I have the following string "not really//" and I want to write a function that replaces every two slashes "//" with two dots ".."
I thought about using map, but then I would iterate through characters and can't know if a slash is going to be followed by another or not. Any clue how this can be done? (without regex)
We can use the replace :: Text -> Text -> Text -> Text function of the Data.Text function. For example:
Prelude Data.Text> replace "//" ".." "not really//"
"not really.."
Here we work however on Texts. If that is a problem, we can also use pack :: String -> Text and unpack :: Text -> String to convert between String and Text. So we can define a function with:
{-# LANGUAGE OverloadedStrings #-}
import Data.Text(pack, unpack, replace)
replacedoubleslash :: String -> String
replacedoubleslash = unpack . replace "//" ".." . pack
But usually for efficient string processing - both in terms of speed and memory - using Text is better than working with Strings.
Explicit recursion looks fine here:
replace :: String -> String
replace ('/':'/':xs) = '.' : '.' : replace xs
replace (x:xs) = x : replace xs
replace "" = ""
This does not scale to long patterns, but for replacing "//" it should work fine.
If you don't want to use any package, this is what I've come with:
isHead':: String -> String -> String -> Bool
isHead' str [] ret = True
isHead' [] key ret = False
isHead' (x:xs) (x':xs') ret = (x == x') && isHead' xs xs' ret
isHead:: String -> String -> Bool
isHead key str= isHead' str key key
remouveXelem:: Int ->[a] -> [a]
remouveXelem i a | i <= 0 = a
remouveXelem _ [] = []
remouveXelem i (x:xs) = remouveXelem (i - 1) xs
replace :: String -> String -> String -> String
replace ori new [] = []
replace ori new s = if isHead ori s then
new ++ replace ori new (remouveXelem (length ori) s)
else head s : replace ori new (tail s)
replace "i" "a" "il etait une fois" => "al etaat une foas"
