TH.hs

{-# language TemplateHaskell #-}
{-# options_ghc -Wno-unused-imports #-}
{-|
Include the value of an environment variable in the binary at compile time.

== Rationale
Users might want to embed secrets (e.g. API keys, database connection strings) inside production artifacts without checking these into the repository.

== Example

@
import IncludeEnv.TH (includeEnv)

$(`includeEnv` \"SHELL\" "shl")
shl :: String

main :: IO ()
main = putStrLn $ unwords ["your current shell :", shl]
@

-}
module IncludeEnv.TH (
  includeEnv
  , includeEnvLenient
  , includeEnvMaybe
  ) where

import System.Environment (lookupEnv)

-- template-haskell
import Language.Haskell.TH (runIO, runQ)
import Language.Haskell.TH.Syntax (Q, Exp(..), Dec(..), Pat(..), Name, mkName, Body(..), Lit(..), reportWarning)
import Language.Haskell.TH.Lib (valD)


-- | Include the value of an environment variable at compile time.
--
-- A fresh variable of type `String` is declared each time this is computation is evaluated.
--
-- Note : will crash with `error` if the environment variable is not found.
includeEnv :: String -- ^ name of environment variable to be looked up
           -> String -- ^ name of new value
           -> Q [Dec]
includeEnv e varname = do
  mstr <- runIO $ lookupEnv e
  case mstr of
    Just str -> decl varname str
    Nothing -> error $ unwords ["Cannot find variable", e, "in the environment."]
    where
      decl :: String -> String -> Q [Dec]
      decl n x = pure [dq] where
        dq = ValD qpat qbody []
        qpat = VarP (mkName n)
        qbody = NormalB (LitE (StringL x))

-- | Like 'includeEnv' but only prints a warning if the environment variable cannot be found.
--
-- NB : If the lookup fails, the declared value will contain an _empty string_ .
includeEnvLenient :: String -- ^ name of environment variable to be looked up
                  -> String -- ^ name of new value
                  -> Q [Dec]
includeEnvLenient e varname = do
  mstr <- runIO $ lookupEnv e
  case mstr of
    Just str -> decl varname str
    Nothing -> do
      reportWarning $ unwords ["*** WARNING : Cannot find variable", e, "in the environment."]
      decl varname ""
    where
      decl :: String -> String -> Q [Dec]
      decl n x = pure [dq] where
        dq = ValD qpat qbody []
        qpat = VarP (mkName n)
        qbody = NormalB (LitE (StringL x))

-- | Like 'includeEnv' but produces a 'Maybe String'
--
-- Use case : The program needs to be compiled against two different environments that may have different sets of environment variables. 'includeEnvMaybe' lets you account for the results of multiple such lookups at runtime.
--
-- @since 0.4.0.0
includeEnvMaybe :: String -- ^ name of environment variable to be looked up
                -> Q Exp
includeEnvMaybe e = do
  mstr <- runIO $ lookupEnv e
  [| mstr |]