Snapshot/expect testing framework for F#
Get Started
$ dotnet add package WoofWare.ExpectReadme
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
}
Updating an individual snapshot
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
}