Haskell使用gitlib操作Git时报HasLgRepo实例缺失错误如何解决

问题修复说明 #

两个编译错误的修复逻辑如下：

1. 第一个报错是因为blobContents是普通字段访问，不属于Monadic操作，不需要用<-绑定，直接用let声明即可
2. 第二个报错是因为lookupBlob依赖仓库上下文，属于ReaderT LgRepo IO 类型的操作，不能放到lift包裹的纯IO环境中执行，仅需要对最终的IO输出操作使用lift即可。

修正后可编译完整代码 #

{-# LANGUAGE OverloadedStrings #-}

module GitOperations where

import Git
import Git.Libgit2 (lgFactory, LgRepo(..) )
import qualified Data.Text as T
import Control.Monad.Trans.Reader
import Control.Monad.Trans
import Data.ByteString.Char8 as BS

repositoryOptions :: RepositoryOptions
repositoryOptions = RepositoryOptions { repoPath = ".git"
                                , repoWorkingDir = Just "."
                                , repoIsBare = False
                                , repoAutoCreate = True}
  
main :: IO ()
main = do
  repo <- openRepository lgFactory repositoryOptions
  runReaderT listHEADFiles repo

listHEADFiles:: ReaderT LgRepo IO ()
listHEADFiles = do 
  ref <- resolveReference $ T.pack "HEAD"
  case ref of
    Nothing -> fail $ "Could not resolve reference named 'HEAD'"
    Just reference -> do
      obj <- lookupObject reference
      case obj of
        CommitObj commit -> do 
          objects <- listAllObjects (Just $ commitOid commit) (commitOid commit)
          mapM_ (\o -> case o of 
                  BlobObjOid boOid -> do
                    blob <- lookupBlob boOid
                    let bc = blobContents blob
                    case bc of 
                      BlobString bs -> lift $ BS.putStrLn bs
                  _ -> return ()
                ) objects
        _ -> fail $ "'HEAD' is not a commit object"

如果需要获取文件路径而非文件内容，可改用treeEntries遍历提交对应的树对象，即可拿到文件路径与Blob的对应关系。

内容的提问来源于stack exchange，提问作者altern