[Feature request] Creating custom constraints

[C0DK]

[Feature request] Creating custom constraints

[CaptnCodr]

Hello @C0DK ,
it depends what type somePerson is.

I assume that somePerson is a complex type like a record or class. In case of that WithName and/or HaveName must support the same type as somePerson.

Even so, I recommend assertions as simply as possible e.g.:

type Person = { Name: string; Age: int }

let somePerson = ... // function that returns: { Name = "test"; Age = 34 }

somePerson.Name
|> should equal "test"

// and perhaps:
somePerson.Age 
|> should equal 34

[C0DK]

I'm new to FSharp so I might just take things to the extreme, but having a test that is simply one expression is neat (in my opinion), so it was simply to not have to access the property. Currently, I was doing:

somePerson
|> withName SomeName
|> getName
|> Should equal SomeName

Should I just use let more rather than doing everything as a singular expression?

[CaptnCodr]

Should I just use let more rather than doing everything as a singular expression?

No, that was just an example on my side for better readability.
You can also write from my example:

somePerson
|> fun p -> p.Name
|> should equal "test"

An extra constraint or operator in FsUnit would be too special, btw.
Use single/short functions to have a little toolset in your implementations.

[C0DK]

alright. Consider it closed then :) I might raise another ticket if i have a more complex case. In a c# project where we use NUnit, we do have custom constraints for specific things, like checking whether a StatusCode is "Succesful" (200 <= x < 300) based on NUnits custom constraints https://docs.nunit.org/articles/nunit/extending-nunit/Custom-Constraints.html - and i think that was my offset. but in this case i do see your point.

Thank you :) I also didn't think of using a fun p -> p.Name rather than an explicit function defined above. neat!