使用OverloadedLabels的QuasiQuotes

Question

我正在尝试使用here包中的OverloadedLabels和QuasiQuotes。使用普通镜头可以工作，但#foo在编译过程中会因解析错误而失败。field @"foo"也是如此。这是不是有更深层次的原因，或者可能是here的内插式解析器中的错误？

{-# language DataKinds #-}
{-# language DeriveGeneric #-}
{-# language DerivingStrategies #-}
{-# language OverloadedLabels #-}
{-# language OverloadedStrings #-}
{-# language TypeApplications #-}
{-# language QuasiQuotes #-}

import Control.Lens
import Data.Text (Text)
import qualified Data.Text.IO as T
import Data.Generics.Product
import Data.Generics.Labels
import Data.String.Here
import GHC.Generics (Generic)

data Test = Test
  { name :: Text
  } deriving stock (Eq, Show, Generic)

_name :: Lens' Test Text
_name f (Test a) = fmap (\a' -> Test a') (f a)

t :: Test
t = Test "test"

test :: IO ()
test = do
  -- ok
  T.putStrLn $ t ^. field @"name"
  T.putStrLn $ t ^. #name
  putStrLn [i|${t ^. _name}|]

  -- parse error
  putStrLn [i|The name is ${t ^. field @"name"}|]
  putStrLn [i|The name is ${t ^. #name}|]

#name的错误

test.hs:36:12: error:
    • Exception when trying to run compile-time code:
        Failed to parse interpolated expression in string: The name is ${t ^. #name}
(line 1, column 25):
0
SrcLoc "" 1 6
Parse error in expression: t ^.

CallStack (from HasCallStack):
  error, called at src/Data/String/Here/Interpolated.hs:64:33 in here-1.2.13-HU0AD0x0dD36rY9YuL1gwE:Data.String.Here.Interpolated
      Code: Language.Haskell.TH.Quote.quoteExp
              i "The name is ${t ^. #name}"
    • In the quasi-quotation: [i|The name is ${t ^. #name}|]
   |
36 |   putStrLn [i|The name is ${t ^. #name}|]
   |

Answer 1

看起来haskell-src-meta还不支持OverloadedLabels。haskell-src-exts解析器有an OverloadedLabels case，但是haskell-src-meta在the ToExp instance for Exp中没有它的用例。我猜来自haskell-src-meta的“不受支持”的错误消息正在被here中的错误处理所吞噬。

Answer 2

为了防止其他人偶然发现这一点，我在haskell-src-meta中创建了一个PR来解决这个问题：https://github.com/DanBurton/haskell-src-meta/pull/19