# Strain

Implement the `keep` and `discard` operation on collections. Given a collection
and a predicate on the collection's elements, `keep` returns a new collection
containing those elements where the predicate is true, while `discard` returns
a new collection containing those elements where the predicate is false.

For example, given the collection of numbers:

- 1, 2, 3, 4, 5

And the predicate:

- is the number even?

Then your keep operation should produce:

- 2, 4

While your discard operation should produce:

- 1, 3, 5

Note that the union of keep and discard is all the elements.

The functions may be called `keep` and `discard`, or they may need different
names in order to not clash with existing functions or concepts in your
language.

## Restrictions

Keep your hands off that filter/reject/whatchamacallit functionality
provided by your standard library!  Solve this one yourself using other
basic tools instead.

## Running tests

In order to run the tests, issue the following command from the exercise
directory:

For running the tests provided, `rebar3` is used as it is the official build and
dependency management tool for erlang now. Please refer to [the tracks installation
instructions](http://exercism.io/languages/erlang/installation) on how to do that.

In order to run the tests, you can issue the following command from the exercise
directory.

```bash
$ rebar3 eunit
```

### Test versioning

Each problem defines a macro `TEST_VERSION` in the test file and
verifies that the solution defines and exports a function `test_version`
returning that same value.

To make tests pass, add the following to your solution:

```erlang
-export([test_version/0]).

test_version() ->
  1.
```

The benefit of this is that reviewers can see against which test version
an iteration was written if, for example, a previously posted solution
does not solve the current problem or passes current tests.

## Questions?

For detailed information about the Erlang track, please refer to the
[help page](http://exercism.io/languages/erlang) on the Exercism site.
This covers the basic information on setting up the development
environment expected by the exercises.

## Source

Conversation with James Edward Gray II [https://twitter.com/jeg2](https://twitter.com/jeg2)

## Submitting Incomplete Solutions
It's possible to submit an incomplete solution so you can see how others have completed the exercise.