[Haskell-cafe] A simple question

[Marc Charpentier]

Good evening,


Doing some calculations in ghci, I encountered a difficulty which I 
cannot resolve.

Here's a distilled version, prepared for the command line:

	let i = 2 in (-1)^i/(2^(10*i)) * (-2^5/(4*i+1))

gives the following error messages

ghci:
	Ambiguous type variable `b' in these top-level constraints:
	  `Fractional b' arising from use of `/' at <interactive>:1
	  `Integral b' arising from use of `^' at <interactive>:1

hugs:
	ERROR - Unresolved overloading
	*** Type       : (Integral a, Fractional a) => a
	*** Expression : let {...} in (-1) ^ i / 2 ^ (10 * i) * negate (2 ^ 5 
/ (4 * i + 1))

However, if I substitute i = 2 in the last fraction, then the 
transformed expression

	let i = 2 in (-1)^i/(2^(10*i)) * (-2^5/(4*2+1))

is accepted.

Note that both

	let i=2 in (-1)^i/(2^(10*i))

and

	let i=2 in  (-2^5/(4*i+1))

are accepted.

Why?

Thank you in advance.


Marc Charpentier