用Hakyll的MonadMetadata进行测试

Question

对于Hakyll代码库，我编写了一些助手方法，并开始围绕新的方法添加一些HSpec单元测试，例如：

-- | Reject an item unless @fieldName@ is set to "true"
unlessEnabled :: MonadMetadata m
              => String
              -> Item a
              -> m Bool
unlessEnabled fieldName item = do
    maybeValue <- getMetadataBool (itemIdentifier item) fieldName
    return $ maybe True not maybeValue

-- | Try to look up a boolean field ("true" maps to @Just True@)
getMetadataBool :: MonadMetadata m
                => Identifier
                -> String
                -> m (Maybe Bool)
getMetadataBool ident name = do
    maybeString <- getMetadataField ident name
    return $ ((== "true") . map toLower) <$> maybeString

现在，为测试创建一个Item或Identifier是相当容易的，但我不知道在运行Hspec时，MonadMetadata在哪里。

我见过testCompiler，感觉它可能是可复制的/有用的( instance)，但我已经超出了我的Haskell深度.

Answer 1

最后(多亏了@Bergi的建议)，我得到了一些有用的东西，然后整理了一下：

{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}

type KeyedValuesOf a = [(String, a)]

-- | Create Metadata from basic list of key-values -- stolen from Hakyll itself.
meta :: Yaml.ToJSON a => KeyedValuesOf a -> Metadata
meta pairs = HMS.fromList [(T.pack k, Yaml.toJSON v) | (k, v) <- pairs]

-- | A 'Reader' class of our own, with a specialised environment
--   for key-value metadata pairs.
newtype MonadMetadataReader a =
    MonadMetadataReader {runMonadMetadataReader :: KeyedValuesOf String -> a}
        deriving (Functor, Applicative, Monad)

-- | Simple 'Metadata' holder.
-- There are never any matches found,
-- and all metadata (for all items)
-- comes from from the Reader-like environment at setup.
instance MonadMetadata MonadMetadataReader where
    getMetadata identifier = MonadMetadataReader meta

    getMatches pattern = return []

它允许进行以下简单的单元测试：

it "enables for a value of \"false\"" $ do
    let enabled = runMonadMetadataReader (unlessEnabled "key" item) [("key", "false")]
    enabled `shouldBe` True

it "disables for a value of \"true\"" $ do
    let enabled = runMonadMetadataReader (unlessEnabled "key" item) [("key", "true")]
    enabled `shouldBe` False

it "enables if no metadata found" $ do
    let enabled = runMonadMetadataReader (unlessEnabled "key" item) []
    enabled `shouldBe` True