proptest.md

Proptest

• everything is built around the concept of a Strategy
• a Strategy can be a collection of other strategies, or a single Strategy
• a Strategy returns a ValueTree that shrinks or complicates a value within that strategy
  • eg in a number range, shrinking would approx try to do a binary search to figure out the failing threshold
• PROPTEST_CASES is equal to 256
• any for built in types
  • any::<u32>() is a Strategy for all u32s
• we can use prop_map to transform strategies
  • eg a function expects to parse strings as integers
  • the Strategy should be a u32 range, then prop_map-ped to a string
  • fn test_do_stuff(v in any::<u32>().prop_map(|v| v.to_string())
  • prop_map creates a new strategy which transforms every generated value with the provided function
  • the main way to generate new types
• prop_compose! to build struct
• prop_oneof! to build enums
  • Just for enums without interior values
  • Strategy + prop_map for enums with interior values
  • proptest will shrink to items earlier int he prop_oneof![...] list!!
    • sort simplest to most complex!
• Filters
  • Local: apply to a single strategy
  • Global: apply to the whole test case, the whole case is regenerated if it doesn't fit the filter
  • local: prop_filter!
  • global: prop_assume!, is called when the test case itself returns Err(TestCaseError::Reject)
  • a test with filter could theoretically run forever!
    • would run into Too many global rejects! error
• prop_recursive! for recursive types
  • first build non-recursive cases with prop_oneof![...]
  • then apply prop_recursive! on top of it to implement the recursive cases
• prop_flat_map combinator for strategies that are generated by other strategies
  • Higher-Order Strategies
  • prop_flat_map returns a strategy
  • used when parameters depend on each other (eg vector and index)
  • prop_compose! also comes in handy here, as it allows us to pass three argument lists (fn vec_and_idx()(...)(...) { ... })
• proptest_derive can be used to automate implementing Arbitrary for simple cases
• proptest stores the seed that was used to produce failing test cases

Notes

• fork and timeout
  • allows to run test cases in subprocess and limit how long they can run
  • #![proptest_config(ProptestConfig { fork: true, timeout: 100, .. PropTestConfig::default()}]
• Performance (Link)