Haskell here docs & interpolated strings via quasiquotation
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src/Data/String Don't stop interpolation when a lone '$' is encountered Dec 16, 2017
test Tidy up Apr 3, 2018



here is a package that adds support for multi-line string literals (a.k.a. "here docs") and string interpolation to Haskell via GHC's QuasiQuotes extension.

The quoted expressions behave like string literals in the source code; that is, they produce values of type String, or, with the OverloadedStrings language extension enabled, values of type IsString a => a.

The package includes six quasiquoters:

  • here: Strips leading and trailing whitespace. This allows you to add a line break after the opening quote bracket, which looks nicer.
  • hereLit: Quotes the here doc literally, with no whitespace stripping
  • hereFile: Quotes a file's contents as a here doc
  • i: Like hereLit, but with the ability to interpolate the values of antiquoted Haskell expressions (bracketed by ${ and })
  • iTrim: Like i, but trimming leading and trailing whitespace as with here
  • template: Applies string interpolation to a file, as a simple template engine


{-# LANGUAGE QuasiQuotes #-}

import Data.Char
import Data.String.Here

main = do let foo = "foo"
          putStrLn [i|"foo", when capitalized, is ${map toUpper foo}!|]
          putStrLn [here|
Hello world,

I am a multiline here doc!


"foo", when capitalized, is FOO!
Hello world,

I am a multiline here doc!