An example of QualifiedStrings to validate the string literal at compile-time

AsciiOnly.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE RequiredTypeArguments #-}
{-# LANGUAGE UndecidableInstances #-}
module AsciiOnly where
import Data.Kind
import Data.Type.Ord
import Data.Proxy
import GHC.TypeLits

type IsAsciiOnly :: Symbol -> Constraint
type IsAsciiOnly s = IsAsciiOnlyHelper (UnconsSymbol s)

type IsAsciiOnlyHelper :: Maybe (Char, Symbol) -> Constraint
type family IsAsciiOnlyHelper m where
  IsAsciiOnlyHelper Nothing = ()
  IsAsciiOnlyHelper (Just '(c, rest)) = (c <= '\x7f', IsAsciiOnly rest)

fromString :: forall (s :: Symbol) -> (IsAsciiOnly s, KnownSymbol s) => String
fromString s = symbolVal (Proxy @s)

Main.hs

{-# LANGUAGE QualifiedStrings #-}
import qualified AsciiOnly

main :: IO ()
main = do
  putStrLn AsciiOnly."Hello world!"
  -- putStrLn AsciiOnly."🤔" -- type error