Extract the first paragraph of body after pandocCompiler processed the input.

teaser.hs

match "posts/*" $ do
    route $ setExtension ".html"
    compile $ pandocCompiler >>= saveSnapshot "content" >>= ...

postCtx = mconcat
    [ Context $ \k i ->
        if k == "teaser" then do
            i' <- loadSnapshot (itemIdentifier i) "content"
            return $ fromMaybe "" $ safeHead $ lines $ itemBody i'
          else
            fail "obscure error"
    ]

Use

field :: String -> (Item a -> Compiler String) -> Context a

E.g.

postCtx = mconcat
    [ field "teaser" $ \i -> do
        i' <- loadSnapshot (itemIdentifier i) "content"
        return $ fromMaybe "" $ safeHead $ lines $ itemBody i'
    ]

The mconcat is also unnecessary in this case, you can just have:

postCtx = field "teaser" $ \i -> do
    i' <- loadSnapshot (itemIdentifier i) "content"
    return $ fromMaybe "" $ safeHead $ lines $ itemBody i'