-- A recursive-descent parser for bit strings which computes
-- the decimal value of its input

-- the type of tokens delivered by the scanner:
data Token = Bit0 | Bit1 | EOF
   deriving (Eq,Show)

-- the semantic value is a pair of the number of digits and the value
-- of the consumed input:
digits :: [Token] -> ([Token],(Int,Int))
digits (t:ts) =
  case t of
    Bit0 -> let (ts1,(l,v)) = digits ts
             in (ts1,(l+1,v))
    Bit1 -> let (ts1,(l,v)) = digits ts
             in (ts1,(l+1,2^l+v))
    EOF  -> (ts,(0,0))

parse :: [Token] -> Int
parse ts = value
  where (_,(_,value)) = digits ts

main1 = parse [Bit1, Bit0, Bit1, Bit1, EOF]

main2 = parse [Bit1, Bit1, Bit1, Bit1, EOF]

main3 = parse [Bit1, Bit0, Bit0, Bit0, Bit0, EOF]