inst/examples/strings/README.md

In muttest: Mutation Testing

Label — Property Check Instead of Exact Value

What this demonstrates

Assertions that check only whether a string is non-empty (or has characters) cannot detect string literal mutations. Replacing a string with "" or "mutant" changes the content but may still satisfy a length check. Asserting the exact expected string kills the mutant.

The function

greet returns a personalised greeting or a fallback for empty names. default_label returns the label if non-empty, or "unknown" for an empty string. Both functions contain string literals that string_empty() and string_fill() will replace.

The weak test

The tests check that the return value is a character and that its length is greater than zero. A mutated string ("" or "mutant") may still satisfy is.character(), and "mutant" satisfies nchar() > 0. Neither check catches the mutation.

The surviving mutant

string_empty() replaces "Hello, stranger!" with "". greet("") now returns "". is.character("") = TRUE and the test passes — but the function is silently broken.

The fix

Assert the exact string that the function should return. expect_equal(greet("Alice"), "Hello, Alice!") fails immediately when the literal is emptied or filled with a placeholder.

Key rule

When a string literal mutant survives, replace length or type checks with expect_equal against the exact expected string.

muttest documentation built on May 14, 2026, 5:10 p.m.