Haskell: Library function to convert hexadecimal to binary notation

Question

Is there a library function to convert a hexadecimal digit (0-F) to its equivalent four-digit binary value? For example, 9 = 1001, C = 1100.

I've looked at Data.Hex, Data.HexString, Numeric, and several other libraries that should be relevant, and I had assumed I would be able to find the function I am looking for. However, I'm quite new to Haskell and may be overlooking something simple.

EDIT: Not a duplicate, IMO. My question is about conversion directly from hexadecimal to binary, which the linked question (and its answers) does not mention.

Answer 1

There isn't a single library function to convert a hex digit to a binary string. You can use readHex from module Numeric to convert a hex digit to an integer and printf from the Text.Printf module to generate a binary string, like so:

import Numeric (readHex)
import Text.Printf (printf)

hexToBin :: Char -> Maybe String
hexToBin c
  = case readHex [c] of
      (x,_):_ -> Just $ printf "%04b" (x::Int)
      _       -> Nothing

giving:

> map hexToBin (['0'..'9'] ++ ['A'..'G'])
[Just "0000",Just "0001",Just "0010",Just "0011",Just "0100",
Just "0101",Just "0110",Just "0111",Just "1000",Just "1001",
Just "1010",Just "1011",Just "1100",Just "1101",Just "1110",
Just "1111",Nothing]