Mutation Testing in Elixir

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mutation testing elixir in Daniel Serrano 2019

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outline • what is mutation testing? • exavier • elixir AST • exunit • mutators • biggest drawback • rough edges • future

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mutation testing who watches the watchmen?

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mutation testing who tests the tests?

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mutation testing ✨ mutation testing ✨

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DeMillo, Lipton and Sayward 1978 mutation testing

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1. write green tests 2. modify code under test 3. check tests now fail 1. 2. 3. code test code test code test mutation testing

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• killed mutant test fails after mutation (expected, good) code mutation testing test

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• surviving mutant test stays green after mutation (unexpected, bad) code test mutation testing

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• threshold mutation testing cover threshold percentage of killed mutants below which mutation testing fails (e.g., 80%) mutation testing

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• elixir mutation testing library • goal • install exavier library • run mix exavier.test • mutate code • run tests • show mutation testing report exavier

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how tho?

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elixir AST

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elixir AST * + 3 1 2 abstract syntax tree tree representation of the abstract syntactic structure of the source code (1+2) * 3

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elixir AST * + 3 1 2 (1+2) * 3 {:*, [line: 1], [ {:+, [line: 1], [ 1, 2 ]}, 3 ]}

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elixir AST * + 3 1 2 (1+2) * 3 {:*, [line: 1], [ {:+, [line: 1], [ 1, 2 ]}, 3 ]}

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elixir AST * + 3 1 2 (1+2) * 3 {:*, [line: 1], [ {:+, [line: 1], [ 1, 2 ]}, 3 ]}

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elixir AST * + 3 1 2 (1+2) * 3 {:*, [line: 1], [ {:+, [line: 1], [ 1, 2 ]}, 3 ]}

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elixir AST * + 3 1 2 {:*, [line: 1], [ {:+, [line: 1], [ 1, 2 ]}, 3 ]} operator metadata arguments

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elixir AST {atom, keyword, list} operator metadata arguments

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• ast.ninja • Kernel.quote/2 demo • Code.string_to_quoted/2 • Code.compile_quoted/2

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elixir AST Arjan Scherpenisse – AST ninja

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• elixir mutation testing library • goal • install exavier library • run mix exavier.test • mutate code • run tests • show mutation testing report exavier

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exunit

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exunit • default elixir testing framework • mix task mix test

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exunit

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exunit

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• elixir mutation testing library • goal • install exavier library • run mix exavier.test • mutate code • run tests • show mutation testing report exavier

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exunit • allows deﬁnition of custom formatters • provides callbacks for events •:test_started •:test_ﬁnished • etc. • e.g., junit_formatter

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exunit

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• elixir mutation testing library • goal • install exavier library • run mix exavier.test • mutate code • run tests • show mutation testing report exavier

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mutators • used by pitest (JVM) and mutant (Ruby) • e.g., following pitest, AOR1 (Arithmetic Operator Replacement Mutator #1) original mutated + - - + * / / * % *

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mutators

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mutators • e.g., following pitest, REMOVE_CONDITIONALS (Remove Conditionals Mutator) “The remove conditionals mutator will remove all conditionals statements such that the guarded statements always execute” original mutated

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mutators

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we’re ready

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exavier • Run code line coverage analysis for each module, sequentially • Mutate the code according to each available mutator For each module, in parallel: For each mutator, sequentially: 1. Mutate code with given mutator 2. Run tests (now against mutated code) 3. Report (% mutants survived vs. killed)

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exavier •mix exavier.test

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biggest drawback • currently mutating a single module all at once and running all tests against mutated module problem?

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biggest drawback • skews number of survived mutants, e.g.:

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rough edges • Macros • might be hard to provide useful output • may have to track macro expansion (if possible) • Edge cases, e.g.: • division by zero (Issue #7) • function references as division (Issue #2)

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future • get back to work on exavier • hopefully with the help of the community ⚠ ﬁrst order of business: ﬁx “biggest drawback”

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the end thank you questions? dnlserrano.dev