One line `deriving Show` for all types in the current file

deriveShow.hs

{-# LANGUAGE TemplateHaskell #-}
import Data.DeriveTH
import Data.Derive.Show
import Language.Haskell.TH.Module.Magic

newtype Foo = Foo (Int, String)
data Bar a = Bar Int | Baz a (Bar a)

foo :: Foo -> Int -> String
foo (Foo (n, s)) x = if n == x then s else ""

$(concat `fmap` (mapM (deriveFromDec makeShow) =<< declarations) )