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decodeelm
Richard Haven
Richard Haven

Reputation: 1154

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

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?

Upvotes: 3

Views: 412

Answers (2)

jfMR
jfMR

Reputation: 24738

There are nowadays specialized Expectations – Expect.ok and Expect.err – for expressing the expected variant (Ok or Err) of a Result value:

Expect.ok : Result a b -> Expectation
Expect.err : Result a b -> Expectation

Given an expectedFailure value of type Result a b, you can set up the expectation for test of expectedFailure being the Err variant as:

\_ -> expectedFailure |> Expect.err

Upvotes: 2

Ryan Plant
Ryan Plant

Reputation: 1047

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).

Upvotes: 5

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