Haskell: [String] to IO ()

Question

I am new to Haskell and I am trying to get a list of values from input and print one item out from the list each line.

func :: [String] -> IO ()

I am having trouble trying to figure out how to print out the item in the list, when the list size is just 1.

func [] = return ()  
func [x] = return x

I am getting this error message when trying to compile the file:

Couldn't match expected type `()' with actual type `String'
    In the first argument of `return', namely `x'
    In the expression: return x

I am completely lost and I have tried searching but haven't found anything. Thanks!

You're not actually printing anything if you just return a value. You need a function like putStrLn for that. — Fred Foo
– Fred Foo, Commented Apr 1, 2013 at 14:20

Lee · Accepted Answer · 2013-04-01 14:24:45Z

9

You can use forM_ for this:

func :: [String] -> IO ()
func l = forM_ l putStrLn

If you want to write your own version directly, you have some problems.

For the empty list, you have nothing to do but create a value of IO (), which you can do with return.

For the non-empty list you want to output the line with putStrLn and then process the rest of the list. The non-empty list is of the form x:xs where x is the head of the list and xs the tail. Your second pattern matches the one-element list.

func [] = return ()
func (x:xs) = putStrLn x >> func xs

answered Apr 1, 2013 at 14:24

Lee

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1 Comment

Benjamin Hodgson Over a year ago

If you prefer the 'origami programming' style, your second implementation could be written as foldr (\s m -> putStrLn s >> m) (return ()), or even foldr ((>>) . putStrLn) (return ()), which I think is rather pretty :)

Benjamin Hodgson · Accepted Answer · 2013-04-01 14:25:50Z

8

func = mapM_ putStrLn

mapM_ applies a monadic function like putStrLn to each element of a list, and discards the return value.

answered Apr 1, 2013 at 14:25

Benjamin Hodgson

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Comments

Whadupapp · Accepted Answer · 2013-04-01 14:24:27Z

7

you actually aren't trying to print anything, you use putStr for that. try something like

print [] = return ()
print (x:xs) = do
                 putStr x
                 print xs

answered Apr 1, 2013 at 14:24

Whadupapp

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2 Comments

user1798403 Over a year ago

Thanks. That actually makes a lot of sense. :)

stusmith Over a year ago

Be aware there's already a function named print in the Prelude: hackage.haskell.org/packages/archive/base/latest/doc/html/…

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