[Haskell-beginners] editing a list

[Jonathan Phillips]

I have a list (more precisely a string) which I'm trying to recurse
through, editing sections as I come to it.  It's obvious how to do it
with a for loop, but I'm doing it all wrong in haskell, as this simple
task is running mind-numbingly slowly.

addOneNum cycles through each line, and when it gets to character 23,
it calls changeNo, which edits the string.  The problem is clearly the
adding on to the end of the string, but the only other way I can see
of doing it is writing the 'new string' backwards and then reverse it
at the end.  This looks like it would make the function even less
legible than it already is though.


addOneNum       :: String -> String -> Integer -> String

addOneNum x [] _        = x
addOneNum x (y:ys) n    = case n of
        23      -> changeNo x (y:ys) n
        _       -> case y of
                '\n'    -> addOneNum (x ++ [y]) ys 1
                _       -> addOneNum (x ++ [y]) ys (n+1)

changeNo        :: String -> String -> Integer -> String
changeNo v (w:x:y:z:xs) n       = case
length(show((read([w,x,y,z])::Integer) + 1)) of
        0 -> addOneNum (v ++ "    ") xs (n+4)
        1 -> addOneNum (v ++ show((read([w,x,y,z])::Integer) + 1) ++ "
  ") xs (n+4)
        2 -> addOneNum (v ++ show((read([w,x,y,z])::Integer) + 1) ++ "
 ") xs (n+4)
        3 -> addOneNum (v ++ show((read([w,x,y,z])::Integer) + 1) ++ "
") xs (n+4)
        4 -> addOneNum (v ++ show((read([w,x,y,z])::Integer) + 1)) xs (n+4)
        _ -> error "asdf"
changeNo w _ n  = error "changeNo error"


Also, while I'm at it, it looks like I want to be using an as-pattern
(or some C-style #define thing):

changeNo v (w:x:y:z:xs) n       = case
length(s@(show((read([w,x,y,z])::Integer) + 1))) of
        0 -> addOneNum (v ++ "    ") xs (n+4)
        1 -> addOneNum (v ++ s ++ "   ") xs (n+4)
        2 -> addOneNum (v ++ s ++ "  ") xs (n+4)
        3 -> addOneNum (v ++ s ++ " ") xs (n+4)
        4 -> addOneNum (v ++ s) xs (n+4)
        _ -> error "asdf"

but that gives me a 'not in scope' error.  Any suggestions?

Cheers,
PhiJ