Re: Help

The problem is fixed. I fixed it using the do notation as Rijk said, it worked perfectly. Now I got two more problems: 1) Should I use the do notation in order to write this result (type IO [[Float]]) to a file? How? 2) The Glasglow compiler doesn let me compile one library because I use the function readFloat (declare in the Prelude of Hugs 98), but Hugs lets me, why? Thanks On Mon, 25 Feb 2002, Keith Wansbrough wrote:

...

I thing that wont works, look: contents :: IO [Char]

parser :: Integral a => [Char] -> [a]

control :: [Float] -> [[Float]]

The two problems are: 1) The input of parser. Doesnt match with the type of input 2) The input of control (or the output of parser). Doesn match with the type of the next function.

The "do" notation used by Rijk fixes the first problem; you should try it.

For the second problem, you want to convert an Integral to a Float - but are you sure? Integers aren't floating point numbers!

If you are sure, then do something like

main = do { contents <- input "twoboxes.dat" return (control (map fromInteger (parser contents))) }

fromInteger has the type Num a => Integer -> a, and since Num Float and Integral Integer, all your type constraints will be satisfied.

HTH.

--KW 8-) -- Keith Wansbrough http://www.cl.cam.ac.uk/users/kw217/ University of Cambridge Computer Laboratory.