diff --git a/src/formatter.cr b/src/formatter.cr
index b9f8381..3203078 100644
--- a/src/formatter.cr
+++ b/src/formatter.cr
@@ -12,7 +12,11 @@ module Tartrazine
     property theme : Theme = Tartrazine.theme("default-dark")
 
     # Format the text using the given lexer.
-    def format(text : String, lexer : Lexer, io : IO? = nil) : String?
+    def format(text : String, lexer : Lexer, io : IO = nil) : Nil
+      raise Exception.new("Not implemented")
+    end
+
+    def format(text : String, lexer : Lexer) : String
       raise Exception.new("Not implemented")
     end
 
diff --git a/src/formatters/ansi.cr b/src/formatters/ansi.cr
index 19d72f3..8efeb25 100644
--- a/src/formatters/ansi.cr
+++ b/src/formatters/ansi.cr
@@ -11,8 +11,13 @@ module Tartrazine
       "#{i + 1}".rjust(4).ljust(5)
     end
 
-    def format(text : String, lexer : Lexer, io : IO? = nil) : String?
-      outp = io.nil? ? String::Builder.new("") : io
+    def format(text : String, lexer : Lexer) : String
+      outp = String::Builder.new("")
+      format(text, lexer, outp)
+      return outp.to_s
+    end
+
+    def format(text : String, lexer : Lexer, outp : IO) : Nil
       tokenizer = Tokenizer.new(lexer, text)
       i = 0
       outp << line_label(i) if line_numbers?
@@ -23,7 +28,6 @@ module Tartrazine
           outp << line_label(i) if line_numbers?
         end
       end
-      return outp.to_s if io.nil?
     end
 
     def colorize(text : String, token : String) : String
diff --git a/src/formatters/html.cr b/src/formatters/html.cr
index 3938820..4217cdd 100644
--- a/src/formatters/html.cr
+++ b/src/formatters/html.cr
@@ -34,13 +34,18 @@ module Tartrazine
                    @weight_of_bold : Int32 = 600)
     end
 
-    def format(text : String, lexer : Lexer, io : IO? = nil) : String?
-      outp = io.nil? ? String::Builder.new("") : io
+    def format(text : String, lexer : Lexer) : String
+      outp = String::Builder.new("")
+      format(text, lexer, outp)
+      return outp.to_s
+    end
+
+
+    def format(text : String, lexer : Lexer, io : IO) : Nil
       pre, post = wrap_standalone
-      outp << pre if standalone?
-      format_text(text, lexer, outp)
-      outp << post if standalone?
-      return outp.to_s if io.nil?
+      io << pre if standalone?
+      format_text(text, lexer, io)
+      io << post if standalone?
     end
 
     # Wrap text into a full HTML document, including the CSS for the theme
diff --git a/src/formatters/json.cr b/src/formatters/json.cr
index 39271c8..c9d6a39 100644
--- a/src/formatters/json.cr
+++ b/src/formatters/json.cr
@@ -4,11 +4,16 @@ module Tartrazine
   class Json < Formatter
     property name = "json"
 
-    def format(text : String, lexer : Lexer, io : IO? = nil) : String?
-      outp = io.nil? ? String::Builder.new("") : io
+    def format(text : String, lexer : Lexer) : String
+      outp = String::Builder.new("")
+      format(text, lexer, outp)
+      return outp.to_s
+    end
+
+
+    def format(text : String, lexer : Lexer, io : IO) : Nil
       tokenizer = Tokenizer.new(lexer, text)
-      outp << Tartrazine::Lexer.collapse_tokens(tokenizer.to_a).to_json
-      return outp.to_s if io.nil?
+      io << Tartrazine::Lexer.collapse_tokens(tokenizer.to_a).to_json
     end
   end
 end