FsCheck: How to generate test data that depends on other test data?

Question

FsCheck has some neat default Arbitrary types to generate test data. However what if one of my test dates depends on another?

For instance, consider the property of string.Substring() that a resulting substring can never be longer than the input string:

[Fact]
public void SubstringIsNeverLongerThanInputString()
{
    Prop.ForAll(
        Arb.Default.NonEmptyString(),
        Arb.Default.PositiveInt(),
        (input, length) => input.Get.Substring(0, length.Get).Length <= input.Get.Length
    ).QuickCheckThrowOnFailure();
}

Although the implementation of Substring certainly is correct, this property fails, because eventually a PositiveInt will be generated that is longer than the genereated NonEmptyString resulting in an exception.

Shrunk: NonEmptyString "a" PositiveInt 2 with exception: System.ArgumentOutOfRangeException: Index and length must refer to a location within the string.

I could guard the comparison with an if (input.Length < length) return true; but that way I end up with lots of test runs were the property isn't even checked.

How do I tell FsCheck to only generate PositiveInts that don't exceed the input string? I presume I have to use the Gen<T> class, but it's interface is just hella confusing to me... I tried the following but still got PositiveInts exceeding the string:

var inputs = Arb.Default.NonEmptyString();
// I have no idea what I'm doing here...
var lengths = inputs.Generator.Select(s => s.Get.Length).ToArbitrary();

Prop.ForAll(
    inputs,
    lengths,
    (input, length) => input.Get.Substring(0, length).Length <= input.Get.Length
).QuickCheckThrowOnFailure();

Soble

fscheck.github.io/FsCheck/TestData.html – Cosmetic

I looked through this site multiple times already and still couldn't figure out a way to do what I want. – Soble

Answer 1

You can create generators which depend on values generated from another using SelectMany. This also allows you to use the LINQ query syntax e.g.

var gen = from s in Arb.Generate<NonEmptyString>()
          from i in Gen.Choose(0, s.Get.Length - 1)
          select Tuple.Create(s, i);

var p = Prop.ForAll(Arb.From(gen), t =>
{
    var s = t.Item1.Get;
    var len = t.Item2;
    return s.Substring(0, len).Length <= s.Length;
});

Check.Quick(p);

Ptah

Is this something I could have found in the official docs? If not could you please add your reference? – Soble

@GoodNightNerdPride - I don't think I've seen much documentation for the C# interop, so I probably found out based on reading the source and an educated guess from the gen builder. F# computation expressions (like gen { ... }) map quite closely to the C# LINQ query syntax since they are built from the same syntactic pattern (although F#'s supports more features). – Ptah

There aren't many examples in the docs for either the F# computation expressions or the LINQ syntax. Here's an example github.com/fscheck/FsCheck/blob/master/examples/… LINQ is mentioned in the second paragraph of the TestData page, and there is a small example here: fscheck.github.io/FsCheck/TestData.html#Generators – Denny