[xperiandri]

Proposed Changes

Adds Assert.fs to the test project: extension members of the MSTest Assert class for F# option, voption and Result values. AssertExtensions is the root module of the file and opens automatically in the tests.

• WantSome, WantValueSome, WantOk and WantError return the unwrapped value or fail the test with the message of the caller. WantOk and WantError add the error or the value to the message. A test goes on with the value in one expression, instead of matching on the case and failing by hand.
• IsSome, IsNone, IsValueSome, IsValueNone, IsOk and IsError check the case and discard the value.
• IsDefaultOf, OkEquals and ErrorEquals compare a value with the default of its type, or the content of a result with an expected value.

The tests that are already in main use the assertions where they took the value out of an option by hand:

• The expected elements of replace and replaceWith of FlatList and FlatListBuilder come from WantValueSome where they came from ValueOption.get.
• FlatListOracles.messageDetail fails the test, where it threw InvalidOperationException, when the action does not throw.
• tryTail of the sequence-like access contract and of the Option namespace tests asserts the case and takes the tail in one expression.

Types of changes

What types of changes does your code introduce to FSharp.Collections.Immutable?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

A change of the test project only: no code of the library changes.

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• Build and tests pass locally
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have added necessary documentation (if appropriate)

Further comments

AssertExtensionsTests has 18 tests. It checks each member on the case that it accepts and on the case that it rejects, and a failure must carry the message of the caller. Three deliberate defects each made tests fail: an IsNone that never fails, a WantOk that drops the message and an OkEquals that never compares. The converted tests keep their expectations, so the number of tests stays the same.

An optional message that the caller leaves out is null, while Assert.Fail takes a text that is not null, so the assertions that fail pass an empty text then. The parameter stays string | null, as an F# optional argument of a reference type.

The test category Assert extensions names the area, as the rules of the test categories ask.

The open pull requests that unwrap options in their tests, such as #31, can take WantSome and WantValueSome from here instead of local helpers.

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