strain

strain/.exercism/config.json

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+{
+  "blurb": "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.",
+  "authors": [
+    "petehuang"
+  ],
+  "contributors": [
+    "angelikatyborska",
+    "Cohen-Carlisle",
+    "devonestes",
+    "DoggettCK",
+    "fxn",
+    "kytrinyx",
+    "neenjaw",
+    "sotojuan"
+  ],
+  "files": {
+    "solution": [
+      "lib/strain.ex"
+    ],
+    "test": [
+      "test/strain_test.exs"
+    ],
+    "example": [
+      ".meta/example.ex"
+    ]
+  },
+  "source": "Conversation with James Edward Gray II",
+  "source_url": "https://twitter.com/jeg2"
+}

strain/.exercism/metadata.json

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+{"track":"elixir","exercise":"strain","id":"b3af0b1256ea40e1a4721bfcd7a862c5","url":"https://exercism.org/tracks/elixir/exercises/strain","handle":"rjNemo","is_requester":true,"auto_approve":false}

strain/.formatter.exs

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+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]

strain/.gitignore

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+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where third-party dependencies like ExDoc output generated docs.
+/doc/
+
+# Ignore .fetch files in case you like to edit your project deps locally.
+/.fetch
+
+# If the VM crashes, it generates a dump, let's ignore it too.
+erl_crash.dump
+
+# Also ignore archive artifacts (built via "mix archive.build").
+*.ez
+
+# Ignore package tarball (built via "mix hex.build").
+strain-*.tar
+

strain/HELP.md

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+# Help
+
+## Running the tests
+
+From the terminal, change to the base directory of the exercise then execute the tests with:
+
+```bash
+$ mix test
+```
+
+This will execute the test file found in the `test` subfolder -- a file ending in `_test.exs`
+
+Documentation:
+
+* [`mix test` - Elixir's test execution tool](https://hexdocs.pm/mix/Mix.Tasks.Test.html)
+* [`ExUnit` - Elixir's unit test library](https://hexdocs.pm/ex_unit/ExUnit.html)
+
+## Pending tests
+
+In test suites of practice exercises, all but the first test have been tagged to be skipped.
+
+Once you get a test passing, you can unskip the next one by commenting out the relevant `@tag :pending` with a `#` symbol.
+
+For example:
+
+```elixir
+# @tag :pending
+test "shouting" do
+  assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"
+end
+```
+
+If you wish to run all tests at once, you can include all skipped test by using the `--include` flag on the `mix test` command:
+
+```bash
+$ mix test --include pending
+```
+
+Or, you can enable all the tests by commenting out the `ExUnit.configure` line in the file `test/test_helper.exs`.
+
+```elixir
+# ExUnit.configure(exclude: :pending, trace: true)
+```
+
+## Useful `mix test` options
+
+* `test/<FILE>.exs:LINENUM` - runs only a single test, the test from `<FILE>.exs` whose definition is on line `LINENUM`
+* `--failed` - runs only tests that failed the last time they ran
+* `--max-failures` - the suite stops evaluating tests when this number of test failures
+is reached
+* `--seed 0` - disables randomization so the tests in a single file will always be ran
+in the same order they were defined in
+
+## Submitting your solution
+
+You can submit your solution using the `exercism submit lib/strain.ex` command.
+This command will upload your solution to the Exercism website and print the solution page's URL.
+
+It's possible to submit an incomplete solution which allows you to:
+
+- See how others have completed the exercise
+- Request help from a mentor
+
+## Need to get help?
+
+If you'd like help solving the exercise, check the following pages:
+
+- The [Elixir track's documentation](https://exercism.org/docs/tracks/elixir)
+- [Exercism's support channel on gitter](https://gitter.im/exercism/support)
+- The [Frequently Asked Questions](https://exercism.org/docs/using/faqs)
+
+Should those resources not suffice, you could submit your (incomplete) solution to request mentoring.
+
+If you're stuck on something, it may help to look at some of the [available resources](https://exercism.org/docs/tracks/elixir/resources) out there where answers might be found.
+If you can't find what you're looking for in the documentation, feel free to ask help in the Exercism's BEAM [gitter channel](https://gitter.im/exercism/xerlang).

strain/HINTS.md

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+# Hints
+
+- `apply` will let you pass arguments to a function, as will `fun.(args)`.

strain/README.md

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+# Strain
+
+Welcome to Strain on Exercism's Elixir Track.
+If you need help running the tests or submitting your code, check out `HELP.md`.
+If you get stuck on the exercise, check out `HINTS.md`, but try and solve it without using those first :)
+
+## Instructions
+
+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.
+
+## Source
+
+### Created by
+
+- @petehuang
+
+### Contributed to by
+
+- @angelikatyborska
+- @Cohen-Carlisle
+- @devonestes
+- @DoggettCK
+- @fxn
+- @kytrinyx
+- @neenjaw
+- @sotojuan
+
+### Based on
+
+Conversation with James Edward Gray II - https://twitter.com/jeg2

strain/lib/strain.ex

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+defmodule Strain do
+  @doc """
+  Given a `list` of items and a function `fun`, return the list of items where
+  `fun` returns true.
+
+  Do not use `Enum.filter`.
+  """
+  @spec keep(list :: list(any), fun :: (any -> boolean)) :: list(any)
+  def keep([], _), do: []
+
+  def keep([h | tail], fun) do
+    if fun.(h) == true do
+      [h | keep(tail, fun)]
+    else
+      keep(tail, fun)
+    end
+  end
+
+  @doc """
+  Given a `list` of items and a function `fun`, return the list of items where
+  `fun` returns false.
+
+  Do not use `Enum.reject`.
+  """
+  @spec discard(list :: list(any), fun :: (any -> boolean)) :: list(any)
+  def discard(list, fun), do: keep(list, &(!fun.(&1)))
+end

strain/mix.exs

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+defmodule Strain.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :strain,
+      version: "0.1.0",
+      # elixir: "~> 1.8",
+      start_permanent: Mix.env() == :prod,
+      deps: deps()
+    ]
+  end
+
+  # Run "mix help compile.app" to learn about applications.
+  def application do
+    [
+      extra_applications: [:logger]
+    ]
+  end
+
+  # Run "mix help deps" to learn about dependencies.
+  defp deps do
+    [
+      # {:dep_from_hexpm, "~> 0.3.0"},
+      # {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
+    ]
+  end
+end

strain/test/strain_test.exs

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+defmodule StrainTest do
+  use ExUnit.Case
+
+  defp is_odd?(n), do: rem(n, 2) == 1
+  defp is_even?(n), do: rem(n, 2) == 0
+  defp noop(_), do: true
+
+  # @tag :pending
+  test "empty keep" do
+    assert Strain.keep([], &noop/1) == []
+  end
+
+  test "keep everything" do
+    assert Strain.keep([1, 2, 3], fn e -> e < 10 end) == [1, 2, 3]
+  end
+
+  test "keep first and last" do
+    assert Strain.keep([1, 2, 3], &is_odd?/1) == [1, 3]
+  end
+
+  test "keep neither first nor last" do
+    assert Strain.keep([1, 2, 3, 4, 5], &is_even?/1) == [2, 4]
+  end
+
+  test "keep strings" do
+    words = ~w(apple zebra banana zombies cherimoya zelot)
+    assert Strain.keep(words, &String.starts_with?(&1, "z")) == ~w(zebra zombies zelot)
+  end
+
+  test "keep arrays" do
+    rows = [
+      [1, 2, 3],
+      [5, 5, 5],
+      [5, 1, 2],
+      [2, 1, 2],
+      [1, 5, 2],
+      [2, 2, 1],
+      [1, 2, 5]
+    ]
+
+    assert Strain.keep(rows, fn row -> 5 in row end) == [
+             [5, 5, 5],
+             [5, 1, 2],
+             [1, 5, 2],
+             [1, 2, 5]
+           ]
+  end
+
+  test "empty discard" do
+    assert Strain.discard([], &noop/1) == []
+  end
+
+  test "discard nothing" do
+    assert Strain.discard([1, 2, 3], fn e -> e > 10 end) == [1, 2, 3]
+  end
+
+  test "discard first and last" do
+    assert Strain.discard([1, 2, 3], &is_odd?/1) == [2]
+  end
+
+  test "discard neither first nor last" do
+    assert Strain.discard([1, 2, 3, 4, 5], &is_even?/1) == [1, 3, 5]
+  end
+
+  test "discard strings" do
+    words = ~w(apple zebra banana zombies cherimoya zelot)
+    assert Strain.discard(words, &String.starts_with?(&1, "z")) == ~w(apple banana cherimoya)
+  end
+
+  test "discard arrays" do
+    rows = [
+      [1, 2, 3],
+      [5, 5, 5],
+      [5, 1, 2],
+      [2, 1, 2],
+      [1, 5, 2],
+      [2, 2, 1],
+      [1, 2, 5]
+    ]
+
+    assert Strain.discard(rows, fn row -> 5 in row end) == [[1, 2, 3], [2, 1, 2], [2, 2, 1]]
+  end
+end

strain/test/test_helper.exs

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+ExUnit.start()
+ExUnit.configure(exclude: :pending, trace: true)