Haskell Pipes and testing with HSpec

Question

I have written a program for a project that uses Pipes, which I love! I'm struggling to unit test my code however.

I have a series of functions of type Pipe In Out IO () (for example) that I wish to test with HSpec. How can I go about this?

For example, suppose I have this domain:

data Person = Person String Int | Unknown deriving (Show, Eq)
data Classification = Friend | Foe | Undecided deriving Show

and this Pipe:

classify :: Pipe Person (Person, Classification) IO ()
classify = do
    p@(Person name _) <- await
    case name of 
      "Alex" -> yield (p, Friend)
      "Bob" -> yield (p, Foe)
      _ -> yield (p, Undecided)

I would like to write a spec:

main = hspec $ do
  describe "readFileP" $ 
    it "yields all the lines of a file"
      pendingWith "How can I test this Pipe? :("

Answer 1

The trick is to use toListM from Pipes ListT monad transformer.

import Pipes
import qualified Pipes.Prelude as P
import Test.Hspec

data Person = Person String Int | Unknown deriving (Show, Eq)
data Classification = Friend | Foe | Undecided deriving (Show, Eq)

classify :: Pipe Person (Person, Classification) IO ()
classify = do
  p@(Person name _) <- await
  case name of 
    "Alex" -> yield (p, Friend)
    "Bob" -> yield (p, Foe)
    _ -> yield (p, Undecided)

The test, using the ListT transformer to convert the pipe to a ListT and asserting using HSpec:

main = hspec $ do
  describe "classify" $ do
    it "correctly finds friends" $ do
      [(p, cl)] <- P.toListM $ each [Person "Alex" 31] >-> classify
      p `shouldBe` (Person "Alex" 31)
      cl `shouldBe` Friend

Note, you don't have to use each, this could be a simple producer with a call to yield.