Haskell getting value of Either from Get monad

Question

I am a Haskell noob feeling stuck on a rather simple function I am attempting to perform. Ultimately my goal is to read a strict ByteString, use the Get monad with a decoder to retrieve the first Word32 from the ByteString, and perform specific functions where Data.Bits.testBit evaluates to True on various parts of the Word32.

Here is my example code:

import Data.List
import Data.Char
import Data.Function
import System.Random
import Data.Bits
import Data.Either
import Data.Binary.Strict.Get
import System.IO as SIO
import Data.ByteString.Char8 as B
import Data.Word (Word32)
import Data.ByteString.UTF8 as BU

dateTemplate = "YYMMDDhhmmss"

convertFromString :: String -> ByteString
convertFromString s = BU.fromString s

mahDecoder :: Get Word32
mahDecoder = do
  first32Bits <- getWord32be
  return first32Bits

main :: IO ()
main = do 
  let a = runGet mahDecoder (convertFromString dateTemplate)
  SIO.putStrLn $ show a

  -- When I uncomment these lines I get the problem

  --case a of
  --  Left val -> SIO.putStrLn "Communist!"
  --  Right val -> SIO.putStrLn $ "Fascist!"

When I run the runGet function on the decoder and pass my ByteString in main, I can see it returns an Either instance like so:

(Right 1499024717,"DDhhmmss")

When I attempt to case on Left or Right then it fails with the following error:

HSStackOverflowExamp.hs:31:5:
    Couldn't match expected type `(Either String Word32, ByteString)'
                with actual type `Either t0 t1'
    In the pattern: Left val
    In a case alternative: Left val -> SIO.putStrLn "Communist!"
    In a stmt of a 'do' block:
      case a of {
        Left val -> SIO.putStrLn "Communist!"
        Right val -> SIO.putStrLn $ "Fascist!" }

Any ideas what I am doing wrong here? I get the feeling like I am supposed to read all bytes from the ByteString with the decoder. To be honest I am not entirely sure what the type of a is here. I still have a very limited understanding of Monads and Monad Transformers. I was hoping that I didn't need to master every one of the Haskell dark arts before I could implement even simple use cases for practice. If the answer is to keep reading then I accept that.

Chad Gilbert Chad Gilbert · Accepted Answer · 2016-12-22T15:35:22

The type of a is a tuple where the first item is an Either. Try changing your case statement to this:

case a of
    (Left val, _) -> SIO.putStrLn "Communist!"
    (Right val, _) -> SIO.putStrLn $ "Fascist!"

Haskell getting value of Either from Get monad

1 Answers