Merge two HashMaps in Rust

rust

Solution

A more up to date answer from this tweet:

use std::collections::HashMap;

// Mutating one map
fn merge1(map1: &mut HashMap<(), ()>, map2: HashMap<(), ()>) {
    map1.extend(map2);
}

// Without mutation
fn merge2(map1: HashMap<(), ()>, map2: HashMap<(), ()>) -> HashMap<(), ()> {
    map1.into_iter().chain(map2).collect()
}

// If you only have a reference to the map to be merged in
fn merge_from_ref(map: &mut HashMap<(), ()>, map_ref: &HashMap<(), ()>) {
    map.extend(map_ref.into_iter().map(|(k, v)| (k.clone(), v.clone())));
}

Rust Playground Link

Problem

So I'm a bit stuck, trying to merge two HashMaps. It's easy to do it inline: ``` fn inline() { let mut first_context = HashMap::new(); first_context.insert("Hello", "World"); let mut second_context = HashMap::new(); second_context.insert("Hey", "There"); let mut new_context = HashMap::new(); for (key, value) in first_context.iter() { new_context.insert(*key, *value); } for (key, value) in second_context.iter() { new_context.insert(*key, *value); } println!("Inline:\t\t{}", new_context); println!("Inline:\t\t{}\t{} [Initial Maps Still Usable]", first_context, second_context); } ``` It's easy enough to make a function: ``` fn abstracted() { fn merge<'a>(first_context: &HashMap<&'a str, &'a str>, second_context: &HashMap<&'a str, &'a str>) -> HashMap<&'a str, &'a str> { let mut new_context = HashMap::new(); for (key, value) in first_context.iter() { new_context.insert(*key, *value); } for (key, value) in second_context.iter() { new_context.insert(*key, *value); } new_context } let mut first_context = HashMap::new(); first_context.insert("Hello", "World"); let mut second_context = HashMap::new(); second_context.insert("Hey", "There"); println!("Abstracted:\t{}", merge(&first_context, &second_context)); println!("Abstracted:\t{}\t{} [Initial Maps Still Usable]", first_context, second_context); } ``` However, I can't seem to get the generic version to work: ``` fn generic() { fn merge<'a, K: Hash + Eq, V>(first_context: &HashMap<&'a K, &'a V>, second_context: &HashMap<&'a K, &'a V>) -> HashMap<&'a K, &'a V> { let mut new_context = HashMap::new(); for (key, value) in first_context.iter() { new_context.insert(*key, *value); } for (key, value) in second_context.iter() { new_context.insert(*key, *value); } new_context } let mut first_context = HashMap::new(); first_context.insert("Hello", "World"); let mut second_context = HashMap::new(); second_context.insert("Hey", "There"); println!("Generic:\t{}", merge(&first_context, &second_context)); println!("Generic:\t{}\t{} [Initial Maps Still Usable]", first_context, second_context); } ``` The above code on play.rust-lang.org. Compiling it: ``` error: the trait `core::kinds::Sized` is not implemented for the type `str` ``` I get that the compiler is confused about the size of the generic value, but I'm not sure why "str" doesn't have a strict memory size? I know its a String slice and not a type, but still this should work, no? Is this a bug? I thought this would be a relatively trivial function. If someone has a good solution, I'd love to learn. Actually ideally, I'd love to see a solution with a `trait Mergeable` and write a decorator for HashMap<&K, &V>, such that I can call `let new_context = first_context.merge(&second_context);` but this can be a different question.

Original source