[Haskell-beginners] Got "Non type-variable argument in the constraint" error for a simple function

[wizard]

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
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