Convert String to Tuple, Special Formatting in Haskell

Question

For a test app, I'm trying to convert a special type of string to a tuple. The string is always in the following format, with an int (n>=1) followed by a character.

Examples of Input String :

"2s"

"13f"

"1b"

Examples of Desired Output Tuples (Int, Char) :

(2, 's')

(13, 'f')

(1, 'b')

Any pointers would be extremely appreciated. Thanks.

Answer 1

You can use readS to parse the int and get the rest of the string:

readTup :: String -> (Int, Char)
readTup s = (n, head rest)
  where [(n, rest)] = reads s

a safer version would be:

maybeReadTup :: String -> Maybe (Int, Char)
maybeReadTup s = do
  [(n, [c])] <- return $ reads s
  return (n, c)

Answer 2

Here's one way to do it:

import Data.Maybe (listToMaybe)

parseTuple :: String -> Maybe (Int, Char)
parseTuple s = do
  (int, (char:_)) <- listToMaybe $ reads s

  return (int, char)

This uses the Maybe Monad to express the possible parse failure. Note that if the (char:_) pattern fails to match (ie, if there is only a number with no character after it), this gets translated into a Nothing result (this is due to how do notation works in Haskell. It calls the fail function of the Monad if pattern matches fail. In the case of Maybe a , we have fail _ = Nothing ). The function also evaluates to Nothing if reads can't read an Int at the beginning of the input. If this happens, reads gives [] which is then turned into Nothing by listToMaybe .