diff --git a/Sources/Core/SourceFile.swift b/Sources/Core/SourceFile.swift
index e0303b7c4..f03bd41b1 100644
--- a/Sources/Core/SourceFile.swift
+++ b/Sources/Core/SourceFile.swift
@@ -33,6 +33,14 @@ public struct SourceFile {
     try self.init(contentsOf: URL(fileURLWithPath: filePath))
   }
 
+  /// Creates an instance with the given `contents`, identified by `fileID`.
+  ///
+  /// `contents` may not reflect what—if anything—is stored on disk at `fileID`.
+  public init(contents: String, fileID: URL) {
+    let storage = Storage(fileID, replaceExisting: true) { contents[...] }
+    self.storage = storage
+  }
+
   /// Creates an instance for the `text` given by a multiline string literal in the given
   /// `swiftFile`, the literal's textual content (the line after the opening quotes) being
   /// startLine.
@@ -344,17 +352,23 @@ extension SourceFile {
     }
 
     /// The owner of all instances of `Storage`.
-    private static var allInstances = SharedMutable<[URL: Storage]>([:])
+    private static let allInstances = SharedMutable<[URL: Storage]>([:])
 
-    /// Creates an alias to the instance with the given `url` if it exists, or creates a new
+    /// Creates an alias to the instance with the given `url` if it exists, replacing the instance's text with the
+    /// result of `makeText()` iff `replaceExisting` is `true`, or creates a new
     /// instance having the given `url` and the text resulting from `makeText()`.
     fileprivate convenience init(
-      _ url: URL, lineStarts: [Index]? = nil, makeText: () throws -> Substring
+      _ url: URL, lineStarts: [Index]? = nil, replaceExisting: Bool = false, makeText: () throws -> Substring
     ) rethrows {
       self.init(
         aliasing: try Self.allInstances.modify { (c: inout [URL: Storage]) -> Storage in
           try modify(&c[url]) { v in
-            let r = try v ?? Storage(url: url, lineStarts: lineStarts, text: makeText())
+            let r: Storage
+            if !replaceExisting && v != nil {
+              r = v!
+            } else {
+              r = try Storage(url: url, lineStarts: lineStarts, text: makeText())
+            }
             v = r
             return r
           }
diff --git a/Tests/HyloTests/SourceFileTests.swift b/Tests/HyloTests/SourceFileTests.swift
index 188b05d24..fc3d4c9e1 100644
--- a/Tests/HyloTests/SourceFileTests.swift
+++ b/Tests/HyloTests/SourceFileTests.swift
@@ -1,5 +1,6 @@
 import Core
 import XCTest
+import Foundation
 
 final class SourceFileTests: XCTestCase {
 
@@ -33,4 +34,12 @@ final class SourceFileTests: XCTestCase {
     XCTAssertEqual(source.line(containing: source.text.endIndex).number, 3)
   }
 
+  func testUpdateSourceFileContent() throws {
+    let url = URL(fileURLWithPath: "foo.hylo")
+    let s1 = SourceFile(contents: "import A", fileID: url)
+    XCTAssertEqual(s1.text, "import A")
+    let s2 = SourceFile(contents: "import B", fileID: url)
+    XCTAssertEqual(s2.text, "import B")
+  }
+
 }