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Poor man's here document

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== Poor man's heredoc / here document ==
 
<haskell>
 
<haskell>
-- Poor man's heredoc / here document
 
 
main = do
 
main = do
 
doc <- here "DATA" "Here.hs" [("variable","some"),("substitution","variables")]
 
doc <- here "DATA" "Here.hs" [("variable","some"),("substitution","variables")]
Line 46: Line 46:
   
 
</haskell>
 
</haskell>
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  +
== Even poorer man's here-doc / here-document ==
  +
  +
If you're just looking to define a multiline string constant, you can just say:
  +
  +
<haskell>
  +
str :: String
  +
str = unlines [
  +
"Here's a multiline string constant.",
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"\tIt's not as convenient as Perl's here-documents,",
  +
"\tbut it does the trick for me."
  +
]
  +
</haskell>
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  +
You can fake interpolation with:
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<haskell>
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hereDocPraise :: String -> String
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hereDocPraise lang = unlines [
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"The language with the best here-document support",
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"in my opinion is " ++ lang ++ "."
  +
]
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</haskell>
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  +
Disadvantages to poorer man's here-docs:
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* You still need to escape special characters.
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* It ends with a newline whether you want one or not.
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  +
----
   
 
See Also
 
See Also

Revision as of 05:20, 20 April 2007

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 ++ "."
    ]

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.

See Also