Add less-than matcher

src/spectator/matchers/be_comparison.cr

@@ -10,8 +10,8 @@ module Spectator::Matchers
     # ```
     # expect(0).to be < 1
     # ```
-    def <(other : ExpectedType) forall ExpectedType
+    def <(expected : ExpectedType) forall ExpectedType
-      raise NotImplementedError.new("be <")
+      LessThanMatcher.new(expected)
     end
 
     # Creates a matcher that checks if a value is less than or equal to an expected value.

src/spectator/matchers/less_than_matcher.cr

@@ -0,0 +1,25 @@
+require "./value_matcher"
+
+module Spectator::Matchers
+  # Matcher that tests whether one value is less than another.
+  # The values are compared with the `<` operator.
+  struct LessThanMatcher(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 : Expectations::ValueExpectationPartial(ActualType)) : Bool forall ActualType
+      partial.actual < 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 less than #{label} (using <)"
+    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 less than #{label} (using <)"
+    end
+  end
+end