WoofWare.Expect

An expect-testing library for F#. (Also known as "snapshot testing".)

Current status

The basic mechanism works. Snapshot updating is vibe-coded with Opus 4 and is purely text-based; I didn't want to use the F# compiler services because that's a pretty heavyweight dependency which should be confined to a separate test runner entity. It's not very well tested, and I expect it to be kind of brittle.

How to use

See the tests.

[<Test>]
let ``This test fails: JSON documents are not equal`` () =
    expect {
        snapshotJson "123"
        return 124
    }

[<Test>]
let ``This test passes: JSON documents are equal`` () =
    expect {
        snapshotJson "  123  "
        return 123
    }

[<Test>]
let ``This test fails: plain text comparison of ToString`` () =
    expect {
        snapshot "  123  "
        return 123
    }

If a snapshot is failing, add a ' to the expect builder and rerun. The rerun will throw, but it will update the snapshot; then remove the ' again to put the test back into "assert snapshot" mode.

[<Test>]
let ``Example of automatically updating`` () =
    // This test fails...
    expect {
        snapshotJson "123"
        return 124
    }

    // so make this change:
    expect' {
        snapshotJson "123"
        return 124
    }

    // and rerunning converts the result to this:
    expect' {
        snapshotJson @"124"
        return 124
    }

    // That test will always throw, because it's not in "assertion" mode but in "update" mode;
    // so finally, remove the `'` again. This test now passes!
    expect {
        snapshotJson @"124"
        return 124
    }

Limitations

• The snapshot updating mechanism requires you to use verbatim string literals. While the test assertions will work correctly if you do snapshot ("foo" + "bar" + f 3), for example, the updating code is liable to do something undefined in that case. Also do not use format strings ($"blah").

Licence

MIT.