collect_failable

A set of traits for collecting values into containers that must uphold invariants during construction or extension. These traits let you propagate structured errors instead of panicking or silently discarding data. Examples include preventing duplicate keys in a HashMap or respecting capacity limits in types like ArrayVec.

Traits

This crate provides several complementary traits for failable collection:

• TryFromIterator – failably build a container from an IntoIterator.
• TryCollectEx – failably collect an IntoIterator into a container.
• TryExtend and TryExtendSafe – failably extend a container with an IntoIterator, with different error guarantees.
• TryExtendOne – failable extend a container with a single item.
• TryUnzip – failably unzip an IntoIterator of pairs into two containers (requires feature tuple, enabled by default).

Additionally, several implementations are provided for common and popular containers. See the implementations section for more details.

Installation

It's on crates.io.

Features

This crate provides the following optional features:

• unsafe (default) – Enables TryFromIterator implementations for arrays using unsafe code.
• tuple (default) – Enables tuple extension (TryExtend for tuples) and TryUnzip trait, requiring a public dependency on the either crate (re-exported as collect_failable::Either).
• arrayvec – Enables TryFromIterator and TryExtend implementations for ArrayVec.
• hashbrown – Enables TryFromIterator and TryExtend implementations for hashbrown::HashMap and hashbrown::HashSet.
• indexmap – Enables TryFromIterator and TryExtend implementations for indexmap::IndexMap and indexmap::IndexSet.

Usage

TryFromIterator and TryCollectEx

Construct a container from an iterator, with errors for invalid input. This behaves like FromIterator but returns Result<Self, E> instead of panicking or ignoring failures.

use std::collections::HashMap;
use collect_failable::{TryFromIterator, TryCollectEx};

// can be called on any type that implements TryFromIterator
let err = HashMap::try_from_iter([(1, 2), (2, 3), (1, 4), (3, 5)]).expect_err("should be Err");
assert_eq!(err.item.0, 1); // err.item is the colliding (K, V) tuple

// For `HashMap` the error contains all the data necessary to reconstruct the consumed iterator
let all_items: Vec<_> = err.into_iter().collect();
assert_eq!(all_items.len(), 4); // all 4 original items are present, though order is not guaranteed

// or collected via the TryCollectEx trait a turbofish may be necessary to disambiguate
let map = [(1, 2), (2, 3)].into_iter().try_collect_ex::<HashMap<_, _>>().expect("should be Ok");
assert_eq!(map, HashMap::from([(1, 2), (2, 3)]));

// or type ascription. Note the Result type can be inferred, just not the collection type.
let map: HashMap<_, _> = [(1, 2), (2, 3)].into_iter().try_collect_ex().expect("should be Ok");
assert_eq!(map, HashMap::from([(1, 2), (2, 3)]));

TryExtend and TryExtendSafe

Extend an existing container with items that may violate its invariants. Two different trait exposes two styles of error behavior:

• TryExtendSafe – strong guarantee on an error, the container must remain unchanged.
• TryExtend – basic guarantee the container may have partially ingested items, but must remain valid.

Use TryExtendSafe if you must avoid mutation on failure; otherwise, prefer the faster TryExtend.

use std::collections::HashMap;
use collect_failable::TryExtendSafe;

let mut map = HashMap::new();
map.try_extend_safe([(1, 2), (2, 3)]).expect("should be Ok");
assert_eq!(map, HashMap::from([(1, 2), (2, 3)]));

// on a failure, the container is not modified
map.try_extend_safe([(1, 3)]).expect_err("should be Err");
assert_eq!(map, HashMap::from([(1, 2), (2, 3)]));

TryExtendOne

Extend a collection with a single item. This trait always provides a strong guarantee: on failure, the collection remains unchanged. Implemented as a seperate trait with no default implementation due to limitations imposed by the trait definition.

TryUnzip

Fallible equivalent of Iterator::unzip. Given an iterator of (A, B) items, produce two collections that implement Default + TryExtend, stopping on the first failure.

Allows unzipping an iterator of pairs into two collections that implement Default and TryExtend.

This is analogous to Iterator::unzip, except allows for failable construction.

use std::collections::{BTreeSet, HashSet};
use collect_failable::TryUnzip;

// Unzip into two different container types
let data = vec![(1, 'a'), (2, 'b'), (3, 'c')];
let (nums, chars): (BTreeSet<i32>, HashSet<char>) = data.into_iter().try_unzip().expect("should be ok");

assert_eq!(nums, BTreeSet::from([1, 2, 3]));
assert_eq!(chars, HashSet::from(['a', 'b', 'c']));

Implementations

Implementations for various containers are provided.

• HashMap, HashSet
• BTreeMap, BTreeSet
• Array (feature unsafe, enabled by default)
• ArrayVec (feature arrayvec)
• hashbrown::HashMap, hashbrown::HashSet (feature hashbrown)
• indexmap::IndexMap, indexmap::IndexSet (feature indexmap)

Tuple Implementations

Tuples of arity 2 implement TryExtend when their inner types do (requires feature tuple, enabled by default). For constructing tuple collections from IntoIterator TryUnzip is available.

Array Implementation

Arrays implement TryFromIterator for IntoIterator that yield exactly the right number of elements. This uses unsafe internally and is gated behind the unsafe feature (enabled by default).

Result Implementation

TryFromIterator is implemented for Result<C, E>, where C implements TryFromIterator<T>, similar to the FromIterator implementation for Result. This allows short-circuiting collection of failable values into a container whose construction is also failable.