Crate fallible_iterator

“Fallible” iterators.

The iterator APIs in the Rust standard library do not support iteration that can fail in a first class manner. These iterators are typically modeled as iterating over Result<T, E> values; for example, the Lines iterator returns io::Result<String>s. When simply iterating over these types, the value being iterated over must be unwrapped in some way before it can be used:

for line in reader.lines() {
    let line = line?;
    // work with line
}

In addition, many of the additional methods on the Iterator trait will not behave properly in the presence of errors when working with these kinds of iterators. For example, if one wanted to count the number of lines of text in a Reader, this might be a way to go about it:

let count = reader.lines().count();

This will return the proper value when the reader operates successfully, but if it encounters an IO error, the result will either be slightly higher than expected if the error is transient, or it may run forever if the error is returned repeatedly!

In contrast, a fallible iterator is built around the concept that a call to next can fail. The trait has an additional Error associated type in addition to the Item type, and next returns Result<Option<Self::Item>, Self::Error> rather than Option<Self::Item>. Methods like count return Results as well.

This does mean that fallible iterators are incompatible with Rust’s for loop syntax, but while let loops offer a similar level of ergonomics:

while let Some(item) = iter.next()? {
    // work with item
}

Fallible closure arguments

Like Iterator, many FallibleIterator methods take closures as arguments. These use the same signatures as their Iterator counterparts, except that FallibleIterator expects the closures to be fallible: they return Result<T, Self::Error> instead of simply T.

For example, the standard library’s Iterator::filter adapter method filters the underlying iterator according to a predicate provided by the user, whose return type is bool. In FallibleIterator::filter, however, the predicate returns Result<bool, Self::Error>:

let numbers = convert("100\n200\nfern\n400".lines().map(Ok::<&str, Box<Error>>));
let big_numbers = numbers.filter(|n| Ok(u64::from_str(n)? > 100));
assert!(big_numbers.count().is_err());

Structs

Chain

An iterator which yields the elements of one iterator followed by another.

Cloned

An iterator which clones the elements of the underlying iterator.

Convert

A fallible iterator that wraps a normal iterator over Results.

Cycle

An iterator which cycles another endlessly.

Enumerate

An iterator that yields the iteration count as well as the values of the underlying iterator.

Filter

An iterator which uses a fallible predicate to determine which values of the underlying iterator should be yielded.

FilterMap

An iterator which both filters and maps the values of the underlying iterator.

FlatMap

An iterator which maps each element to another iterator, yielding those iterator’s elements.

Flatten

An iterator which flattens an iterator of iterators, yielding those iterators’ elements.

Fuse

An iterator that yields Ok(None) forever after the underlying iterator yields Ok(None) once.

Inspect

An iterator which passes each element to a closure before returning it.

Iterator

A normal (non-fallible) iterator which wraps a fallible iterator.

Map

An iterator which applies a fallible transform to the elements of the underlying iterator.

MapErr

An iterator which applies a transform to the errors of the underlying iterator.

Peekable

An iterator which can look at the next element without consuming it.

Rev

An iterator which yields elements of the underlying iterator in reverse order.

Scan

An iterator which applies a stateful closure.

Skip

An iterator which skips initial elements.

SkipWhile

An iterator which skips initial elements based on a predicate.

StepBy

An iterator which steps through the elements of the underlying iterator by a certain amount.

Take

An iterator which yields a limited number of elements from the underlying iterator.

TakeWhile

An iterator which yields elements based on a predicate.

Zip

An iterator that yields pairs of this iterator’s and another iterator’s values.

Traits

DoubleEndedFallibleIterator

A fallible iterator able to yield elements from both ends.

FallibleIterator

An Iterator-like trait that allows for calculation of items to fail.

FromFallibleIterator

Conversion from a fallible iterator.

IntoFallibleIterator

Conversion into a FallibleIterator.

Functions

convert

Converts an Iterator<Item = Result<T, E>> into a FallibleIterator<Item = T, Error = E>.