Question

Trying to convert a bytestring to a hex ascii string display

wordtoascii :: Int -> String
wordtoascii y =
  showIntAtBase 16 intToDigit ( fromEnum  y) ""

bs2string :: Data.ByteString.ByteString -> String
bs2string bs = do
  Prelude.map( wordtoascii,
               (unpack bs))

Type error:

Couldn't match expected type `a -> b'
       against inferred type `(Int -> String, [GHC.Word.Word8])'
In the first argument of `Prelude.map', namely
    `(wordtoascii, (unpack bs))'
In the expression: Prelude.map (wordtoascii, (unpack bs))
In the expression: do { Prelude.map (wordtoascii, (unpack bs)) }

Answer 1

This is not the syntax for what you think it is.

Prelude.map( wordtoascii,
            (unpack bs))

That is the same as:

let x = (wordtoascii, unpack bs)
in map x

Remove the parentheses and the comma.

map wordtoascii (unpack bs)

However, this is also wrong. Because the type of the above expression is [String], not String. You want concatMap, which is like map but splices the results together in one string.

concatMap wordtoascii (unpack bs)

Or, even better,

bs2string = concatMap wordtoascii . unpack

The comma is for creating tuples, lists, and records. For example, (1, 7) :: (Int, Int) could be cartesian coordinates. The comma does not appear in function calls.

Typically, ByteString is imported only as a qualified import, since so many functions clash with Prelude. This eliminates the need for Prelude. qualification on the functions that do clash.

import qualified Data.ByteString as S
bs2string = map wordtoascii . S.unpack

The S stands for Strict. BS is also a common choice, it stands for for Bytestring (Strict).