Port of elm-microthesis to Haskell.
Both are just (purely functional) ports of Minithesis, which is itself a distilled down version the core idea in Hypothesis.
That idea is:
- when generating, remember integers we picked from the PRNG
- give generators the ability to run on hardcoded lists of those integers (instead of running on the PRNG)
- there is a single internal shrinker, which instead of working on the generated valus, works on the recording of the PRNG history
- after every shrink attempt the generators run again (with <100% success rate), and the generated value is tested for the user property.
- since the internal shrinker knows nothing about the generators, shrinker and generators agree to follow the rule "Shortlex-smaller recording leads to simpler generated values"
See also my talk describing this approach: How to do Property based Testing Shrinkers Right @ HaskellX 2022
Tests in app/Main.elm:
M.test "Shrunk values hold generation invariants"
(fmap (* 100) $ Gen.uniformInt 20)
(\n -> n < 321)
M.test "Monadic bind can shrink both 'before' and 'after'"
(naiveListGen (Gen.uniformInt 20))
(\list -> list == List.sort list)
result in test results like the following:
Seed: 718886798816897144
Shrunk values hold generation invariants
Fails with counterexample: 400
Recording: Recording [4]
Shrink history:
- Step {value = 1400, recording = Recording [14], shrinkerUsed = "Initial"}
- Step {value = 400, recording = Recording [4], shrinkerUsed = "Minimize with binary search (i=0)"}
Monadic bind can shrink both 'before' and 'after'
Fails with counterexample: [1,0]
Recording: Recording [2,1,0]
Shrink history:
- Step {value = [14,4,16,12,12], recording = Recording [5,14,4,16,12,12], shrinkerUsed = "Initial"}
- Step {value = [4,16,12,12], recording = Recording [4,4,16,12,12], shrinkerUsed = "Delete chunk and maybe decrement previous (i=1..1)"}
- Step {value = [4,16,0,0], recording = Recording [4,4,16,0,0], shrinkerUsed = "Replace chunk with zero (i=3..4)"}
- Step {value = [4,0,0,0], recording = Recording [4,4,0,0,0], shrinkerUsed = "Replace chunk with zero (i=2..3)"}
- Step {value = [0,4,0,0], recording = Recording [4,0,4,0,0], shrinkerUsed = "Sort chunk (i=1..2)"}
- Step {value = [0,1,0,0], recording = Recording [4,0,1,0,0], shrinkerUsed = "Minimize with binary search (i=2)"}
- Step {value = [0,1,0], recording = Recording [3,0,1,0,0], shrinkerUsed = "Minimize with binary search (i=0)"}
- Step {value = [0,1,0], recording = Recording [3,0,1,0], shrinkerUsed = "Delete chunk and maybe decrement previous (i=4..4)"}
- Step {value = [1,0], recording = Recording [2,1,0], shrinkerUsed = "Delete chunk and maybe decrement previous (i=1..1)"}