Question

I am trying to learn some Haskell and I find it difficult. I am having some issues with my
current project. The idea is that I have to go through a String and substitute certain chars
with new substrings. For instance if I have a String “FLXF” and I want to replace every F
with a substring called “FLF” the result should be “FLFLXFLF”. Now I have been working on this
specific problem for hours. I have been reading up on types, different functions that might come in handy (map, fold, etc) and yet I have not been able to solve this problem.

The code below is some of the different tries I have had:

apply :: String -> String
apply []     = []
apply (x:xs) = if (x == 'F')
               then do show "Hello"
                       apply xs
               else (apply (xs))

This example here I was just trying to show hello every time I encountered a ‘F’, but all it shows is “”, so this clearly does not work. I am really not sure an if else statement is the way to go here. I was also thinking the function map might do the trick. Here the code I was thinking about could look something like this:

map (\x y -> if y == 'F' then "FLD" else y) "FLF"

but that gives me a type error. So as you can see I am lost. Excuse me my poor knowledge to Haskell, but I am still new to it. I really hope some of you can help me out here or give me a push in the right direction. Feel free to ask questions if I have been unclear about something.

Thank you in advance!

John

Answer 1

map (\x y -> if y == 'F' then "FLD" else y) "FLF"

This is nearly right.

First… why does the function take two arguments?

map (\y -> if y == 'F' then "FLD" else y) "FLF"

The remaining type error is because the then branch gives a String, but the else branch gives a Char (the two branches must each give a value of the same type). So we’ll make the else branch give a String instead (recall that String is a synonym for [Char]):

map (\y -> if y == 'F' then "FLD" else [y]) "FLF"

Now the problem is that this gives you a [String] value instead of a String. So we’ll concatenate all those strings together:

concat (map (\y -> if y == 'F' then "FLD" else [y]) "FLF")

This combination of concat and map is common enough that there’s a standard function that combines them.

concatMap (\y -> if y == 'F' then "FLD" else [y]) "FLF"