Haskell trying to mutate string in [String]

Question

movex [] a s = []    
movex (x:xs) a s
| elem a x = moveNow x a s
| otherwise = x : (movex xs a s)
where
  moveNow x a s
    | s == 'l' = moveNow2 x a
    where
        moveNow2 [] _ = []
        moveNow2 (x:y:xs) a
          | x == ' ' && y == a = a : x : moveNow2 (y:xs) a
          | otherwise = x : moveNow2 (y:xs) a

<- This is what I got right now

I am trying to make a function that iterates through [string], finds the right string and then mutates it.

given input

func ["abc", "dfg"] f l -- move f in this list 1 space left --

expected output

["abc", "fdg"]

Right now I am stuck at movex function that gives me error

Couldn't match expected type `Char' with actual type `[Char]'
In the first argument of `(:)', namely `x'
In the expression: x : (movex xs a s)

| elem a x = moveNow x a s but what about xs? How can you drop it? Perhaps try giving your functions explicit type signatures. — n. m. could be an AI
– n. m. could be an AI, Commented Dec 23, 2015 at 13:28
use for condition x==' ' is to check whether x is a space character or not — John Smith
– John Smith, Commented Dec 26, 2015 at 22:23

user2512323 · Accepted Answer · 2015-12-23 13:36:03Z

1

Direct solution to the error is to replace the line

| elem a x = moveNow x a s

With

| elem a x = moveNow x a s : movex xs a s

Or, probably

| elem a x = moveNow x a s : xs

Depending on what you want to do after the first match: continue looking for certain character, or leave other strings untouched.

Your moveNow function has return type String, or [Char], while movex has [String], or [[Char]], that's why compiler complains.

To avoid such problems(or fix them easier) consider writing explicit type signatures, like so:

movex :: [String]->String->String->[String]

answered Dec 23, 2015 at 13:36

user2512323

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1 Comment

John Smith Over a year ago

Replacing with | elem a x = moveNow x a s : xs is what I was after. Thanks!

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