Haskell Put IO String

Question

I am trying to read and print the output from the "readProcess" command mapped onto a list of filenames:

files <- readProcess "ls" [] []
let mdList = map ( \file -> do
    md <- readProcess "mdls" [file] []
    return md ) $ splitOn "\n" files in
    map (\md -> putStrLn md) mdList
putStrLn "Complete"

Each time I try to map putStrLn onto the mdList, I get this error:

Couldn't match type ‘IO String’ with ‘[Char]’

I have read many StackOverflow answers that seem to just use putStrLn on an IO String but I am unable to do so. Also, I am new to Haskell so any other tips are also appreciated.

An IO String value is not a string. It doesn't make sense to want to put an IO String, for the same reason you wouldn't want to eat a cookbook. — leftaroundabout
– leftaroundabout, Commented Oct 30, 2017 at 16:17
I am aware than an IO String is not a string. I am asking how to write an IO String. — genghiskhan
– genghiskhan, Commented Oct 30, 2017 at 16:20
@genghiskhan What would you expect to get when you "write" an IO String? Machine code? Assembly instructions? What? — Thomas M. DuBuisson
– Thomas M. DuBuisson, Commented Oct 30, 2017 at 16:23
@genghiskhan yes, it is equivalent to a do block. But if you shove a do block in a function argument, it'll always be a monadic action, not a pure value such as the function might accept. — leftaroundabout
– leftaroundabout, Commented Oct 30, 2017 at 16:30
The type of map putStrLn is [String] -> [IO ()], but every statement in your do block must have type IO X for some X (and clearly [IO ()] can't match IO X for any X). However, mapM_ putStrLn :: [String] -> IO (). — user2407038
– user2407038, Commented Oct 30, 2017 at 17:01

chi · Accepted Answer · 2017-10-30 21:15:46Z

You are using

map :: (a -> b) -> [a] -> [b]

which specializes to

map :: (a -> IO b) -> [a] -> [IO b]

The final result, [IO b] is not what we need. That is a list of IO actions, i.e. the equivalent of a list of non-executed no-arguments imperative functions. Instead, we want a single IO action, whose result is a list of b: that would be IO [b] instead of [IO b].

The library provides that as well:

mapM :: (a -> IO b) -> [a] -> IO [b]

or, if we don't care about collecting the results

mapM_ :: (a -> IO b) -> [a] -> IO ()

The library also provides variants with flipped arguments:

for  :: [a] -> (a -> IO b) -> IO [b]
for_ :: [a] -> (a -> IO b) -> IO ()

So, the original code can be fixed as follows:

import Data.Foldable

files <- readProcess "ls" [] []
for_ files $ \file -> do
    md <- readProcess "mdls" [file] []
    putStrLn md
putStrLn "Complete"

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