Add `have_key` matcher

README.md

@@ -114,7 +114,7 @@ In no particular order, features that have been implemented and are planned:
     - [X] Equality matchers - `eq`, `ne`, `be ==`, `be !=`
     - [X] Comparison matchers - `be <`, `be <=`, `be >`, `be >=`, `be_within[.of]`, `be_close`
     - [X] Type matchers - `be_a`
-    - [ ] Collection matchers - `contain`, `have`, `contain_exactly[.in_order|.in_any_order]`, `match_array[.in_order|.in_any_order]`, `start_with`, `end_with`, `be_empty`, `has_key`, `has_value`, `all`, `all_satisfy`
+    - [ ] Collection matchers - `contain`, `have`, `contain_exactly[.in_order|.in_any_order]`, `match_array[.in_order|.in_any_order]`, `start_with`, `end_with`, `be_empty`, `have_key`, `have_value`, `all`, `all_satisfy`
     - [X] Truthy matchers - `be`, `be_true`, `be_truthy`, `be_false`, `be_falsey`, `be_nil`
     - [ ] Error matchers - `raise_error`
     - [ ] Yield matchers - `yield_control[.times]`, `yield_with_args[.times]`, `yield_with_no_args[.times]`, `yield_successive_args`

src/spectator/dsl/matcher_dsl.cr

@@ -395,6 +395,23 @@ module Spectator::DSL
       ::Spectator::Matchers::HaveMatcher.new({{expected.splat.stringify}}, {{expected}})
     end
 
+    # Indicates that some set, such as a `Hash`, has a given key.
+    # The `has_key?` method is used for this check.
+    #
+    # Examples:
+    # ```
+    # expect({foo: "bar"}).to have_key(:foo)
+    # expect({"lucky" => 7}).to have_key("lucky")
+    # ```
+    macro have_key(expected)
+      ::Spectator::Matchers::HasKeyMatcher.new({{expected.stringify}}, {{expected}})
+    end
+
+    # ditto
+    macro has_key(expected)
+      have_key({{expected}})
+    end
+
     # Indicates that some value should have a set of attributes matching some conditions.
     # A list of named arguments are expected.
     # The names correspond to the attributes in the instance to check.

src/spectator/matchers/have_key_matcher.cr

@@ -0,0 +1,25 @@
+require "./value_matcher"
+
+module Spectator::Matchers
+  # Matcher that tests whether a `Hash` (or similar type) has a given key.
+  # The set is checked with the `has_key?` method.
+  struct HaveKeyMatcher(ExpectedType) < ValueMatcher(ExpectedType)
+    # Determines whether the matcher is satisfied with the value given to it.
+    # True is returned if the match was successful, false otherwise.
+    def match?(partial)
+      partial.actual.has_key?(expected)
+    end
+
+    # Describes the condition that satisfies the matcher.
+    # This is informational and displayed to the end-user.
+    def message(partial)
+      "Expected #{partial.label} to have key #{label}"
+    end
+
+    # Describes the condition that won't satsify the matcher.
+    # This is informational and displayed to the end-user.
+    def negated_message(partial)
+      "Expected #{partial.label} to not have key #{label}"
+    end
+  end
+end