Elm testing: how to check if a Result is an Err?

Question

I'm writing tests for functions that return a Result. How do I test that it "is" an Err (or an Ok, for that matter) ?

\() -> Expect.equal expectedFailure (Err _)

does not work.

How does one decode a non-parameter?

This won't help you now, but there is an `Expect.err` that has been added for this exact case and will be in the next release https://github.com/elm-community/elm-test/pull/90 — robertjlooby, Jan 02 '17 at 18:41

Ryan Plant · Accepted Answer · 2017-01-01T12:07:34.813

5

There may well be a more elegant solution I've missed, but I personally would just write a helper function.

resultOk result =
    case result of
        Ok _ -> True
        Err _ -> False

then in your tests

Expect.true "expected this to be OK" (resultOk <| Ok "All good")
Expect.false "expected this to be an error" (resultOk <| Err "Oh no!")

Expect.true and Expect.false take a string to print if the test fails, and then an expression that should be true (in the case of Expect.true) or false (in the case of Expect.false).

edited Jan 01 '17 at 12:07

answered Jan 01 '17 at 08:30

Ryan Plant

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I wondered if I could decode the value inside the function if it was not a parameter. I think your way is more direct and I am creating helper functions like this. Thanks – Richard Haven Jan 02 '17 at 05:18

Elm testing: how to check if a Result is an Err?

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