我如何putStrLn Data.ByteString.Internal.ByteString？

Question

I'm learning haskell and decided to try writing some small test programs to get use to Haskell code and using modules. Currently I'm trying to use the first argument to create a password hash using the Cypto.PasswordStore. To test out my program I'm trying to create a hash from the first argument and then print the hash to screen.

import Crypto.PasswordStore
import System.Environment

main = do
    args <- getArgs
    putStrLn (makePassword (head args) 12)

I'm getting the following error:

testmakePassword.hs:8:19:
    Couldn't match expected type `String'
            with actual type `IO Data.ByteString.Internal.ByteString'
    In the return type of a call of `makePassword'
    In the first argument of `putStrLn', namely
      `(makePassword (head args) 12)'
    In a stmt of a 'do' block: putStrLn (makePassword (head args) 12)

I've been using the following links as references but I am now just trial-erroring to no avail. http://hackage.haskell.org/packages/archive/bytestring/0.9.0.4/doc/html/Data-ByteString-Internal.html http://hackage.haskell.org/packages/archive/pwstore-purehaskell/2.1/doc/html/Crypto-PasswordStore.html

Answer 1

You haven't imported ByteString, so it's trying to use the String version of putStrLn. I've provided toBS for the String->ByteString conversion.

Try

import Crypto.PasswordStore
import System.Environment
import qualified Data.ByteString.Char8 as B

toBS = B.pack

main = do
    args <- getArgs
    makePassword (toBS (head args)) 12 >>= B.putStrLn

Answer 2

You have to do two things differently. First, makePassword is in IO, so you need to bind the result to a name and then pass the name to the IO function. Secondly, you need to import IO functions from Data.ByteString

import Crypto.PasswordStore
import System.Environment
import qualified Data.ByteString as B

main = do
    args <- getArgs
    pwd <- makePassword (B.pack $ head args) 12
    B.putStrLn pwd

Or, if you won't be using the password result anywhere else, you can use bind to connect the two functions directly:

main = do
    args <- getArgs
    B.putStrLn =<< makePassword (B.pack $ head args) 12