Applying putStr to each item of a list

Question

I'm trying to print out a list of integers in this format ``` [1] [2] [3] ``` So my function takes a list of integers and returns an IO () like so: ``` import System.IO printVector :: [Integer] -> IO () printVector (x:xs) = putStr("[" ++ show(x) ++ "]" ++ "
") : printVector xs ``` But ghc gives me the error: ``` lin-test.hs:5:22: Couldn't match expected type `IO ()' with actual type `[a0]' In the expression: putStr ("[" ++ show (x) ++ "]" ++ "") : printVector xs In an equation for `printVector': printVector (x : xs) = putStr ("[" ++ show (x) ++ "]" ++ "") : printVector xs Failed, modules loaded: none. ``` Now it is my understanding that the function would run through the list, taking the first item 'x' first, and then with : printVector xs it would recursively call the rest of the list applying the same putStr function to each item in the list. But I think my problem is where I do : printVector xs? Can anyone point out what i'm doing wrong here?

Accepted Answer

You need to map over the list but, as these are IO actions, you also need to execute them (as `map` will return a list of IO action without executing them, see following example).

```
sequence $ map (putStrLn . show) [1,2,3,4]
```

There is a function doing that already, it's `mapM`. So the example can be simplify as :

```
mapM (putStrLn . show) [1,2,3,4]
```

Another thing you can do is using `mapM_` which uses `sequence_` instead and will ignore the result of executing the IO action to each of the elements. Therefore, the return type will be `IO ()` instead of `IO [()]` (previous example).

```
mapM_ (putStrLn . show) [1,2,3,4]
```

Applying putStr to each item of a list

Solution

Problem