Replace every Number in String with a String in Haskell

Question

I think my desire is quite forward.

So I have a string e.g. "abc1abc2abc3" I want to replace every number with the Char 'X' time the given Number. So "abc1abc2abc3" -> "abcXabcXXabcXXX"

I know i only get Integers <9 so I thought doing something like:

myFunction givenString = let
    replaceEveryNumber '1' = 'X'
    replaceEveryNumber '8' = "XXXXXXXX"
    replaceEveryNumber c = c
    in map replaceEveryNumber myFunction

And yeah this does not really work because of different types.

I could also apply a similiar function to a [String] for my desire. So I could change from the code above '1' = 'X' to "1" = "X" and so on, but the problem here is that if I have ["abc2abc", "8"] the result is ["abc2abc", "XXXXXXXX"] so it ignores the char in the String in !!0, which I guess makes sense but I still don't know how to do it correctly.

I hope someone can help me, thanks!

willeM_ Van Onsem · Accepted Answer · 2021-02-28 07:51:40Z

There are a few problems here. You are not mapping over givenString, but over myFunction. Since myFunction is a function, that will not work. We thus should replace the mapping expression with:

myFunction givenString = let … in map replaceEveryNumber givenString

Furthermore the replaceEveryNumber can only return values of one type, your function however returns a Char, and the other lines return a String. Since your function should be able to return a sequence of multiple Chars, we thus should return a string for every line. For c we will need to wrap it in a singleton list to convert the Char to a String with that character:

myFunction givenString = let
    replaceEveryNumber '1' = "X"
    replaceEveryNumber '2' = "XX"
    replaceEveryNumber '3' = "XXX"
    replaceEveryNumber '4' = "XXXX"
    replaceEveryNumber '5' = "XXXXX"
    replaceEveryNumber '6' = "XXXXXX"
    replaceEveryNumber '7' = "XXXXXXX"
    replaceEveryNumber '8' = "XXXXXXXX"
    replaceEveryNumber c = [c]
    in map replaceEveryNumber givenString

Now this will generate a list of Strings, for example:

Prelude> myFunction "abc1abc2abc3"
["a","b","c","X","a","b","c","XX","a","b","c","XXX"]

the only thing that we still need to do is concatenate these strings. We can make use of concat :: [[a]] -> [a], but we can also work with concatMap :: (a -> [b]) -> [a] -> [b]:

myFunction givenString = let
    replaceEveryNumber '1' = "X"
    replaceEveryNumber '2' = "XX"
    replaceEveryNumber '3' = "XXX"
    replaceEveryNumber '4' = "XXXX"
    replaceEveryNumber '5' = "XXXXX"
    replaceEveryNumber '6' = "XXXXXX"
    replaceEveryNumber '7' = "XXXXXXX"
    replaceEveryNumber '8' = "XXXXXXXX"
    replaceEveryNumber c = [c]
    in concatMap replaceEveryNumber givenString

Finally we do not need to write clauses for each digit. We can work with functions like replicate and digitToInt. I leave that as an exercise.

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