Convert a string representing a binary number to a base 10 string haskell

Question

I have the string \"1001\" and I want the string \"9\".

The numeric library has the (rather clunky) showIntAtBase, but I haven\'t been able to find the opposite.

Answer 1

Because

1001 = 1 * 2^0 + 0 * 2^1 + 0 * 2^2 + 1 * 2^3 = 1 + 0 + 0 + 8 = 9

┌───┬───┬───┬───┐
│1  │0  │0  │1  │
├───┼───┼───┼───┤
│2^3│2^2│2^1│2^0│
└───┴───┴───┴───┘

so obviously:

fromBinary :: String -> Int
fromBinary str = sum $ zipWith toDec (reverse str) [0 .. length str]
  where toDec a b = digitToInt a * (2 ^ b)