inst/examples/boolean/README.md

In muttest: Mutation Testing

Guard — Type Check Instead of Value Check

What this demonstrates

Assertions that check the type of a return value rather than its content cannot detect boolean literal mutations. A TRUE → FALSE swap changes the value the function returns but not its type. Replacing type assertions with value assertions kills the mutant.

The function

is_valid returns FALSE for NULL and TRUE for everything else. all_valid returns TRUE only if every element in a list passes is_valid. Both functions contain boolean literals (TRUE, FALSE) that boolean literal mutators will flip.

The weak test

The tests only verify that the return value is a logical (is.logical(...)). A flipped boolean is still a logical — the type check passes for both the original and the mutant.

The surviving mutant

FALSE → TRUE survives. When is_valid(NULL) returns TRUE instead of FALSE, is.logical(TRUE) is still TRUE. The test cannot tell the difference.

The fix

Replace type assertions with value assertions using expect_true and expect_false. These check the actual boolean value returned, not just its type.

Key rule

When a boolean literal mutant survives, replace is.logical() type checks with expect_true() / expect_false() value checks.

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