Basic koans for the Enum module.

lib/koans/06_enum.ex

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+defmodule Enums do
+  use Koans
+
+  koan "can tell you how many elements there are" do
+    assert Enum.count([1,2,3,4]) == 4
+  end
+
+  koan "enumerable can tell you if all elements have a certain property" do
+    assert Enum.all?([1,2,3], fn element -> element < 5 end)
+
+    refute Enum.all?([1, 2, 3], fn element -> element > 5 end)
+  end
+
+  koan "can tell you if there is _some_ element in the list for which a property holds" do
+    assert Enum.any?([1,2,3], fn element -> rem(element, 2) == 0 end)
+
+     Enum.any?([1,2,3], fn element -> rem(element, 5) == 0 end)
+  end
+
+  koan "can tell if an element is in the enumerable" do
+    assert Enum.member?([1,2,3], 1)
+
+    refute Enum.member?([1,2,3], 30)
+  end
+
+  koan "converts elements by calling a function for each element" do
+    assert Enum.map([1,2,3], fn element -> element * 10 end) == [10,20,30]
+    assert Enum.map([1,2,3], fn element -> rem(element, 2) == 0 end) == [false, true, false]
+  end
+
+  koan "only keeps elements that match a certain criteria" do
+    assert Enum.filter([1,2,3], fn element -> rem(element, 2) == 1 end) == [1,3]
+  end
+
+  koan "removes elements that match a certain criteria" do
+    assert Enum.reject([1,2,3], fn element -> rem(element, 2) == 1 end) == [2]
+  end
+
+  koan "can take a number of elements" do
+    assert Enum.take([1,2,3,4,5], 3) == [1,2,3]
+    assert Enum.take([1,2,3,4,5], 10) == [1,2,3,4,5]
+  end
+
+  koan "can take as long as a property holds" do
+    assert Enum.take_while([1,2,3,4,5], fn element -> element < 4 end) == [1,2,3]
+  end
+
+  koan "can drop a number of elements" do
+    assert Enum.drop([-1,0,1,2,3], 2) == [1,2,3]
+  end
+
+  koan "can drop as long as a property holds" do
+    assert Enum.drop_while([-1,0,1,2,3], fn element -> element <= 0 end) == [1,2,3]
+  end
+
+  koan "can group elements by things they have in common" do
+    assert Enum.group_by([1,2,3,4,5,6], fn element -> rem(element, 3) end) == %{ 0 => [6, 3], 1 => [4, 1], 2 => [5, 2]}
+
+    odd_or_even = fn element -> if rem(element, 2) == 0 do :even else :odd end end
+
+    assert Enum.group_by([1,2,3,4], odd_or_even) == %{ :odd => [3,1], :even => [4,2] }
+  end
+
+  koan "can merge two lists by picking pairs" do
+    numbers = [1,2,3]
+    letters = [:a, :b, :c]
+
+    assert Enum.zip(numbers, letters) == [{1, :a}, {2, :b}, {3, :c}]
+
+    more_numbers = numbers ++ [4,5]
+    assert Enum.zip(more_numbers, letters) == [{1, :a}, {2, :b}, {3, :c}]
+  end
+
+  koan "can find elements by properties" do
+    assert Enum.find([1,2,3], fn element -> rem(element,2) == 0 end) == 2
+    assert Enum.find([1,2,3], fn element -> rem(element,5) == 0 end) == nil
+    assert Enum.find([1,2,3], :no_such_element, fn element -> rem(element,5) == 0 end) == :no_such_element
+  end
+
+  koan "can reduce stuff" do
+    assert Enum.reduce([1,2,3], 0, fn(element, accumulator) -> element + accumulator end) == 6
+
+    assert Enum.reduce(["1", "2", "3"], "", fn(element, accumulator) -> element <> accumulator end) == "321"
+  end
+end