That's surprising since it's fairly straightforward. The type of `3` gets inferred as `Num a => a` because there's nothing to constrain it further. The type o

That's surprising since it's fairly straightforward. The type of `3` gets inferred as `Num a => a` because there's nothing to constrain it further. The type of `div 6 2` gets inferred as `Integral a => a` because the type of `div` is `Integral a => a -> a -> a`. The type of `3 + 1.0` gets inferred as `Fractional a => a` because of the decimal point. Then the same thing happens in the last statement so the type of the subexpression `(div 6 2)` (still) gets inferred to `Integral a => a` and the type of `1.0` (still) gets inferred to `Fractional a => a` because of the decimal point. So at that point you have an expression that's trying to add something constrained to being a whole number to something constrained to being a fractional number and that is ill typed because there is no type you could choose for the overall expression that would meet both sets of constraints. Edit: When I first replied there was text at the bottom of the message saying that no one could explain why this happens. My comment was addressing that portion of the original message. The behavior itself is not surprising for the same reasons.