Question

import Data.ByteString as B
import Data.ByteString.Internal as I
import Data.Bits
main = do input <- getLine
      let bs = B.pack input
      let intermediatebs  =  unfoldrN ((B.length bs)*2) unfld 0
      where unfld i
      |Prelude.rem i 2 == 0 = Just (top4 $ B.index bs (i/2) , i+1)
      |otherwise =  Just (bottom4 $ B.index bs (i/2) , i+1)
      top4bits x = shiftR x 4
      bottom4bits x = x .&. 0x0F
      top4 x = convertASCIIword8 $ top4bits x
      bottom4 x = convertASCIIword8 $ bottom4bits x
      convertASCIIword8 x
      |x <= 9 = I.c2w '0' + x
      |otherwise = I.c2w 'A' + (x-10)
      let outputbs = B.map I.w2c $ intermediatebs
      in do putStrLn (outputbs)

i am getting this compilation error
The last statement in a ‘do’ construct must be an expression:
let intermediatebs = unfoldrN ((B.length bs) * 2) unfld 0

Answer 1

It’s hard to see exactly what you want here. It is possible to make your code parse with only indentation changes:

import Data.ByteString as B
import Data.ByteString.Internal as I
import Data.Bits
main = do
  input <- getLine
  let bs = B.pack input
  let intermediatebs  =  unfoldrN ((B.length bs)*2) unfld 0
       where
        unfld i
          | Prelude.rem i 2 == 0 = Just (top4 $ B.index bs (i/2) , i+1)
          | otherwise =  Just (bottom4 $ B.index bs (i/2) , i+1)
        top4bits x = shiftR x 4
        bottom4bits x = x .&. 0x0F
        top4 x = convertASCIIword8 $ top4bits x
        bottom4 x = convertASCIIword8 $ bottom4bits x
        convertASCIIword8 x
          | x <= 9 = I.c2w '0' + x
          | otherwise = I.c2w 'A' + (x-10)
  let outputbs = B.map I.w2c $ intermediatebs
  putStrLn (outputbs)

although it won’t compile due to ambiguous getLine and putStrLn occurrences. You may want to import qualified where appropriate. Key observations:

• Do-blocks, let-blocks etc. start from the left edge of the first thing inside them, regardless of where the keyword is itself. E.g.

  do x
     y
  -- ^ because of where the x is, all future lines have to line up with it
  
  let x = ...
      y = ...
  -- ^ if you have code appearing left of here, the let-block ends
  

  As a consequence, I often give do and where their own line before starting a block. In the example I gave above, you can see that even if I moved the do-block into a function with a longer or shorter name, the indentation wouldn’t change.

• Lines in the same block that start indented are a continuation of the previous line. Guards are a continuation so need to be indented, likewise where for a let assignment needs to be indented further than the assignment itself.

• The in do at the end of your block was redundant, so I removed it (alternatively, just indenting it would have worked too).