How to print out bytes from binary file in Haskell?

Question

My print statement keeps throwing error, don't really understand what is going on.

import Data.ByteString.Lazy as BS
import Data.Word
import Data.Bits

readByte :: String -> IO [Word8]
readByte fp = do
    contents <- BS.readFile fp
    return $ Prelude.take 5 $ unpack contents

main :: IO ()
main = do
    input <- readByte "DATA.BIN"
    print "Byte 0: " ++ [input!!0]

Getting the below error:

Couldn't match expected type `[()]' with actual type `IO ()'
In the return type of a call of `print'
In the first argument of `(++)', namely `print "Byte 0: "'

A note on terminology. print isn't "throwing [an] error", that would imply you are executing the program. What is happening is the compiler is emitting a type checking error regarding the print statement. — Thomas M. DuBuisson
– Thomas M. DuBuisson, Commented Sep 29, 2016 at 6:07

Alec · Accepted Answer · 2016-09-29 05:57:15Z

2

Haskell is parsing print "Byte 0: " ++ [input!!0] as (print "Byte 0: ") ++ [input!!0], which is probably not what you intended. You might want

main :: IO ()
main = do
    input <- readByte "DATA.BIN"
    putStrLn $ "Byte 0: " ++ show (input!!0)

instead

answered Sep 29, 2016 at 5:57

Alec

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3 Comments

Vlam Over a year ago

If I store input!!0 into a variable first and then use it in the putStrLn, I get "Couldn't match type Word8' with IO a0'" error.

Vlam Over a year ago

Doing this databasetype <- input!!0 to assign the variable and then Prelude.putStrLn $ "DBType: " ++ show (databasetype) to print out. Getting the "Couldn't match type Word8' with IO a0'" error.

Alec Over a year ago

@Vlam Ah. So you will want to have let databasetype = input!!0 for the assignment. <- actually is part of do-notation, which is something different. I recommend you read up on do-notation a bit.

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