diff --git a/src/spectator/abstract_expression.cr b/src/spectator/abstract_expression.cr
index b58c1cc..6ab4cbf 100644
--- a/src/spectator/abstract_expression.cr
+++ b/src/spectator/abstract_expression.cr
@@ -34,7 +34,7 @@ module Spectator
 
     # Produces a string representation of the expression.
     # This consists of the label (if one is available) and the value.
-    def to_s(io)
+    def to_s(io : IO) : Nil
       if (label = @label)
         io << label << ": "
       end
@@ -43,7 +43,7 @@ module Spectator
 
     # Produces a detailed string representation of the expression.
     # This consists of the label (if one is available) and the value.
-    def inspect(io)
+    def inspect(io : IO) : Nil
       if (label = @label)
         io << label << ": "
       end
diff --git a/src/spectator/anything.cr b/src/spectator/anything.cr
index e4d7b34..aa25e3c 100644
--- a/src/spectator/anything.cr
+++ b/src/spectator/anything.cr
@@ -13,12 +13,12 @@ module Spectator
     end
 
     # Displays "anything".
-    def to_s(io)
+    def to_s(io : IO) : Nil
       io << "anything"
     end
 
     # Displays "<anything>".
-    def inspect(io)
+    def inspect(io : IO) : Nil
       io << "<anything>"
     end
   end
diff --git a/src/spectator/context.cr b/src/spectator/context.cr
index c3213e3..7fc126b 100644
--- a/src/spectator/context.cr
+++ b/src/spectator/context.cr
@@ -10,12 +10,12 @@ abstract class SpectatorContext
   # and that the Crystal compiler instantiates a `#to_s` and/or `#inspect` for each of those types,
   # an explosion in method instances can be created.
   # The compile time is drastically reduced by using a dummy string instead.
-  def to_s(io)
+  def to_s(io : IO) : Nil
     io << "Context"
   end
 
   # :ditto:
-  def inspect(io)
+  def inspect(io : IO) : Nil
     io << "Context<" << self.class << '>'
   end
 end
diff --git a/src/spectator/error_result.cr b/src/spectator/error_result.cr
index 4babc2a..a4531fb 100644
--- a/src/spectator/error_result.cr
+++ b/src/spectator/error_result.cr
@@ -16,7 +16,7 @@ module Spectator
     end
 
     # One-word description of the result.
-    def to_s(io)
+    def to_s(io : IO) : Nil
       io << "error"
     end
 
diff --git a/src/spectator/example.cr b/src/spectator/example.cr
index 8f99e93..06e0f19 100644
--- a/src/spectator/example.cr
+++ b/src/spectator/example.cr
@@ -191,7 +191,7 @@ module Spectator
 
     # Constructs the full name or description of the example.
     # This prepends names of groups this example is part of.
-    def to_s(io)
+    def to_s(io : IO) : Nil
       name = @name
 
       # Prefix with group's full name if the node belongs to a group.
@@ -210,7 +210,7 @@ module Spectator
     end
 
     # Exposes information about the example useful for debugging.
-    def inspect(io)
+    def inspect(io : IO) : Nil
       super
       io << ' ' << result
     end
@@ -286,7 +286,7 @@ module Spectator
 
       # Constructs the full name or description of the example.
       # This prepends names of groups this example is part of.
-      def to_s(io) : Nil
+      def to_s(io : IO) : Nil
         @example.to_s(io)
       end
     end
diff --git a/src/spectator/example_group.cr b/src/spectator/example_group.cr
index 0481fc4..bb702e4 100644
--- a/src/spectator/example_group.cr
+++ b/src/spectator/example_group.cr
@@ -112,7 +112,7 @@ module Spectator
 
     # Constructs the full name or description of the example group.
     # This prepends names of groups this group is part of.
-    def to_s(io)
+    def to_s(io : IO) : Nil
       # Prefix with group's full name if the node belongs to a group.
       return unless parent = @group
 
diff --git a/src/spectator/example_group_hook.cr b/src/spectator/example_group_hook.cr
index bd6bac8..aee357f 100644
--- a/src/spectator/example_group_hook.cr
+++ b/src/spectator/example_group_hook.cr
@@ -42,7 +42,7 @@ module Spectator
 
     # Produces the string representation of the hook.
     # Includes the location and label if they're not nil.
-    def to_s(io)
+    def to_s(io : IO) : Nil
       io << "example group hook"
 
       if (label = @label)
diff --git a/src/spectator/example_hook.cr b/src/spectator/example_hook.cr
index edebf26..6bc77a0 100644
--- a/src/spectator/example_hook.cr
+++ b/src/spectator/example_hook.cr
@@ -37,7 +37,7 @@ module Spectator
 
     # Produces the string representation of the hook.
     # Includes the location and label if they're not nil.
-    def to_s(io)
+    def to_s(io : IO) : Nil
       io << "example hook"
 
       if (label = @label)
diff --git a/src/spectator/example_procsy_hook.cr b/src/spectator/example_procsy_hook.cr
index 8a64f17..16bc970 100644
--- a/src/spectator/example_procsy_hook.cr
+++ b/src/spectator/example_procsy_hook.cr
@@ -39,7 +39,7 @@ module Spectator
 
     # Produces the string representation of the hook.
     # Includes the location and label if they're not nil.
-    def to_s(io)
+    def to_s(io : IO) : Nil
       io << "example hook"
 
       if (label = @label)
diff --git a/src/spectator/fail_result.cr b/src/spectator/fail_result.cr
index 36ea8fb..c283a80 100644
--- a/src/spectator/fail_result.cr
+++ b/src/spectator/fail_result.cr
@@ -55,7 +55,7 @@ module Spectator
     end
 
     # One-word description of the result.
-    def to_s(io)
+    def to_s(io : IO) : Nil
       io << "fail"
     end
 
diff --git a/src/spectator/formatting/components/comment.cr b/src/spectator/formatting/components/comment.cr
index b398840..30f4293 100644
--- a/src/spectator/formatting/components/comment.cr
+++ b/src/spectator/formatting/components/comment.cr
@@ -16,7 +16,7 @@ module Spectator::Formatting::Components
     end
 
     # Writes the comment to the output.
-    def to_s(io)
+    def to_s(io : IO) : Nil
       io << "# " << @content
     end
   end
diff --git a/src/spectator/formatting/components/example_command.cr b/src/spectator/formatting/components/example_command.cr
index 8b3c0b9..b1246db 100644
--- a/src/spectator/formatting/components/example_command.cr
+++ b/src/spectator/formatting/components/example_command.cr
@@ -9,7 +9,7 @@ module Spectator::Formatting::Components
     end
 
     # Produces output for running the previously specified example.
-    def to_s(io)
+    def to_s(io : IO) : Nil
       io << "crystal spec "
 
       # Use location for argument if it's available, since it's simpler.
diff --git a/src/spectator/formatting/components/failure_command_list.cr b/src/spectator/formatting/components/failure_command_list.cr
index 7da5ed2..7eab4f5 100644
--- a/src/spectator/formatting/components/failure_command_list.cr
+++ b/src/spectator/formatting/components/failure_command_list.cr
@@ -10,7 +10,7 @@ module Spectator::Formatting::Components
     end
 
     # Produces the list of commands to run failed examples.
-    def to_s(io)
+    def to_s(io : IO) : Nil
       io.puts "Failed examples:"
       io.puts
       @failures.each do |failure|
diff --git a/src/spectator/formatting/components/profile.cr b/src/spectator/formatting/components/profile.cr
index 2a56e0d..b98d8f5 100644
--- a/src/spectator/formatting/components/profile.cr
+++ b/src/spectator/formatting/components/profile.cr
@@ -9,7 +9,7 @@ module Spectator::Formatting::Components
     end
 
     # Produces the output containing the profiling information.
-    def to_s(io)
+    def to_s(io : IO) : Nil
       io << "Top "
       io << @profile.size
       io << " slowest examples ("
diff --git a/src/spectator/formatting/components/result_block.cr b/src/spectator/formatting/components/result_block.cr
index bf4b3d2..ddd9c47 100644
--- a/src/spectator/formatting/components/result_block.cr
+++ b/src/spectator/formatting/components/result_block.cr
@@ -41,7 +41,7 @@ module Spectator::Formatting::Components
     private abstract def content(io)
 
     # Writes the component's output to the specified stream.
-    def to_s(io)
+    def to_s(io : IO) : Nil
       title_line(io)
       # Ident over to align with the spacing used by the index.
       indent(index_digit_count + 2) do
diff --git a/src/spectator/formatting/components/runtime.cr b/src/spectator/formatting/components/runtime.cr
index 9f28813..9638d93 100644
--- a/src/spectator/formatting/components/runtime.cr
+++ b/src/spectator/formatting/components/runtime.cr
@@ -15,7 +15,7 @@ module Spectator::Formatting::Components
     # #:##:##
     # # days #:##:##
     # ```
-    def to_s(io)
+    def to_s(io : IO) : Nil
       millis = @span.total_milliseconds
       return format_micro(io, millis * 1000) if millis < 1
 
diff --git a/src/spectator/formatting/components/stats.cr b/src/spectator/formatting/components/stats.cr
index ab76c7c..47e1063 100644
--- a/src/spectator/formatting/components/stats.cr
+++ b/src/spectator/formatting/components/stats.cr
@@ -11,7 +11,7 @@ module Spectator::Formatting::Components
     end
 
     # Displays the stats.
-    def to_s(io)
+    def to_s(io : IO) : Nil
       runtime(io)
       totals(io)
       if seed = @report.random_seed?
diff --git a/src/spectator/formatting/components/tap_profile.cr b/src/spectator/formatting/components/tap_profile.cr
index 64c36b0..4154e07 100644
--- a/src/spectator/formatting/components/tap_profile.cr
+++ b/src/spectator/formatting/components/tap_profile.cr
@@ -10,7 +10,7 @@ module Spectator::Formatting::Components
     end
 
     # Produces the output containing the profiling information.
-    def to_s(io)
+    def to_s(io : IO) : Nil
       io << "# Top "
       io << @profile.size
       io << " slowest examples ("
diff --git a/src/spectator/formatting/components/totals.cr b/src/spectator/formatting/components/totals.cr
index 941b6ee..4063cae 100644
--- a/src/spectator/formatting/components/totals.cr
+++ b/src/spectator/formatting/components/totals.cr
@@ -31,7 +31,7 @@ module Spectator::Formatting::Components
     end
 
     # Writes the counts to the output.
-    def to_s(io)
+    def to_s(io : IO) : Nil
       io << @examples << " examples, " << @failures << " failures"
 
       if @errors > 0
diff --git a/src/spectator/location.cr b/src/spectator/location.cr
index 688fdb6..7b06eb9 100644
--- a/src/spectator/location.cr
+++ b/src/spectator/location.cr
@@ -59,7 +59,7 @@ module Spectator
     # ```text
     # FILE:LINE
     # ```
-    def to_s(io)
+    def to_s(io : IO) : Nil
       io << path << ':' << line
     end
   end
diff --git a/src/spectator/matchers/attributes_matcher.cr b/src/spectator/matchers/attributes_matcher.cr
index 8ecd714..c66ecc4 100644
--- a/src/spectator/matchers/attributes_matcher.cr
+++ b/src/spectator/matchers/attributes_matcher.cr
@@ -15,7 +15,7 @@ module Spectator::Matchers
       extend self
 
       # Text displayed when a method is undefined.
-      def inspect(io)
+      def inspect(io : IO) : Nil
         io << "<Method undefined>"
       end
     end
diff --git a/src/spectator/node.cr b/src/spectator/node.cr
index c5a64b6..807c8df 100644
--- a/src/spectator/node.cr
+++ b/src/spectator/node.cr
@@ -66,12 +66,12 @@ module Spectator
 
     # Constructs the full name or description of the node.
     # This prepends names of groups this node is part of.
-    def to_s(io)
+    def to_s(io : IO) : Nil
       display_name.to_s(io)
     end
 
     # Exposes information about the node useful for debugging.
-    def inspect(io)
+    def inspect(io : IO) : Nil
       # Full node name.
       io << '"' << self << '"'
 
diff --git a/src/spectator/pass_result.cr b/src/spectator/pass_result.cr
index 2b62383..20e3b04 100644
--- a/src/spectator/pass_result.cr
+++ b/src/spectator/pass_result.cr
@@ -24,7 +24,7 @@ module Spectator
     end
 
     # One-word description of the result.
-    def to_s(io)
+    def to_s(io : IO) : Nil
       io << "pass"
     end
 
diff --git a/src/spectator/pending_result.cr b/src/spectator/pending_result.cr
index 03700d9..cff38c5 100644
--- a/src/spectator/pending_result.cr
+++ b/src/spectator/pending_result.cr
@@ -43,7 +43,7 @@ module Spectator
     end
 
     # One-word description of the result.
-    def to_s(io)
+    def to_s(io : IO) : Nil
       io << "pending"
     end