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  "path": "/t/mutation-testing-in-haskell/14199#post_7",
  "publishedAt": "2026-06-09T06:08:46.000Z",
  "site": "https://discourse.haskell.org",
  "textContent": "andremarianiello:\n\n> The question is: does this mutation cause a change in behavior that is caught by a test?\n\nWhy should I care?\n\nThe basic assumption seems to be that a test suite must fail if the code is altered semantically.\nSuppose there are two data types, `A` and `B` and a function `f :: A -> B` that is to be tested. Suppose both `A` and `B` are finite with `n` and `m` total elements, respectively. Then `f` has `m^n - 1` possible mutations. A naive test suite, adding one test after another, might need in the same order of magnitude of test cases, a cleverly designed suite only `log(m^n)` test cases to rule out all undesired mutations. With infinite types and general recursion, this even becomes infeasible.\nConsider the example in the announcement. The tests cover the design space of `Int -> Int -> Bool` while the semantics seem to be concerned with both integers in a finite range from 0 to some number, e.g. 20. Hence there are infinitely many mutations of `canCastFireball` that could be distinguished by test cases yet to be written, but that are semantically identical in the design space that the programmer cares about. If a mutation alters the behaviour at `level = -42`, does that mean we ought to add a test for that?\n\nWhat I am trying to convey is that a spec should pin down the desired behaviour as excactly as possible, and if done right the code could be extracted from it. If a mutation does not fail the tests, it does not necessarily mean the tests are bad. It only means that the desired behaviour is exhibited by a non-singleton set of possibilities. The question is, do my tests let only the desired programs pass?\nMutation tests could indeed be very helpful with that. I would expect a mutation test to nudge the code into a direction specified by a “spec for tests”. Perhaps sydtest does just that, but one can not tell from the announcement.",
  "title": "Mutation Testing in Haskell"
}