Cucumber for Haskell
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Cucumber for Haskell

An effort to bring Cucumber-ish BDD to Haskell.

Want to help? Join in at #hspec on freenode!


Let's start with a simple feature:

Feature: Beta reduction of lambda terms

  In order to be able to reduce lambda terms
  As a programmer
  I want a function that reduces lambda terms

  Scenario: Reducing a simple lambda term
    Given a lambda term "(λx.x)y"
    When I parse it
    And I reduce it
    And I pretty-print it
    Then the result should be "y"

This should result in the following test term:

test = (show . reduce . parse) "(λx.x)y" `shouldBe` "y"


test = flip shouldBe "y" (show (reduce (parse (id "(λx.x)y"))))

So how would we give step definitions for that? Currently I think Template Haskell is our best bet. (I do not really like it! If you have any other ideas, please let me know.)

$(given "a lambda term \"([^\"]*)\"") = id

$(when "I parse it") = parse

$(when "I reduce it") = reduce

$(when "I pretty-print it") = show

$(then_ "the result should be \"([^\"]*)\"") = flip shouldBe

This would then expand to something like:

given_0 :: String -> Maybe String
given_0 = case match input "a lambda term \"([^\"]*)\"" of
  [x] -> Just (id x)
  _   -> Nothing

when_0 :: String -> Maybe (String -> Term)
when_0 input
  | input == "I parse it" = Just parse
  | otherwise             = Nothing

when_1 :: String -> Maybe (Term -> Term)
when_1 input
  | input == "I reduce it" = Just reduce
  | otherwise              = Nothing

when_2 :: Show a => String -> Maybe (a -> String)
when_2 input
  | input == "I pretty-print it" = Just show
  | otherwise                    = Nothing

then_0 :: String -> Maybe Expectation
then_0 input = case match input "the result should be \"([^\"]*)\"" of
  [x] -> Just (flip shouldBe x)
  _   -> Nothing

In theory, constructing the test term from that is easy:

import Data.Maybe
import Control.Applicative

test = fromJust $
      then_0  "the result should be \"y\""
  <*> when_2  "I pretty-print it"
  <*> when_1  "I reduce it"
  <*> when_0  "I parse it"
  <*> given_0 "a lambda term \"(λx.x)y\""

What to do if multiple things are given?

The resulting values could than be applied to the term we get from the first When step.



Other efforts to implement a Cucumber-ish BDD framework for Haskell: