take-some

A simple library that provides a way to obtain some value from various collections.

Sometimes one simply needs to "take" an element from a collection, no matter which element will it be. If you are that one, please feel free to use the crate that aims to make your (yes, yours!) life a tiny bit easier.

Now let's see how it works. Let's say you want to implement a hash map that is statically guaranteed by the rust's type system to be non-empy at all the times. The most straightforward way to do so is to declare it as a pair of an item and the rest of the items, like struct NonEmptyHashMap<K, V, S> (((K, V), ::std::collections::HashMap<K, V,S>)).

So far, so good. You will obviously want to create some nice user API for it, and that will include an instantiation of you type with a normal HashMap from the standard library. And here shines the take-some crate!

```rust extern crate take_some;

use std::collections::HashMap; use std::hash::{BuildHasher, Hash}; use take_some::TakeSome;

struct NonEmptyHashMap { first: (K, V), rest: HashMap, }

impl NonEmptyHashMap where K: Eq + Hash, S: BuildHasher, { fn fromstdhashmap(mut map: HashMap) -> Option { map.take_some().map(|first| NonEmptyHashMap { first, rest: map, }) }

// An alternative implementation that takes advantage of the `TakeSome::split_at_some`
// method.
fn from_std_hashmap2(map: HashMap<K, V, S>) -> Option<Self> {
    map.split_at_some().map(|(first, rest)| NonEmptyHashMap {first, rest}).ok()
}

}

fn main() { let map: HashMap = vec![("first".into(), "lol".into()), ("second".into(), "lul".into())] .intoiter().collect(); let nonemptymap = NonEmptyHashMap::fromstdhashmap(map); assert!(nonemptymap.issome());

let empty_map: HashMap<String, String> = HashMap::new();
let non_empty_map = NonEmptyHashMap::from_std_hashmap(empty_map);
// It is entirely impossible (hail to the type system!) to create an instance of the
// `NonEmptyHashMap` with an empty map!
assert!(non_empty_map.is_none());

} ```

And that's it! Yes, it is that simple.

If you'd like to implement the trait for your custom collections, have a look at the source code of the crate (btree_map.rs, hash_set.rs, vec.rs and so forth), or take a look at an example in the examples directory where we implement the trait for a "foreign" type.

License: MIT/Apache-2.0