An Observer examines some particular part of the world so that a claim can be evaluated with respect to it.

Observe some aspect of the model of the program whose behavior is being described in this scenario.

For example, if the program model looks like { score: 27 }, then you could observe the score like so:

Spec.it "has the correct score" (
  observeModel .score
    |> Spec.expect 
      (Spec.Claim.isEqual Debug.toString 27)
)

Check out Spec.Markup, Spec.Http, Spec.Port, and Spec.Witness for observers that evaluate claims with respect to the world outside the program.

Create a new Observer that will evaluate a Claim with respect to the successful Result observed by the given Observer. If the Result fails then the Claim will be rejected.

Create a new Observer that will map the Report created if the Claim is rejected.

Create a new Observer that evaluates a claim with respect to a particular aspect of the world observed by the given observer.

Consider an Observer model (Maybe HtmlElement), which observes all aspects of a Maybe HtmlElement. Create an Observer that focuses on just the text of an existing HtmlElement like so:

Spec.Markup.observeElement
  |> Spec.Markup.query << by [ id "some-element" ]
  |> focus Spec.Claim.isSomethingWhere
  |> focus Spec.Markup.text
  |> Spec.expect (
    Spec.Claim.isEqual Debug.toString "Something cool!"
  )

If the element doesn't exist, then the isSomethingWhere claim would fail, and subsequent claims would not be evaluated by the observer.

You could then abstract this into a helper function like so:

expectText : Claim String -> Observer model (Maybe HtmlElement) -> Expectation model
expectText claim observer =
  observer
    |> focus Spec.Claim.isSomethingWhere
    |> focus Spec.Markup.text
    |> Spec.expect claim

And then you could write:

Spec.Markup.observeElement
  |> Spec.Markup.query << by [ id "some-element" ]
  |> expectText (
    Spec.Claim.isEqual Debug.toString "Something cool!"
  )