[Haskell-beginners] How do I map a String and a IO String?

[Jona Ekenberg]

2017-06-29 15:16 GMT+02:00 Francesco Ariis <fa-ml at ariis.it>:

> On Thu, Jun 29, 2017 at 02:33:22PM +0200, Jona Ekenberg wrote:
> > I want to replace lines with the content of a file if the line is a
> filename
>
> Hello Jonas, you want to user `return`:
>
>     λ> :t return
>     return :: Monad m => a -> m a
>
> which lifts a simple value inside a monad (in this case, a -> IO a), like
> this:
>
>     lineOrIO :: String -> IO String
>     lineOrIO cs | (isPrefixOf "./" cs) = readFile cs
>                 | otherwise            = return cs
>
> If this is not a school assignment, consider replacing `isPrefixOf "./"`
> with something from `System.Directory`.
>
> Does this help?


Thank you for your help Francesco!

I tried writing it like this:

> lineOrIo :: String -> IO String
> lineOrIo cs | (isPrefixOf "./" cs) = readFile cs
>             | otherwise            = return cs
>
> printLines path = do
>   file <- readFile path
>   lines <- map lineOrIo (lines file)
>   print lines

But when evaluating I get this error:

PrintComments.lhs:20:14-38: error: …
    • Couldn't match type ‘[]’ with ‘IO’
      Expected type: IO (IO String)
        Actual type: [IO String]
    • In a stmt of a 'do' block: lines <- map lineOrIo (lines file)
      In the expression:
        do { file <- readFile path;
             lines <- map lineOrIo (lines file);
             print lines }
      In an equation for ‘printLines’:
          printLines path
            = do { file <- readFile path;
                   lines <- map lineOrIo (lines file);
                   print lines }
Compilation failed.

Sadly I am not yet very used to the error messages, so I don't understand
what ghci is telling me.
As far as I can tell (lines file) should give me an array of strings, which
I turn into an array of IO String.
Could that be the error? It shouldn't be [IO String] but instead IO
[String]? How do I turn the former into the latter?

Kind regards,
Jona

PS. It is not a school assignment, so I'll make sure to check out
System.Directory.


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