Dear all,

I just started to learn Haskell with learnyouahaskell.com and at the very beginning, I met a strange issue with following simple function:

-- why does work with "toZero 10" but not for "toZero -10"?
toZero :: (Integral t) => t -> [t]
toZero 0 = [0]
toZero x = if x > 0 then x : toZero (x - 1)
                    else x : toZero (x + 1)

This function works as expected for positive arguments, e.g., "toZero 10" gives me [10,9,8,7,6,5,4,3,2,1,0]. However, GHCI will raise following error if I give it a negative argument, e.g., "toZero -10":

*Main> toZero -10

<interactive>:12:1:
Non type-variable argument in the constraint: Num (t -> [t])
(Use FlexibleContexts to permit this)
When checking that ‘it’ has the inferred type
it :: forall t. (Integral t, Num (t -> [t])) => t -> [t]

This seems strange to me as 10 and -10 has exactly the same type "Num a => a". I've done with chapter 1~10 of learnyouahaskell.com but still has no idea on why this error. Anybody can help to explain this?

Thanks a lot.

Regards

Zhiyi Xie