String Interpolation in Haskell

Ruby has a special syntac in String literal which name is String Interpolation that you can write the result of a expression in a string easily.

"hello, #{name}!"

The notation is equivalent to the following notation.

"hello, " + name + "!"

That's not only available in Ruby but also in CoffeeScript.

I wrote this for Haskell in Haskell.

{-# LANGUAGE OverloadedStrings #-}
import qualified Data.Text as T
import qualified Data.Text.IO as T
import qualified Data.Attoparsec.Text as P
import Control.Applicative

main = T.putStrLn $ stringInterpolate "as{df \"jkl#{x}jkw\"jl}kf"

(<<) = T.append

stringInterpolate :: T.Text -> T.Text
stringInterpolate x = case P.parseOnly stringInterpolate' x of
                           Left x -> T.pack x
                           Right x -> x

stringInterpolate' :: P.Parser T.Text
stringInterpolate' = P.try $
  T.concat <$> P.many (block <|> string <|> T.singleton `fmap` P.notChar '}')

block :: P.Parser T.Text
block = P.try $ do
  x <- (P.char '{') *> stringInterpolate' <* (P.char '}')
  return $ "{" << x << "}"

-- "jklsfj#{...}sdjfkl"
string :: P.Parser T.Text
string = P.try $ do
  x <- (P.char '"') *> stringContent <* (P.char '"')
  return $ "(\"" << x << "\")"

-- jklsfj#{...}sdjfkl
stringContent :: P.Parser T.Text
stringContent = P.try $
  T.concat <$> P.many (interpolate <|> T.singleton `fmap` P.notChar '"')

-- #{...}
interpolate :: P.Parser T.Text
interpolate = P.try $ do
  x <- P.string "#{" *> stringInterpolate' <* P.char '}'
  return $ "\" ++ " << x << " ++ \""

String concatination in Haskell is ++. The program above converts string literals like the below.

"hello, #{name}!"

("hello, " ++ name ++ "!")

You can write arbitrary expression in the placeholder.

"hello, #{[ toUpper c | c <- ['a'..'z'] ]}!"

("hello, " ++ [ toUpper c | c <- ['a'..'z'] ] ++ "!")