    type Error = ();

    fn try_from(value: CheckedValue<'a>) -> Result<Self, Self::Error> {
        match value {
            CheckedValue::String(x) => Ok(x),
            _ => Err(()),
        }
    }
}

impl<'a> TryFrom<CheckedValue<'a>> for &'a [u8] {
    type Error = ();

    fn try_from(value: CheckedValue<'a>) -> Result<Self, Self::Error> {
        match value {
            CheckedValue::Bytes(x) => Ok(x),
            _ => Err(()),
        }
    }
}

impl<'a> TryFrom<CheckedValue<'a>> for () {
    type Error = ();

    fn try_from(value: CheckedValue<'a>) -> Result<Self, Self::Error> {
        match value {
            CheckedValue::Empty => Ok(()),
            _ => Err(()),
        }
    }
}

/// Our very dumb (but type-safe) key/value store.
struct KeyValueStore;
impl KeyValueStore {
    fn set_key_value<'a, T>(&mut self, key: TypedKey<T>, value: T)
    where
        T: Into<CheckedValue<'a>>,
    {
        match value.into() {
            CheckedValue::Empty => println!("{} <- (empty)", key.0),
            CheckedValue::Integer(x) => println!("{} <- {:?}", key.0, x),
            CheckedValue::String(x) => println!("{} <- {:?}", key.0, x),
            CheckedValue::Bytes(x) => println!("{} <- {:?}", key.0, x),
        }
    }

    fn get_key_value_raw(&mut self, key: &str) -> Result<CheckedValue<'static>, ()> {
        match key {
            "host" => Ok(CheckedValue::String("example.com")),
            "port" => Ok(CheckedValue::Integer(1234)),
            "auth" => Ok(CheckedValue::Bytes(b"password")),
            _ => Err(()),
        }
    }

    fn get_key_value<T>(&mut self, key: TypedKey<T>) -> Result<T, ()>
    where
        T: TryFrom<CheckedValue<'static>, Error = ()>,
    {
        self.get_key_value_raw(key.0)?.try_into()
    }
}