Max File Size

While doing some refactoring work on solution from this guy¹, I realized a pretty trivial thing about Manod method >>=.

Take a look at its definition:

│ Prelude> :t (>>=)
│ (>>=) :: (Monad m) => m a -> (a -> m b) -> m b

Both a and b ought to be some Monadic type! In other words, all computation glued together by >>= have same Manodic type.

It is really straightforward however I did not have that in mind.

Since I did want to define a function take a String and return a list of String while its implementation has several computations that return IO type. Obviously the function failed to compiled. Therefore I changed it to return IO [String].

│ -- FilePath is from package System.IO and it is synonyms of String
│ getFilesInDir :: FilePath -> IO [FilePath]
│ getFilesInDir inp = do
│   isDir <- doesDirectoryExist inp
│   files <- if isDir then
│   │   │   │   (do
│   │   │   │   │names <- getDirectoryContents inp
│   │   │   │   │forM [ inp </> x | x <- names, isNotSpecialDir x ] getFilesInDir)
│   │   │   │else return [[inp]]
│   return $ concat files

PS:

• Full source code here² which is cool as give me several suggestions.
• Actually it is possible to have multiple Monad in one do levarage Monad Transformers.