tv th-env: init at 1.0.0

tv/5pkgs/haskell/th-env/default.nix

@@ -0,0 +1,10 @@
+{ mkDerivation, base, stdenv, template-haskell, text }:
+mkDerivation {
+  pname = "th-env";
+  version = "1.0.0";
+  src = ./.;
+  libraryHaskellDepends = [ base template-haskell text ];
+  homepage = "https://stackoverflow.com/q/57635686";
+  license = "unknown";
+  hydraPlatforms = stdenv.lib.platforms.none;
+}

tv/5pkgs/haskell/th-env/src/THEnv.hs

@@ -0,0 +1,49 @@
+{-# 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

tv/5pkgs/haskell/th-env/th-env.cabal

@@ -0,0 +1,20 @@
+name: th-env
+version: 1.0.0
+-- license: https://creativecommons.org/licenses/by-sa/4.0/
+license: OtherLicense
+author: https://stackoverflow.com/users/9348482
+homepage: https://stackoverflow.com/q/57635686
+maintainer: tv <tv@krebsco.de>
+build-type: Simple
+cabal-version: >=1.10
+
+library
+  hs-source-dirs: src
+  build-depends:
+    base,
+    template-haskell,
+    text
+  exposed-modules:
+    THEnv
+  default-language: Haskell2010
+  ghc-options: -O2 -Wall