Question

I’m working on learning some Haskell (please excuse the newbie error)-

This routine errors out. My understanding of the do & <- syntax is that they extract the non-Monad type from the monad. So that understanding is flawed: what’s the correct understanding here?

exister :: String -> Bool
exister path = do
  fileexist <- doesFileExist path 
  direxist <- doesDirectoryExist path
  return fileexist || direxist 

error

ghc -o joiner joiner.hs

joiner.hs:53:2:
    Couldn't match expected type `Bool' against inferred type `m Bool'
    In the first argument of `(||)', namely `return fileexist'
    In the expression: return fileexist || direxist
    In the expression:
        do { fileexist <- doesFileExist path;
             direxist <- doesDirectoryExist path;
               return fileexist || direxist }

Answer 1

First problem: The line return fileexist || direxist is parsed as (return fileexist) || direxist, and you can’t pass m Bool as the first argument of ||. Change it to return (fileexist || direxist).

Second problem: You claim the return type of exister is Bool, but the compiler infers that it must be IO Bool. Fix it. (The do and <- syntax let you extract the a value from an m a value, but only if you promise to return an m a value.)