NeatInterpolation provides a quasiquoter for producing strings with a simple interpolation of input values. It removes the excessive indentation from the input and accurately manages the indentation of all lines of interpolated variables. But enough words, the code shows it better.

Consider the following declaration:

{-# LANGUAGE QuasiQuotes #-}

import NeatInterpolation
import Data.Text (Text)

f :: Text -> Text -> Text
f a b =
  [trimming|
    function(){
      function(){
        $a
      }
      return $b
    }
  |]

Executing the following:

main = Text.putStrLn $ f "1" "2"

will produce this (notice the reduced indentation compared to how it was declared):

function(){
  function(){
    1
  }
  return 2
}

Now let's test it with multiline string parameters:

main = Text.putStrLn $ f
  "{\n  indented line\n  indented line\n}"
  "{\n  indented line\n  indented line\n}"

We get

function(){
  function(){
    {
      indented line
      indented line
    }
  }
  return {
    indented line
    indented line
  }
}

See how it neatly preserved the indentation levels of lines the variable placeholders were at?

If you need to separate variable placeholder from the following text to prevent treating the rest of line as variable name, use escaped variable:

f name = [trimming|this_could_be_${name}_long_identifier|]

So

f "one" == "this_could_be_one_long_identifier"

If you want to write something that looks like a variable but should be inserted as-is, escape it with another $:

f word = [trimming|$$my ${word} $${string}|]

results in

f "funny" == "$my funny ${string}"