module Main where

import qualified Data.ByteString.Lazy as S
import qualified Data.Text as T
import qualified Data.Text.Lazy.Encoding as E

import Data.Text.Template

-- | Create 'Context' from association list.
context :: [(T.Text, T.Text)] -> Context
context assocs x = maybe err id . lookup x $ assocs
  where err = error $ "Could not find key: " ++ T.unpack x

main :: IO ()
main = S.putStr $ E.encodeUtf8 $ substitute helloTemplate helloContext
  where
    helloTemplate = T.pack "Hello, $name!\n"
    helloContext  = context [(T.pack "name", T.pack "Joe")]