Semantic confusion

Author: LukeMathWalker

src/SUMMARY.md

@@ -434,6 +434,7 @@
 - [Welcome](idiomatic/welcome.md)
 - [Leveraging the Type System](idiomatic/leveraging-the-type-system.md)
   - [Newtype Pattern](idiomatic/leveraging-the-type-system/newtype-pattern.md)
+    - [Semantic Confusion](idiomatic/leveraging-the-type-system/newtype-pattern/semantic-confusion.md)
 
 ---
 

src/idiomatic/leveraging-the-type-system/newtype-pattern/semantic-confusion.md

@@ -0,0 +1,72 @@
+---
+minutes: 5
+---
+
+# Semantic Confusion
+
+There is room for confusion whenever a function takes multiple arguments of the
+same type:
+
+```rust
+# struct LoginError;
+pub fn login(username: &str, password: &str) -> Result<(), LoginError> {
+    // [...]
+    # Ok(())
+}
+
+# let password = "password";
+# let username = "username";
+// In another part of the codebase, we swap arguments by mistake.
+// Bug (best case), security vulnerability (worst case)
+login(password, username);
+```
+
+The newtype pattern can be used to prevent this class of errors at compile time:
+
+```rust
+pub struct Username(String);
+pub struct Password(String);
+# struct LoginError;
+
+pub fn login(username: &Username, password: &Password) -> Result<(), LoginError> {
+    // [...]
+    # Ok(())
+}
+
+# let password = Password("password".into());
+# let username = Username("username".into());
+// Compiler error 🎉
+login(password, username);
+```
+
+<details>
+
+- Run both examples to show students the successful compilation for the original
+  example, and the compiler error returned by the modified example.
+
+- Stress the _semantic_ angle. The newtype pattern should be leveraged to use
+  distinct types for distinct concepts, thus ruling out this class of errors
+  entirely.
+
+- Nonetheless, note that there are legitimate scenarios where a function may
+  take multiple arguments of the same type. In those scenarios, if correctness
+  is of paramount important, consider using a struct with named fields as input:
+  ```rust
+  pub struct LoginArguments {
+      pub username: &str,
+      pub password: &str,
+  }
+  # fn login(i: LoginArguments) {}
+  # let password = "password";
+  # let username = "username";
+
+  // No need to check the definition of the `login` function to spot the issue.
+  login(LoginArguments {
+      username: password,
+      password: username,
+  })
+  ```
+  Users are forced, at the callsite, to assign values to each field, thus
+  increasing the likelihood of spotting bugs.
+
+</details>