diff --git a/Sources/Compiler/Implementation/CompilerImpl.swift b/Sources/Compiler/Implementation/CompilerImpl.swift
index f4062a6d..f899eca0 100644
--- a/Sources/Compiler/Implementation/CompilerImpl.swift
+++ b/Sources/Compiler/Implementation/CompilerImpl.swift
@@ -15,6 +15,9 @@ internal final class CompilerImpl: ASTVisitor, StatementVisitor, ExpressionVisit
   internal typealias StatementResult = Void
   internal typealias ExpressionResult = Void
 
+  // For Swift 6.1 compatibility with Foundation.
+  internal typealias Expression = VioletParser.Expression
+
   /// Program that we are compiling.
   private let ast: AST
   /// Name of the file that this code object was loaded from.
diff --git a/Tests/CompilerTests/ASTCreator.swift b/Tests/CompilerTests/ASTCreator.swift
index d605fa2f..155f8166 100644
--- a/Tests/CompilerTests/ASTCreator.swift
+++ b/Tests/CompilerTests/ASTCreator.swift
@@ -12,6 +12,9 @@ internal protocol ASTCreator {}
 
 extension ASTCreator {
 
+  // For Swift 6.1 compatibility with Foundation.
+  typealias Expression = VioletParser.Expression
+
   private var id: ASTNodeId {
     // We have to increment 'id', because it is used as a key in 'SymbolTable'.
     let value = _id
diff --git a/Tests/CompilerTests/CompileTestCase.swift b/Tests/CompilerTests/CompileTestCase.swift
index 033b1007..3b8456a2 100644
--- a/Tests/CompilerTests/CompileTestCase.swift
+++ b/Tests/CompilerTests/CompileTestCase.swift
@@ -7,6 +7,9 @@ import VioletBytecode
 /// Base class for all of the compiler tests
 internal class CompileTestCase: XCTestCase, ASTCreator {
 
+  // For Swift 6.1 compatibility with Foundation.
+  typealias Expression = VioletParser.Expression
+
   internal var builder = ASTBuilder()
 
   // MARK: - Compile
diff --git a/Tests/CompilerTests/SymbolTableTestCase.swift b/Tests/CompilerTests/SymbolTableTestCase.swift
index 51e5e076..b23eb6c7 100644
--- a/Tests/CompilerTests/SymbolTableTestCase.swift
+++ b/Tests/CompilerTests/SymbolTableTestCase.swift
@@ -6,6 +6,9 @@ import VioletParser
 /// Base class for all of the symbol table tests
 class SymbolTableTestCase: XCTestCase, ASTCreator {
 
+  // For Swift 6.1 compatibility with Foundation.
+  typealias Expression = VioletParser.Expression
+
   var builder = ASTBuilder()
 
   // MARK: - Create
diff --git a/Tests/RapunzelTests/Lyrics.swift b/Tests/RapunzelTests/Lyrics.swift
index 6cc1dee3..9231638e 100644
--- a/Tests/RapunzelTests/Lyrics.swift
+++ b/Tests/RapunzelTests/Lyrics.swift
@@ -51,7 +51,7 @@ extension Lyrics {
           "And it's warm and real and bright",
           "And the world has somehow shifted",
           "All at once everything looks different",
-          "Now that I see you"
+          "Now that I see you",
         ]
       ),
 
@@ -68,7 +68,7 @@ extension Lyrics {
           "Now she's here shining in the starlight",
           "Now she's here suddenly I know",
           "If she's here it's crystal clear",
-          "I'm where I'm meant to go"
+          "I'm where I'm meant to go",
         ]
       ),
       Part(
@@ -95,9 +95,9 @@ extension Lyrics {
           "All at once everything is different",
           "Now that I see you",
 
-          "Now that I see you"
+          "Now that I see you",
         ]
-      )
+      ),
     ]
   }
 }