Type Level String Concatenation

Example.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}

import TypeLevelString

import GHC.TypeLits
import Language.Haskell.TH
import Language.Haskell.TH.Syntax

type family (++) (as :: [k]) (bs :: [k]) :: [k] where
  (++) a '[] = a
  (++) '[] b = b
  (++) (a ': as) bs = a ': (as ++ bs)

type Foo = [tstr|"Foo"|]
type Bar = [tstr|"Bar"|]
type FooBar = Foo ++ Bar

TypeLevelString.hs

{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE ScopedTypeVariables #-}

module TypeLevelString (
  tstr,
) where

import Text.Read
import Data.Char

import Language.Haskell.TH
import Language.Haskell.TH.Quote

stype :: String -> Q Type
stype str = do
  case readEither str of
    Left err -> fail (show err)
    Right n -> do
      let chars = fmap (fromIntegral . ord) n
      let tcons x y = AppT (AppT PromotedConsT x) y
      let tnil = PromotedNilT
      return $ foldr tcons tnil (fmap (LitT . NumTyLit) chars)

tstr :: QuasiQuoter
tstr = QuasiQuoter undefined undefined stype undefined