Haskell,GHC 8:动态加载/导入模块

Question

我需要有类似的东西

-- Main.hs
module Main where

main :: IO ()
main = do
  <import Plugin>
  print Plugin.computation

有插件就好

-- Plugin.hs
module Plugin where

computation :: Int
computation = 4

但是,我需要将插件与主应用程序一起编译.他们需要一起部署.只有模块的导入(而不是编译)才能动态发生.

我发现动态加载已编译的Haskell模块 - GHC 7.6,它在GHC 8.0.2中运行得很好,除了它在执行应用程序时需要插件的源文件在当前工作目录中.

---

编辑(07.12.2017)

是否可以使用GHC API从String而不是文件加载模块？http://hackage.haskell.org/package/ghc-8.2.1/docs/GHC.html#t:Target表明这是可能的,但文档有很多漏洞,我找不到实际做到这一点的方法.如果可以实现,我可以使用file-embed将插件源文件包含到已编译的二进制文件中.例:

module Main where

-- Dynamic loading of modules
import GHC
import GHC.Paths ( libdir )
import DynFlags
import Unsafe.Coerce

import Data.Time.Clock (getCurrentTime)
import StringBuffer

pluginModuleNameStr :: String
pluginModuleNameStr = "MyPlugin"

pluginSourceStr :: String
pluginSourceStr = unlines
  [ "module MyPlugin where"
  , "computation :: Int"
  , "computation = 4"
  ]

pluginModuleName :: ModuleName
pluginModuleName = mkModuleName pluginModuleNameStr

pluginSource :: StringBuffer
pluginSource = stringToStringBuffer pluginSourceStr

main :: IO ()
main = do
    currentTime <- getCurrentTime
    defaultErrorHandler defaultFatalMessager defaultFlushOut $ do
      result <- runGhc (Just libdir) $ do
        dflags <- getSessionDynFlags
        setSessionDynFlags dflags
        let target = Target { targetId = TargetModule $ pluginModuleName
                            , targetAllowObjCode = True
                            , targetContents = Just ( pluginSource
                                                    , currentTime
                                                    )
                            }
        setTargets [target]
        r <- load LoadAllTargets
        case r of
          Failed    -> error "Compilation failed"
          Succeeded -> do
            setContext [IIDecl $ simpleImportDecl pluginModuleName]
            result <- compileExpr ("MyPlugin.computation")
            let result' = unsafeCoerce result :: Int
            return result'
      print result

然而,这导致了

<command-line>: panic! (the 'impossible' happened)
  (GHC version 8.0.2 for x86_64-apple-darwin):
    module ‘MyPlugin’ is a package module

---

编辑(08.12.2017)

我可以通过将源写入临时文件然后像链接的帖子(动态加载编译的Haskell模块 - GHC 7.6)一样加载它,将"插件"直接编译成最终的二进制文件.但是,如果插件从Hackage导入包,则这不会很好:

module Main where

import Control.Monad.IO.Class (liftIO)
import DynFlags
import GHC
import GHC.Paths (libdir)
import System.Directory (getTemporaryDirectory, removePathForcibly)
import Unsafe.Coerce (unsafeCoerce)

pluginModuleNameStr :: String
pluginModuleNameStr = "MyPlugin"

pluginSourceStr :: String
pluginSourceStr = unlines
  [ "module MyPlugin where"
  , "import Data.Aeson"
  , "computation :: Int"
  , "computation = 4"
  ]

writeTempFile :: IO FilePath
writeTempFile = do
  dir <- getTemporaryDirectory
  let file = dir ++ "/" ++ pluginModuleNameStr ++ ".hs"
  writeFile file pluginSourceStr
  return file

main :: IO ()
main = do
  moduleFile <- writeTempFile
  defaultErrorHandler defaultFatalMessager defaultFlushOut $ do
    result <- runGhc (Just libdir) $ do
      dflags <- getSessionDynFlags
      setSessionDynFlags dflags
      target <- guessTarget moduleFile Nothing
      setTargets [target]
      r <- load LoadAllTargets
      liftIO $ removePathForcibly moduleFile
      case r of
        Failed -> error "Compilation failed"
        Succeeded -> do
          setContext [IIDecl $ simpleImportDecl $ mkModuleName pluginModuleNameStr]
          result <- compileExpr "MyPlugin.computation"
          let result' = unsafeCoerce result :: Int
          return result'
    print result

有没有办法在例如MyPlugin包含语句时加载包import Data.Aeson？如果我将它添加到插件字符串,它将失败

/var/folders/t2/hp9y8x6s6rs7zg21hdzvhbf40000gn/T/MyPlugin.hs:2:1: error:
    Failed to load interface for ‘Data.Aeson’
    Perhaps you meant Data.Version (from base-4.9.1.0)
    Use -v to see a list of the files searched for.
haskell-loader-exe: panic! (the 'impossible' happened)
  (GHC version 8.0.2 for x86_64-apple-darwin):
  Compilation failed
CallStack (from HasCallStack):
  error, called at app/Main.hs:40:19 in main:Main

---

我的请求的原因是数据库支持:我们使用Persistent来访问数据库,并且需要动态导入来支持多个数据库(MySQL,PostgreSQL和SQLite),同时仍然允许最终用户只安装三个数据库服务器中的一个(带有换句话说:如果用户仅使用PostgreSQL,则不要求用户安装所有这些内容.只有在用户实际配置主应用程序以使用该模块时,才应加载特定于数据库的模块.

如果我不这样做import Database.Persist.MySQL,那么应用程序不需要安装MySQL.否则,应用程序失败,例如,

dyld: Library not loaded: 
/usr/local/opt/mysql/lib/libmysqlclient.20.dylib

在macOS上.

Answer 1

从外观上看，具有匹配模块名称的文件必须存在 - 文件的内容是什么似乎并不重要。

在 Linux 上，我什至可以将其设置为 /dev/null 的符号链接，并且一切正常，但其自身的符号链接则不行。