Suppose I have the following function
printVariance :: [Float] -> IO ()
printVariance [] = return ()
printVariance (x:xs)
| x >= avg (x:xs) = print (x - avg (x:xs)) >> printVariance xs
| otherwise = printVariance xs
Which gets a list, checks which elements are bigger than the average and prints their value - avg.
My problem is that the avg value changes every step. How can I define it just once and use its value for the recursion?
Move the recursion into a helper function. That function can either take the average as a parameter or you could define locally to printVariance and define another local variable holding the average, which the function could then access.
In code:
printVariance :: [Float] -> IO ()
printVariance xs = loop xs
where
average = avg xs
loop [] = return ()
loop (x:xs)
| x >= average = print (x - average) >> loop xs
| otherwise = loop xs
PS: It would be good design to separate the IO from the program logic. So I'd recommend you make your function simply produce a list of the values you want, rather than printing them and move the IO into a separate function (or just main).
PPS: You're not really calculating the variance, so I'd recommend naming the function something else.
whereor aletbinding where you'd define youravgas something along the lines of(foldr (+) 0 xs) / length xs.