Untested, but you might try:
instance (Num t) => YesNo t where ....
-deech
On Tue, Mar 15, 2011 at 2:21 PM, Amitava Shee
While reading "Learn You a Haskell for Great Good!" I came across the YesNo type class
I tried a minimal version as below
module Kind where
class Yesno a where yesno :: a -> Bool
instance Yesno Int where yesno 0 = False yesno _ = True
I was surprised to get an error
*Kind> :load kind.hs [1 of 1] Compiling Kind ( kind.hs, interpreted ) Ok, modules loaded: Kind. *Kind> yesno 10
<interactive>:1:6: Ambiguous type variable `t' in the constraints: `Num t' arising from the literal `10' at <interactive>:1:6-7 `Yesno t' arising from a use of `yesno' at <interactive>:1:0-7 Probable fix: add a type signature that fixes these type variable(s)
Turns out 10 in this instance is an Integer and I have not defined Yesno over Integer
Easy fix - just define an instance over Integer
instance Yesno Integer where yesno 0 = False yesno _ = True
My question - Is there a way to avoid this kind of boilerplate? What is the idiomatic way?
Thanks & Regards, Amitava Shee
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