How can I resolve this ambiguous type variable error?

Question

The following code compiles just fine, but I cannot use my function:

CODE:

g :: (Fractional b, Integral b) => Int -> b -> b
g 1 x = x / (g 2 x + 1)
g 100 x = 2401*x/100
g n x = ((map (\z -> (ceiling z)^2) (1:[0.5,1..]))!!(n-1))*x / ((g (n+1) x) + fromIntegral n)

ERROR:

Ambiguous type variable `t' in the constraints:
`Integral t' arising from a use of `g' at <interactive>:1:0-6
`Fractional t' arising from a use of `g' at <interactive>:1:0-6
Probable fix: add a type signature that fixes these type variable(s)

Why is this happening, and how can I work around this? I am running GHC 6.10.4 under Windows, if that is at all relevant.

I have already taken a look at this related question, but don't think it addresses my need.

Answer 1

I'm not quite sure what the function is supposed to do, but your problem seems to be that you use ceiling, which has type

(RealFrac a, Integral b) => a -> b

This forces the entire result to be in the class Integral, which is probably not what you want. Adding a FromIntegral and thus changing the last line to

g n x = ((map (\z -> (fromIntegral $ ceiling z)^2) (1:[0.5,1..]))!!(n-1))*x / ((g (n+1) x) + fromIntegral n)

Makes the function compile and gives it the type

g :: (Fractional b) => Int -> b -> b