Generator is not used during shrinking.

[niilohlin]

Version

SwiftCheck (0.10.0)

Environment

Xcode 9.4.1. iOS target.

Description

When a property fails with a custom Gen<T> the generator is not used while shrinking. And therefore calling the test block without it.

Steps To Reproduce

sample code:

func stringDoesNotContainA(_ string: String) -> Bool {
    precondition(!string.isEmpty)
    return !string.contains("a" as Character)
}

class SwiftCheckTests: XCTestCase {
    func testSwiftCheck() {
        property("string is not \"a\"") <- forAll(String.arbitrary.suchThat { !$0.isEmpty }, pf: stringDoesNotContainA)
    }
}

Expected Result

The function stringDoesNotContainA is expected to only be called with non empty strings.

Actual Result

After the test fails. The function is called with an empty String and execution halts.

[niilohlin]

After looking thorough the code and the QuickCheck code I realize that what I'm asking to do is probably not possible to do automatically. But instead forAllShrink can be used with a custom shrinker that filters the values according to my predicate. Like this:

func stringDoesNotContainA(_ string: String) -> Bool {
    precondition(!string.isEmpty)
    return !string.contains("a" as Character)
}

class SwiftCheckTests: XCTestCase {
    func testSwiftCheck() {
        let predicate = { (str: String) -> Bool in
            return !str.isEmpty
        }
        let stringShrinker = { (str: String) -> [String] in
            String.shrink(str).filter(predicate)
        }

        property("string is not \"a\"") <- forAllShrink(String.arbitrary.suchThat(predicate) , shrinker: stringShrinker, f: stringDoesNotContainA)
    }
}

This code will work as expected.

[CodaFi]

Hi @niilohlin, apologies for the delay in getting to this.

This is the expected behavior. shrink is defined as a static requirement on the type that conforms to arbitrary. We do not derive shrinkers from generators as other, more recent, frameworks may try to do. Instead, you should write a modifier type for strings that pass your predicate and add your own shrinker there.