positive binary number

binnum.hs

import Text.Printf
import Numeric
import Data.Char
-- positive binary number
-- https://gist.github.com/yamasushi/472c444409e8bd519e76bf0325398715

-- B -> B1 0 { B.syn = 2 * B1.syn }
--    | B1 1 { B.syn = 2 * B1.syn + 1}
--    | 1 {B.syn = 1}
--
-- B -> 1 {R.inh = 1} R {B.syn = R.syn}
-- R -> 0 {R1.inh = R.inh * 2} R1 {R.syn = R1.syn}
-- R -> 1 {R1.inh = R.inh * 2 + 1} R1 {R.syn = R1.syn}
-- R -> ε {R.syn = R.inh}

test :: Int -> IO()
test num =
  do
    let str = showIntAtBase 2 intToDigit num ""
    printf "%b(%d) --- %b\n" (num :: Int) (num::Int) (fst. b $ str)

b :: String -> (Int,String)
b ('1':rest) = r 1 rest

r :: Int->String->(Int,String)
r inh [] = (inh,[]) -- ε
r inh ('0':rest) = r (inh*2)   rest
r inh ('1':rest) = r (inh*2+1) rest
r inh xs = (inh,xs) -- ε