Implement `be_a` matcher

src/spectator/dsl/matcher_dsl.cr

@@ -28,5 +28,20 @@ module Spectator::DSL
     macro be(expected)
       ::Spectator::Matchers::CaseMatcher.new({{expected.stringify}}, {{expected}})
     end
+
+    # Indicates that some value should be of a specified type.
+    # The `#is_a?` method is used for this check.
+    # A type name or type union should be used for `expected`.
+    #
+    # Examples:
+    # ```
+    # expect("foo").to be_a(String)
+    #
+    # x = Random.rand(2) == 0 ? "foobar" : 5
+    # expect(x).to be_a(Int32 | String)
+    # ```
+    macro be_a(expected)
+      ::Spectator::Matchers::TypeMatcher({{expected}}).new({{expected.stringify}}, nil)
+    end
   end
 end

src/spectator/matchers/type_matcher.cr

@@ -0,0 +1,25 @@
+require "./value_matcher"
+
+module Spectator::Matchers
+  # Matcher that tests a value is of a specified type.
+  # The values are compared with the `#is_a?` method.
+  struct TypeMatcher(Expected) < ValueMatcher(Nil)
+    # 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 : Expectations::ValueExpectationPartial(ActualType)) : Bool forall ActualType
+      partial.actual.is_a?(Expected)
+    end
+
+    # Describes the condition that satisfies the matcher.
+    # This is informational and displayed to the end-user.
+    def message(partial : Expectations::ValueExpectationPartial(ActualType)) : String forall ActualType
+      "Expected #{partial.label} to be a #{label}"
+    end
+
+    # Describes the condition that won't satsify the matcher.
+    # This is informational and displayed to the end-user.
+    def negated_message(partial : Expectations::ValueExpectationPartial(ActualType)) : String forall ActualType
+      "Expected #{partial.label} to not be a #{label}"
+    end
+  end
+end