[英]How to properly communicate compile-time information to Template Haskell functions?
我需要将一些信息从编译脚本传达到Template Haskell中。 当前,编译脚本将信息保存在系统环境中,因此我只是使用包装在runIO
System.Environment.getEnvironment
来runIO
。 是否有更好的方法,例如将一些参数传递给ghc
(类似于C预处理器的-D...
),或者为此专门在TH中设计了某种方法?
由于有这么多人对此问题感兴趣,所以我将添加当前的方法,也许有人会觉得它有用。 最好的方法可能是TH允许在GHC的命令行上读取-D
参数,但是似乎目前没有实现。
一个简单的模块允许TH读取编译时环境。 辅助功能还允许读取文件。 例如,从环境中读取配置文件的路径,然后读取该文件。
{-# LANGUAGE TemplateHaskell #-}
module THEnv
(
-- * Compile-time configuration
lookupCompileEnv
, lookupCompileEnvExp
, getCompileEnv
, getCompileEnvExp
, fileAsString
) where
import Control.Monad
import qualified Data.Text as T
import qualified Data.Text.IO as T
import Language.Haskell.TH
import Language.Haskell.TH.Syntax (Lift(..))
import System.Environment (getEnvironment)
-- Functions that work with compile-time configuration
-- | Looks up a compile-time environment variable.
lookupCompileEnv :: String -> Q (Maybe String)
lookupCompileEnv key = lookup key `liftM` runIO getEnvironment
-- | Looks up a compile-time environment variable. The result is a TH
-- expression of type @Maybe String@.
lookupCompileEnvExp :: String -> Q Exp
lookupCompileEnvExp = (`sigE` [t| Maybe String |]) . lift <=< lookupCompileEnv
-- We need to explicly type the result so that things like `print Nothing`
-- work.
-- | Looks up an compile-time environment variable and fail, if it's not
-- present.
getCompileEnv :: String -> Q String
getCompileEnv key =
lookupCompileEnv key >>=
maybe (fail $ "Environment variable " ++ key ++ " not defined") return
-- | Looks up an compile-time environment variable and fail, if it's not
-- present. The result is a TH expression of type @String@.
getCompileEnvExp :: String -> Q Exp
getCompileEnvExp = lift <=< getCompileEnv
-- | Loads the content of a file as a string constant expression.
-- The given path is relative to the source directory.
fileAsString :: FilePath -> Q Exp
fileAsString = do
-- addDependentFile path -- works only with template-haskell >= 2.7
stringE . T.unpack . T.strip <=< runIO . T.readFile
可以这样使用:
{-# LANGUAGE TemplateHaskell #-}
import THEnv
main = print $( lookupCompileEnvExp "DEBUG" )
然后:
runhaskell Main.hs
打印Nothing
; DEBUG="yes" runhaskell Main.hs
打印Just "yes"
。
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