Move more failure block chunks to their own structs

src/spectator/formatters/failure_block.cr

@@ -26,6 +26,7 @@ module Spectator::Formatters
     # Creates the block of text describing the failure.
     def to_s(io)
       inner_indent = integer_length(@index) + 2 # +2 for ) and space after number.
+
       indent do
         title(io)
         indent(inner_indent) do
@@ -42,7 +43,7 @@ module Spectator::Formatters
     # 1) Example name
     # ```
     private def title(io)
-      line(io, "#{@index}) #{@result.example}")
+      line(io, NumberedItem.new(@index, @result.example))
     end
 
     # Produces the message line of the failure block.
@@ -53,7 +54,7 @@ module Spectator::Formatters
     # The indentation of this line starts directly under
     # the example name from the title line.
     private def message(io)
-      line(io, Color.failure("Failure: #{@result.error}"))
+      line(io, FailureMessage.color(@result))
     end
 
     # Produces the values list of the failure block.

src/spectator/formatters/failure_message.cr

@@ -0,0 +1,35 @@
+module Spectator::Formatters
+  # Produces a stringified failure or error message.
+  private struct FailureMessage
+    # Creates the failure message.
+    def initialize(@result : FailedResult)
+    end
+
+    # Appends the message to the output.
+    def to_s(io)
+      io << @result.call(Label)
+      io << @result.error
+    end
+
+    # Creates a colorized version of the message.
+    def self.color(result)
+      result.call(Color) { |result| new(result) }
+    end
+
+    # Interface for `Result#call` to invoke.
+    # This is used to get the correct prefix for failures and errors.
+    private module Label
+      extend self
+
+      # Returns the prefix for a failure message.
+      def failure
+        "Failure: "
+      end
+
+      # Returns the prefix for an error message.
+      def error
+        "Error: "
+      end
+    end
+  end
+end

src/spectator/formatters/numbered_item.cr

@@ -0,0 +1,16 @@
+module Spectator::Formatters
+  # Produces a stringified value with a numerical prefix.
+  private struct NumberedItem(T)
+    # Creates the numbered item.
+    def initialize(@number : Int32, @text : T)
+    end
+
+    # Appends the numbered item to the output.
+    def to_s(io)
+      io << @number
+      io << ')'
+      io << ' '
+      io << @text
+    end
+  end
+end