Add have_size matcher

src/spectator/dsl/matcher_dsl.cr

@@ -470,6 +470,16 @@ module Spectator::DSL
       ::Spectator::Matchers::ArrayMatcher.new({{expected}}, {{expected.stringify}})
     end
 
+    # Indicates that some set should have a specified size.
+    #
+    # Example:
+    # ```
+    # expect([1, 2, 3]).to have_size(3)
+    # ```
+    macro have_size(expected)
+      ::Spectator::Matchers::SizeMatcher.new({{expected}}, {{expected.stringify}})
+    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/size_matcher.cr

@@ -0,0 +1,43 @@
+require "./value_matcher"
+
+module Spectator::Matchers
+  # Matcher that tests whether a set has a specified number of elements.
+  # The set's `#size` method is used for this check.
+  struct SizeMatcher(ExpectedType) < ValueMatcher(ExpectedType)
+    # Determines whether the matcher is satisfied with the partial given to it.
+    # `MatchData` is returned that contains information about the match.
+    def match(partial)
+      actual = partial.actual.size
+      values = ExpectedActual.new(expected, label, actual, partial.label)
+      MatchData.new(actual == expected, values)
+    end
+
+    # Match data specific to this matcher.
+    private struct MatchData(ExpectedType, ActualType) < MatchData
+      # Creates the match data.
+      def initialize(matched, @values : ExpectedActual(ExpectedType, ActualType))
+        super(matched)
+      end
+
+      # Information about the match.
+      def named_tuple
+        {
+          expected: NegatableMatchDataValue.new(@values.expected),
+          actual:   @values.actual,
+        }
+      end
+
+      # Describes the condition that satisfies the matcher.
+      # This is informational and displayed to the end-user.
+      def message
+        "#{@values.actual_label} has #{@values.expected_label} elements"
+      end
+
+      # Describes the condition that won't satsify the matcher.
+      # This is informational and displayed to the end-user.
+      def negated_message
+        "#{@values.actual_label} does not have #{@values.expected_label} elements"
+      end
+    end
+  end
+end