Haskell: Couldn't match type ‘[Char]’ with ‘Text’

Question

For some reason- which I'm hoping to find out by virtue of asking this question- having recently implemented a cadre of updates to a haskell script, including the addition of:

import System.Directory (doesFileExist, removeFile,getPermissions)

which serves to facilitate the following function:

validFile :: FilePath -> IO Bool
validFile path = do
    exists <- (doesFileExist path)
    if exists
    then (readable <$> getPermissions path)
    else return False

invoked as:

pwds <- case cfgPasswords of
    Just passPath -> do
         pathChecksOut <- validFile passPath
         when (not pathChecksOut) $ 
             errorL' ("Failed to access file at : " ++ passPath)
         (map (Just . T.unpack) . lines) <$> readFileUtf8 passPath
    Nothing       -> return $ replicate (length cfgPublicKeys) Nothing

I'm not able to anymore build the project on my machine.

The error I got was Couldn't match type ‘[Char]’ with ‘Text’, and it pointed me to the following line:

errorL' ("Failed to access file at : " ++ passPath)

It seems there's a question about StackOverflow that tries to address a similar issue, this one. In an attempt to follow that advice I adapted the line like so errorL' ("Failed to access file at : " ++ (passPath :: Text)), but still I wan't able to build the project.

The exact console output I received after implementing the changes you recommend in your last post looks like this:

The full file and the progress of adding this feature (rather bug!) is well encapsulated in this gist file.

Answer 1

To convert from a String (which is what FilePath is) to a Text you need to explicitly use pack: a type annotation alone won't do.

You will also need to use append (or Monoid's (<>)) instead of (++) which is list specific.