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.