diff --git a/src/spectator/abstract_expression.cr b/src/spectator/abstract_expression.cr
index 2a58e06..3bb20f2 100644
--- a/src/spectator/abstract_expression.cr
+++ b/src/spectator/abstract_expression.cr
@@ -6,25 +6,30 @@ module Spectator
   # It consists of a label and the value of the expression.
   # The label should be a string recognizable by the user,
   # or nil if one isn't available.
+  #
   # This base class is provided so that all generic sub-classes can be stored as this one type.
   # The value of the expression can be retrieved by downcasting to the expected type with `#cast`.
+  #
+  # NOTE: This is intentionally a class and not a struct.
+  # If it were a struct, changes made to the value held by an instance may not be kept when passing it around.
+  # See commit ca564619ad2ae45f832a058d514298c868fdf699.
   abstract class AbstractExpression
     # User recognizable string for the expression.
     # This can be something like a variable name or a snippet of Crystal code.
     getter label : Label
 
     # Creates the expression.
-    # The *label* is usually the Crystal code evaluating to the `#value`.
+    # The *label* is usually the Crystal code evaluating to the `#raw_value`.
     # It can be nil if it isn't available.
     def initialize(@label : Label)
     end
 
-    # Retrieves the real value of the expression.
-    abstract def value
+    # Retrieves the evaluated value of the expression.
+    abstract def raw_value
 
-    # Attempts to cast `#value` to the type *T* and return it.
+    # Attempts to cast `#raw_value` to the type *T* and return it.
     def cast(type : T.class) : T forall T
-      value.as(T)
+      raw_value.as(T)
     end
 
     # Produces a string representation of the expression.
@@ -35,7 +40,7 @@ module Spectator
         io << ':'
         io << ' '
       end
-      io << value
+      raw_value.to_s(io)
     end
 
     # Produces a detailed string representation of the expression.
@@ -46,7 +51,7 @@ module Spectator
         io << ':'
         io << ' '
       end
-      value.inspect(io)
+      raw_value.inspect(io)
     end
   end
 end
diff --git a/src/spectator/block.cr b/src/spectator/block.cr
index 312681a..5deb82c 100644
--- a/src/spectator/block.cr
+++ b/src/spectator/block.cr
@@ -1,4 +1,4 @@
-require "./abstract_expression"
+require "./expression"
 require "./label"
 require "./lazy"
 
@@ -8,7 +8,7 @@ module Spectator
   # It consists of a label and parameterless block.
   # The label should be a string recognizable by the user,
   # or nil if one isn't available.
-  class Block(T) < AbstractExpression
+  class Block(T) < Expression(T)
     # Cached value returned from the block.
     @value = Lazy(T).new
 
diff --git a/src/spectator/dsl/assertions.cr b/src/spectator/dsl/assertions.cr
index 791ad73..d868d16 100644
--- a/src/spectator/dsl/assertions.cr
+++ b/src/spectator/dsl/assertions.cr
@@ -1,7 +1,8 @@
 require "../assertion"
 require "../assertion_failed"
-require "../expression"
+require "../block"
 require "../source"
+require "../value"
 
 module Spectator::DSL
   # Methods and macros for asserting that conditions are met.
@@ -50,7 +51,7 @@ module Spectator::DSL
         {{actual}}
       end
 
-      %expression = ::Spectator::Expression.new(%actual, {{actual.stringify}})
+      %expression = ::Spectator::Value.new(%actual, {{actual.stringify}})
       %source = ::Spectator::Source.new({{actual.filename}}, {{actual.line_number}})
       ::Spectator::Assertion::Target.new(%expression, %source)
     end
diff --git a/src/spectator/expression.cr b/src/spectator/expression.cr
index b12efce..0937b8e 100644
--- a/src/spectator/expression.cr
+++ b/src/spectator/expression.cr
@@ -1,22 +1,18 @@
 require "./abstract_expression"
-require "./label"
 
 module Spectator
   # Represents an expression from a test.
   # This is typically captured by an `expect` macro.
-  # It consists of a label and the value of the expression.
+  # It consists of a label and a typed expression.
   # The label should be a string recognizable by the user,
   # or nil if one isn't available.
-  class Expression(T) < AbstractExpression
-    # Raw value of the expression.
-    getter value
+  abstract class Expression(T) < AbstractExpression
+    # Retrieves the underlying value of the expression.
+    abstract def value : T
 
-    # Creates the expression.
-    # Expects the *value* of the expression and a *label* describing it.
-    # The *label* is usually the Crystal code evaluating to the *value*.
-    # It can be nil if it isn't available.
-    def initialize(@value : T, label : Label)
-      super(label)
+    # Retrieves the evaluated value of the expression.
+    def raw_value
+      value
     end
   end
 end
diff --git a/src/spectator/value.cr b/src/spectator/value.cr
new file mode 100644
index 0000000..9d04871
--- /dev/null
+++ b/src/spectator/value.cr
@@ -0,0 +1,22 @@
+require "./expression"
+require "./label"
+
+module Spectator
+  # Represents a value from a test.
+  # This is typically captured by an `expect` macro.
+  # It consists of a label and the value of the expression.
+  # The label should be a string recognizable by the user,
+  # or nil if one isn't available.
+  class Value(T) < Expression(T)
+    # Raw value of the expression.
+    getter value : T
+
+    # Creates the value.
+    # Expects the *value* of the expression and a *label* describing it.
+    # The *label* is usually the Crystal code evaluating to the *value*.
+    # It can be nil if it isn't available.
+    def initialize(@value : T, label : Label)
+      super(label)
+    end
+  end
+end