2017-06-29 15:16 GMT+02:00 Francesco Ariis <fa-ml@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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