A first look at HMock

The other day I found Chris Smith's HMock: First Rate Mocks in Haskell (link to hackage) and thought it could be nice see if it can clear up some of the tests I have in a few of the Haskell projects at work. All the projects follow the pattern of defining custom monads for effects (something like final tagless) with instances implemented on a stack of monads from MTL. It's a pretty standard thing in Haskell I'd say, especially since the monad stack very often ends up being ReaderT MyConfig IO.

I decided to try it first on a single such custom monad, one for making HTTP requests:

class Monad m => MonadHttpClient m where
  mHttpGet :: String -> m (Status, ByteString)
  mHttpPost :: (Typeable a, Postable a) => String -> a -> m (Status, ByteString)

Yes, the underlying implementation uses wreq, but I'm not too bothered by that shining through. Also, initially I didn't have that Typeable a constraint on mHttpPost, it got added after a short exchange about KnownSymbol with Chris.

To dip a toe in the water I thought I'd simply write tests for the two effects themselves. First of all there's an impressive list of extensions needed, and then the monad needs to be made mockable:

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE ImportQualifiedPost #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}

makeMockable ''MonadHttpClient

After that, writing a test with HMock for mHttpGet was fairly straight forward, I could simply follow the examples in the package's documentation. I'm using tasty for organising the tests though:

httpGetTest :: TestTree
httpGetTest = testCase "Get" $ do
  (s, b) <- runMockT $ do
    expect $ MHttpGet "url" |-> (status200, "result")
    mHttpGet "url"
  status200 @=? s
  "result" @=? b

The effect for sending a POST request was slightly trickier, as can be seen in the issue linked above, but with some help I came up with the following:

httpPostTest :: TestTree
httpPostTest = testCase "Post" $ do
  (s, b) <- runMockT $ do
    expect $ MHttpPost_ (eq "url") (typed @ByteString anything) |-> (status201, "result")
    mHttpPost "url" ("hello" :: ByteString)
  status201 @=? s
  "result" @=? b