The main function...
val :: String -> Int
val s = valx 0 s
This works by calling its auxiliary function...
valx n  = n
valx n (c:cs) =
case c of
'+' -> n + valx 0 cs
'-' -> n - valx 0 cs
_ -> valx ((n * 10) + (ord c - ord '0')) cs
*Main Data.Char> val "1+2+34-5+67-8+9"
The reason is that doing it recursively implicitlty brackets from the end of the string, so we are working out 1+(2+(34-(5+(67-(8+9))))) but really we want to work out (((((1+2)+34)-5)+67)-8)+9
OK so I want an auxiliary function that takes an accumulator for the whole expression, and as I go along a smaller accumulator for the value of the number I'm working through. And a number -1 or +1 for the last sign character I encountered.
At the start, the accumulators are zero and the sign is +1.
Each time I hit either a + or a - character I either add or subtract the value of my current number to or from the accumulator. Also I set the sign for the next number, and set the little accumulator back to
When I hit a digit I use it to increase the value of the current number.
So I get this:
eval :: String -> Int
eval s = evalx 0 0 1 s
evalx :: Int -> Int -> Int -> String -> Int
evalx acc cur sgn "" =
acc + (sgn * cur)
evalx acc cur sgn (c:cs) =
case c of
'+' -> evalx (acc + (sgn * cur)) 0 1 cs
'-' -> evalx (acc + (sgn * cur)) 0 (-1) cs
_ -> evalx acc ((cur * 10) + (ord(c) - ord('0'))) sgn cs
*Main Data.Char> eval "1+2+34-5+67-8+9"
This one does work.