Fluent language in FsStory

A nice feature for a story runner is to be able to provide arguments in a story sentence, like this:

ATableWithNumberOfLegs 4

This example is not hard to understand, but there’s some mental translation going on, since the order of the words is all screwed up. Let’s try another one:

TheNumberOfLegsOfATableIs 4

Better, but still not good. First, it’s longer than the previous example. Second, even if the grammar is correct, the word order is, well, unusual.. 😉

What about this?

ATableWith 4 Legs

Now we’re talking!

Here’s a bigger and more complete example:

[<Fact>]
let tableLegsScenario =
given (ATableWith 4 Legs)
|> whens (ICutOf 1 Leg)
|> thens (ItHasOnly 3 LegsLeft)

Note: As far as I know, RSpec/Cucumber is the only story runner(s) that is able to use variables inside a story sentence.

The nice thing about this is that I didn’t have to change FsStory itself to make it work. Not a single line. So, what’s the trick?

If you split up a story sentence like this, you have to prepare the step definition code (the behind-the-scenes code) in a certain way:

let ATableWith = fun n _ -> … do something here …

..or if you prefer, without a lambda:

let ATableWith n _ = … do something here …

Then you have to define Legs:

let Legs = id

As you see, in this case, it was just a matter of discovering the usage, rather than implementing it in the language. Honestly, I had no idea of this usage when I started to write on FsStory. This is clearly one of the reasons why I like internal DSLs!

Exercise: How would you implement the step definition for the following story sentence?

given (ATableWith 4 LegsAnd 2 Chairs)

Cheers!

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About gustafnk

Developer at Jayway, REST/Hypermedia, AWD, Software apprentice, Dvorak user, Christian atheist, Zizek fan. I'm on twitter: @gustaf_nk
This entry was posted in bdd, F#, FsStory. Bookmark the permalink.

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