diff --git a/spec/ameba/formatter/todo_formatter_spec.cr b/spec/ameba/formatter/todo_formatter_spec.cr
index cc7dd32c2..448cd10ad 100644
--- a/spec/ameba/formatter/todo_formatter_spec.cr
+++ b/spec/ameba/formatter/todo_formatter_spec.cr
@@ -13,10 +13,12 @@ module Ameba
 
   private def create_todo
     with_formatter do |formatter|
-      s = Source.new "a = 1", "source.cr"
-      s.add_issue DummyRule.new, {1, 2}, "message"
-      file = formatter.finished([s])
-      file ? File.read(file.path) : ""
+      source = Source.new "a = 1", "source.cr"
+      source.add_issue DummyRule.new, {1, 2}, "message"
+
+      formatter.finished([source])
+
+      File.exists?(CONFIG_PATH) ? File.read(CONFIG_PATH) : ""
     end
   end
 
@@ -97,8 +99,9 @@ module Ameba
             s1.add_issue DummyRule.new, {2, 2}, "message1"
             s2.add_issue DummyRule.new, {2, 2}, "message2"
 
-            file = formatter.finished([s1, s2]).should_not be_nil
-            content = File.read(file.path)
+            formatter.finished([s1, s2])
+
+            content = File.read(CONFIG_PATH)
             content.should contain <<-CONTENT
               # Problems found: 3
               # Run `ameba --only Ameba/DummyRule` for details
diff --git a/src/ameba/cli/cmd.cr b/src/ameba/cli/cmd.cr
index f87030785..3d6040d10 100644
--- a/src/ameba/cli/cmd.cr
+++ b/src/ameba/cli/cmd.cr
@@ -5,7 +5,7 @@ require "option_parser"
 module Ameba::Cli
   extend self
 
-  def run(args = ARGV)
+  def run(args = ARGV) : Nil
     opts = parse_args args
     location_to_explain = opts.location_to_explain
     autocorrect = opts.autocorrect?
@@ -151,7 +151,7 @@ module Ameba::Cli
     opts
   end
 
-  private def configure_rules(config, opts)
+  private def configure_rules(config, opts) : Nil
     case
     when only = opts.only
       config.rules.each(&.enabled = false)
@@ -162,7 +162,7 @@ module Ameba::Cli
     config.update_rules(opts.except, enabled: false)
   end
 
-  private def configure_formatter(config, opts)
+  private def configure_formatter(config, opts) : Nil
     if name = opts.formatter
       config.formatter = name
     end
@@ -171,12 +171,12 @@ module Ameba::Cli
       opts.without_affected_code?
   end
 
-  private def configure_describe_opts(rule_name, opts)
+  private def configure_describe_opts(rule_name, opts) : Nil
     opts.describe_rule = rule_name.presence
     opts.formatter = :silent
   end
 
-  private def configure_explain_opts(loc, opts)
+  private def configure_explain_opts(loc, opts) : Nil
     location_to_explain = parse_explain_location(loc)
     opts.location_to_explain = location_to_explain
     opts.globs = [location_to_explain[:file]]
diff --git a/src/ameba/formatter/base_formatter.cr b/src/ameba/formatter/base_formatter.cr
index 43364e07d..dc7b65287 100644
--- a/src/ameba/formatter/base_formatter.cr
+++ b/src/ameba/formatter/base_formatter.cr
@@ -15,22 +15,22 @@ module Ameba::Formatter
 
     # Callback that indicates when inspecting is started.
     # A list of sources to inspect is passed as an argument.
-    def started(sources); end
+    def started(sources) : Nil; end
 
     # Callback that indicates when source inspection is started.
     # A corresponding source is passed as an argument.
     #
     # WARNING: This method needs to be MT safe
-    def source_started(source : Source); end
+    def source_started(source : Source) : Nil; end
 
     # Callback that indicates when source inspection is finished.
     # A corresponding source is passed as an argument.
     #
     # WARNING: This method needs to be MT safe
-    def source_finished(source : Source); end
+    def source_finished(source : Source) : Nil; end
 
     # Callback that indicates when inspection is finished.
     # A list of inspected sources is passed as an argument.
-    def finished(sources); end
+    def finished(sources) : Nil; end
   end
 end
diff --git a/src/ameba/formatter/disabled_formatter.cr b/src/ameba/formatter/disabled_formatter.cr
index ecab4aa74..ef14da542 100644
--- a/src/ameba/formatter/disabled_formatter.cr
+++ b/src/ameba/formatter/disabled_formatter.cr
@@ -1,7 +1,7 @@
 module Ameba::Formatter
   # A formatter that shows all disabled lines by inline directives.
   class DisabledFormatter < BaseFormatter
-    def finished(sources)
+    def finished(sources) : Nil
       output << "Disabled rules using inline directives:\n\n"
 
       sources.each do |source|
diff --git a/src/ameba/formatter/dot_formatter.cr b/src/ameba/formatter/dot_formatter.cr
index 22eafc527..600de0a10 100644
--- a/src/ameba/formatter/dot_formatter.cr
+++ b/src/ameba/formatter/dot_formatter.cr
@@ -10,7 +10,7 @@ module Ameba::Formatter
     @mutex = Mutex.new
 
     # Reports a message when inspection is started.
-    def started(sources)
+    def started(sources) : Nil
       @started_at = Time.monotonic
 
       output.puts started_message(sources.size)
@@ -18,13 +18,13 @@ module Ameba::Formatter
     end
 
     # Reports a result of the inspection of a corresponding source.
-    def source_finished(source : Source)
+    def source_finished(source : Source) : Nil
       sym = source.valid? ? ".".colorize(:green) : "F".colorize(:red)
       @mutex.synchronize { output << sym }
     end
 
     # Reports a message when inspection is finished.
-    def finished(sources)
+    def finished(sources) : Nil
       output.flush
       output << "\n\n"
 
diff --git a/src/ameba/formatter/explain_formatter.cr b/src/ameba/formatter/explain_formatter.cr
index e963d98f4..0ee208504 100644
--- a/src/ameba/formatter/explain_formatter.cr
+++ b/src/ameba/formatter/explain_formatter.cr
@@ -30,7 +30,7 @@ module Ameba::Formatter
     end
 
     # Reports the explanations at the *@location*.
-    def finished(sources)
+    def finished(sources) : Nil
       source = sources.find(&.path.==(@location.filename))
       return unless source
 
@@ -40,7 +40,7 @@ module Ameba::Formatter
       explain(source, issue)
     end
 
-    private def explain(source, issue)
+    private def explain(source, issue) : Nil
       return unless location = issue.location
 
       output << '\n'
diff --git a/src/ameba/formatter/flycheck_formatter.cr b/src/ameba/formatter/flycheck_formatter.cr
index 491567e4a..5de8030f5 100644
--- a/src/ameba/formatter/flycheck_formatter.cr
+++ b/src/ameba/formatter/flycheck_formatter.cr
@@ -2,7 +2,7 @@ module Ameba::Formatter
   class FlycheckFormatter < BaseFormatter
     @mutex = Mutex.new
 
-    def source_finished(source : Source)
+    def source_finished(source : Source) : Nil
       source.issues.each do |issue|
         next if issue.disabled?
         next if issue.correctable? && config[:autocorrect]?
diff --git a/src/ameba/formatter/json_formatter.cr b/src/ameba/formatter/json_formatter.cr
index 42e310ee3..a65d5fb3c 100644
--- a/src/ameba/formatter/json_formatter.cr
+++ b/src/ameba/formatter/json_formatter.cr
@@ -66,11 +66,11 @@ module Ameba::Formatter
     @result = AsJSON::Result.new
     @mutex = Mutex.new
 
-    def started(sources)
+    def started(sources) : Nil
       @result.summary.target_sources_count = sources.size
     end
 
-    def source_finished(source : Source)
+    def source_finished(source : Source) : Nil
       json_source = AsJSON::Source.new source.path
 
       source.issues.each do |issue|
@@ -92,7 +92,7 @@ module Ameba::Formatter
       end
     end
 
-    def finished(sources)
+    def finished(sources) : Nil
       @result.to_json @output
     end
   end
diff --git a/src/ameba/formatter/todo_formatter.cr b/src/ameba/formatter/todo_formatter.cr
index ce8d58080..2744963ad 100644
--- a/src/ameba/formatter/todo_formatter.cr
+++ b/src/ameba/formatter/todo_formatter.cr
@@ -8,7 +8,7 @@ module Ameba::Formatter
     def initialize(@output = STDOUT, @config_path = Config::DEFAULT_PATH)
     end
 
-    def finished(sources)
+    def finished(sources) : Nil
       super
 
       issues = sources.flat_map(&.issues)
@@ -22,12 +22,12 @@ module Ameba::Formatter
         return
       end
 
-      generate_todo_config(issues).tap do |file|
-        @output.puts "Created #{file.path}"
-      end
+      generate_todo_config(issues)
+
+      @output.puts "Created #{@config_path}"
     end
 
-    private def generate_todo_config(issues)
+    private def generate_todo_config(issues) : Nil
       File.open(@config_path, mode: "w") do |file|
         file << header
 
@@ -41,7 +41,6 @@ module Ameba::Formatter
           file << "\n# Run `ameba --only #{rule.name}` for details"
           file << rule_todo
         end
-        file
       end
     end
 
diff --git a/src/ameba/presenter/rule_collection_presenter.cr b/src/ameba/presenter/rule_collection_presenter.cr
index e833df11d..1b5b6795c 100644
--- a/src/ameba/presenter/rule_collection_presenter.cr
+++ b/src/ameba/presenter/rule_collection_presenter.cr
@@ -1,6 +1,6 @@
 module Ameba::Presenter
   class RuleCollectionPresenter < BasePresenter
-    def run(rules)
+    def run(rules) : Nil
       rules = rules.to_h do |rule|
         name = rule.name.split('/')
         name = "%s/%s" % {
diff --git a/src/ameba/presenter/rule_presenter.cr b/src/ameba/presenter/rule_presenter.cr
index a790ac4b7..9c7da467a 100644
--- a/src/ameba/presenter/rule_presenter.cr
+++ b/src/ameba/presenter/rule_presenter.cr
@@ -1,6 +1,6 @@
 module Ameba::Presenter
   class RulePresenter < BasePresenter
-    def run(rule)
+    def run(rule) : Nil
       output.puts
       output_title "Rule info"
       output_paragraph "%s of a %s severity [enabled: %s]" % {
diff --git a/src/ameba/runner.cr b/src/ameba/runner.cr
index 1923e0a01..03f11b590 100644
--- a/src/ameba/runner.cr
+++ b/src/ameba/runner.cr
@@ -107,7 +107,7 @@ module Ameba
       @formatter.finished @sources
     end
 
-    private def run_source(source)
+    private def run_source(source) : Nil
       @formatter.source_started source
 
       # This variable is a 2D array used to track corrected issues after each