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Poor man's here document

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m (added reference to Quasitquotation)
(quasiquoting method from zygoloid)
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* You still need to escape special characters.
 
* You still need to escape special characters.
 
* It ends with a newline whether you want one or not.
 
* It ends with a newline whether you want one or not.
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  +
== Quasiquoting ==
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<haskell>-- Str.hs
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module Str(str) where
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import Language.Haskell.TH
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import Language.Haskell.TH.Quote
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str = QuasiQuoter stringE (return . LitP . stringL)
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-- Main.hs
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{-# LANGUAGE QuasiQuotes #-}
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module Main where
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import Str
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foo = [$str|This is a multiline string.
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It's many lines long.
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It contains embedded newlines. And weird stuff:
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łe¶→łeđø→ħe¶ŋø→nđe”øn
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It ends here: |]
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main = putStrLn foo
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-- ghci Str.hs -XQuasiQuotes
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{-
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-- Note we can only do single-line quotations here
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*Str> [$str|foo bar baz|]
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"foo bar baz"
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-}</haskell>
   
 
==See also==
 
==See also==

Revision as of 15:06, 18 February 2010

Contents

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 (return . 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