How to replace a string with another in haskell

Question

I want to replace a string from an input file with a different string. I was searching for a method but it seems i can only alter the string character by character. For example in the my code below

replace :: String -> String 
replace [] = [] 
replace (x:xs) = if x == '@' then 'y':replace xs --y is just a random char
                             else x:replace xs

searching :: String -> IO String
searching filename = do
    text <- readFile filename
    return(replace text)


main :: IO ()
main = do

  n <- searching "test.sf"
  writeFile "writefile.html" n

I want to find the first occurrence of the string "@title", but i cant seem to find a method to do so as mentioned before, i can only access the char '@'. Is there a method for doing such a task.

possible duplicate of How can I replace a substring of a string with another in Haskell without using external Libraries like MissingH? — Roman Cheplyaka
– Roman Cheplyaka, Commented Feb 16, 2013 at 8:54
I think, that was already discussed here: stackoverflow.com/questions/14880299/… — kaan
– kaan, Commented Feb 16, 2013 at 8:54
This is not really a duplicate. The other question specifically excludes using other libraries, which is a reasonable answer to this question. — C. A. McCann
– C. A. McCann, Commented Feb 20, 2013 at 16:34
For a general solution to this problem: hackage.haskell.org/package/replace-megaparsec — James Brock
– James Brock, Commented May 10, 2020 at 7:24

josejuan · Accepted Answer · 2013-02-16 08:59:25Z

22

You can use Data.List.Utils replace, it's lazy and you can process a big file with some like:

main = getContents >>= putStr . replace "sourceString" "destinationString"

That's all!

A possible replace function could be

rep a b s@(x:xs) = if isPrefixOf a s

                     -- then, write 'b' and replace jumping 'a' substring
                     then b++rep a b (drop (length a) s)

                     -- then, write 'x' char and try to replace tail string
                     else x:rep a b xs

rep _ _ [] = []

another smart way (from Data.String.Utils)

replace :: Eq a => [a] -> [a] -> [a] -> [a]
replace old new l = join new . split old $ l

edited Feb 16, 2013 at 8:59

answered Feb 16, 2013 at 8:54

josejuan

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2 Comments

Conrad Parker Over a year ago

Alternatively, using Data.List.Split from the split package which is part of Haskell Platform, define replace old new = intercalate new . splitOn old.

Răzvan Flavius Panda Over a year ago

A better option would be to use replace :: Text -> Text -> Text -> Text from the text package: hackage.haskell.org/package/text-1.2.3.0/docs/…

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