Replace a substring from a string in Haskell

Question

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)

Answer 1

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.

Answer 2

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.