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Poor man's here document

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1 Poor man's heredoc / here document

main = do
  doc <- here "DATA" "Here.hs" [("variable","some"),("substitution","variables")]
  putStrLn doc
  html <- here "HTML" "Here.hs" [("code",doc)]
  putStrLn html
 
here tag file env = do
  txt <- readFile file
  let (_,_:rest) = span (/="{- "++tag++" START") (lines txt)
      (doc,_) = span (/="   "++tag++" END -}") rest
  return $ unlines $ map subst doc
  where
    subst ('$':'(':cs) = case span (/=')') cs of 
      (var,')':cs) -> maybe ("$("++var++")") id (lookup var env) ++ subst cs
      _ -> '$':'(':subst cs
    subst (c:cs) = c:subst cs
    subst "" = ""
 
{- DATA START
 
this is a poor man's here-document
 
with quotes ", and escapes \, 
and line-breaks, and layout
without escaping \" \\ \n,
without concatenation.
 
oh, and with $(variable) $(substitution), $(too).
 
   DATA END -}
 
{- HTML START
 
<html>
<head><title>very important page</title></head>
<body>
<verb>
$(code)
</verb>
</body>
</html>
 
   HTML END -}

2 Even poorer man's here-doc / here-document

If you're just looking to define a multiline string constant, you can just say:

str :: String
str = unlines [
    "Here's a multiline string constant.",
    "\tIt's not as convenient as Perl's here-documents,",
    "\tbut it does the trick for me."
    ]

You can fake interpolation with:

hereDocPraise :: String -> String
hereDocPraise lang = unlines [
    "The language with the best here-document support",
    "in my opinion is " ++ lang ++ "."
    ]

2.1 Disadvantages to poorer man's here-docs

  • You still need to escape special characters.
  • It ends with a newline whether you want one or not.

3 Quasiquoting

-- Str.hs
module Str(str) where
 
import Language.Haskell.TH
import Language.Haskell.TH.Quote
 
str = QuasiQuoter stringE (litP . stringL)
 
-- Main.hs
{-# LANGUAGE QuasiQuotes #-}
module Main where
 
import Str
 
foo = [$str|This is a multiline string.
It's many lines long.
 
 
It contains embedded newlines. And weird stuff:
 
łe¶→łeđø→ħe¶ŋø→nđe”øn
 
It ends here: |]
 
main = putStrLn foo
 
-- ghci Str.hs -XQuasiQuotes
{-
-- Note we can only do single-line quotations here
 
*Str> [$str|foo bar baz|]
"foo bar baz"
 
-}

4 See also