Haskell getArgs 类型是 `Char` 而不是 `[Char`]

Question

I am trying to get command line arguments to work with Haskell. I currently have

args ← getArgs
-- opens text file, uses handle for text
handle ←  openFile args ReadMode

However, when I compile I am told that args is of type Char , not [Char] , therefore, I cannot open the file. Is there another way to take in arguments in Haskell or am I taking mine in incorrectly?

Answer 1

openFile :: FilePath -> IOMode -> IO Handle takes a FilePath and an IOMode and gives an IO Handle .

This means that

do
  args <- getArgs
  handle <- openFile args ReadMode
  ...

is claiming that args has type FilePath . However, the type of getArgs :: IO [String] means that args is a [String] , not a FilePath . This means that you are calling openFile with a list of strings rather than a file path.

To fix this, first we must know that FilePath is just a type synonym for String , which means that we must take an element of args rather than the whole list.

Here is an example that does so using pattern matching:

do
  [arg] <- getArgs
  handle <- openFile arg ReadMode
  ...

However, if the caller of your program provides the wrong number of arguments, this will cause an obscure runtime error (generated by fail from the pattern matching failure). A more robust program might handle these cases with more descriptive failure messages:

do
  args <- getArgs
  case args of
    [] -> error "must supply a file to open"
    [arg] -> do handle <- openFile arg ReadMode
                ...
    _ -> error "too many arguments"

Answer 2

As it turns out, it is a simple matter of parsing, the following code works

args <- getArgs
let incomming = head args
handle ←  openFile incomming ReadMode

you have to parse the argument.