A.hs

{-# LANGUAGE TemplateHaskell #-}

module A where

import           Language.Haskell.TH

makeType :: String -> DecsQ
makeType t = do
    let name = mkName t
    pure [DataD [] name [] Nothing [NormalC name []] []]

useType :: String -> String -> DecsQ
useType t f = do
    let tname = mkName t
        fname = mkName f
    sequence
        [ sigD fname [t| $(conT tname) -> IO () |]
        , funD fname [clause [wildP] (normalB [| print $(stringE t) |]) []]
        ]

makeAndUse :: String -> DecsQ
makeAndUse t =
    concat <$> sequenceA
        [ makeType t
        , useType t ("show" ++ t)
        ]

C.hs

{-# LANGUAGE TemplateHaskell #-}

import           A

-- makeType "Hello"
-- useType "Hello" "showHello"
makeAndUse "Hello"

main :: IO ()
main = showHello Hello