{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ViewPatterns #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE DataKinds #-}

import GHC.TypeLits
import Data.Proxy (Proxy(..))

pattern Append :: forall (p :: Symbol). KnownSymbol p => String -> String
pattern Append full <- (takePrefix (symbolVal (Proxy @p)) -> Just full)
  where Append full = symbolVal (Proxy @p) ++ full

takePrefix :: String -> String -> Maybe String
takePrefix prefix full
  | let (pre, post) = splitAt (length prefix) full
  , pre == prefix
  = Just post
  | otherwise
  = Nothing

foo :: String -> IO ()
foo (Append @"this" s) = putStrLn $ "this and then: " ++ s
foo s = putStrLn $ "something else: " ++ s

main :: IO ()
main = do
  foo "this is a string"
  foo "those aren't"